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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Absorption
The passing of a substance or force into the body of another substance.

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Absorption Chiller
A type of air cooling device that uses absorption cooling to cool interior spaces.

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Accumulator
A component of a heat pump that stores liquid and keeps it from flooding the compressor. The accumulator takes the strain off the compressor and improves the reliability of the system.

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Adjustable Speed Drive
An electronic device that controls the rotational speed of motor-driven equipment such as fans, pumps, and compressors. Speed control is achieved by adjusting the frequency of the voltage applied to the motor.

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Air
The mixture of gases that surrounds the earth and forms its atmosphere composed of, by volume, 21 percent oxygen, 78 percent nitrogen.

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Air Change
A measure of the rate at which the air in an interior space is replace by outside (or conditioned) air by ventilation and infiltration; usually measured in cubic feet per time interval (hour), divided by the volume of air in the room.

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Air Conditioner
A device for conditioning air in an interior space. A Room Air Conditioner is a unit designed for installation in the wall or window of a room to deliver conditioned air without ducts. A Unitary Air Conditioner is composed of one or more assemblies that usually include an evaporator or cooling coil, a compressor and condenser combination, and possibly a heating apparatus. A Central Air Conditioner is designed to provide conditioned air from a central unit to a whole house with fans and ducts.

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Air Diffuser
An air distribution outlet, typically located in the ceiling, which mixes conditioned air with room air.

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Air Infiltration Measurement
A building energy auditing technique used to determine and/or locate air leaks in a building shell or envelope.

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Air-Source Heat Pump
A type of heat pump that transfers heat from outdoor air to indoor air during the heating season, and works in reverse during the cooling season.

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Air-to-Air Heat Pump
see Air-Source Heat Pump.

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Air-to-Water Heat Pump
A type of heat pump that transfers heat in outdoor air to water for space or water heating.

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Ambient Air
The air external to a building or device.

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Ambient Temperature
The temperature of a medium, such as gas or liquid, which comes into contact with or surrounds an apparatus or building element.

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Annual Fuel Utilization Efficiency (AFUE)
The measure of seasonal or annual efficiency of a residential heating furnace or boiler. It takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls.

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Appliance Energy Efficiency Ratings
The ratings under which specified appliances convert energy sources into useful energy, as determined by procedures established by the U.S. Department of Energy.

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Appliance Standards
Standards established by the U.S. Congress for energy consuming appliances in the National Appliance Energy Conservation Act (NAECA) of 1987, and as amended in the National Appliance Energy Conservation Amendments of 1988, and the Energy Policy Act of 1992 (EPAct). NAECA established minimum standards of energy efficiency for refrigerators, refrigerator-freezers, freezers, room air conditioners, fluorescent lamp ballasts, incandescent reflector lamps, clothes dryers, clothes washers, dishwashers, kitchen ranges and ovens, pool heaters, television sets (withdrawn in 1995), and water heaters. The EPAct added standards for some fluorescent and incandescent reflector lamps, plumbing products, electric motors, and commercial water heaters and Heating, Ventilation, and Air Conditioning (HVAC) systems. It also allowed for the future development of standards for many other products. The U.S. Department of Energy (DOE) is responsible establishing the standards and the procedures that manufacturers must use to test their models. These procedures are published in the Code of Federal Regulations (10 CFR, Ch. II, Part 430), January 1, 1994 (Federal Register).

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ASHRAE
Abbreviation for the American Society of Heating, Refrigeration, and Air-Conditioning Engineers.

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ASTM
Abbreviation for the American Society for Testing and Materials, which is responsible for the issue of many standard methods used in the energy industry.

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Atmospheric Pressure
The pressure of the air at sea level; one standard atmosphere at zero degrees centigrade is equal to 14.695 pounds per square inch (1.033 kilograms per square centimeter).

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Automatic Damper
A device that cuts off the flow of hot or cold air to or from a room as controlled by a thermostat.

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Available Heat
The amount of heat energy that may be converted into useful energy from a fuel.

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Average Demand
The demand on, or the power output of, an electrical system or any of its parts over an interval of time, as determined by the total number of kilowatt-hours divided by the units of time in the interval.

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Average Cost
The total cost of production divided by the total quantity produced.

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Axial Fans
Fans in which the direction of the flow of the air from inlet to outlet remains unchanged; includes propeller, tubaxial, and vaneaxial type fans.

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Backdrafting
The flow of air down a flue/chimney and into a house caused by low indoor air pressure that can occur when using several fans or fireplaces and/or if the house is very tight.

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Baffle
A device, such as a steel plate, used to check, retard, or divert a flow of a material.

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Balance Point
An outdoor temperature, usually 20 to 45 degrees Fahrenheit, at which a heat pump's output equals the heating demand. Below the balance point, supplementary heat is needed.

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Basal Metabolism
The amount of heat given off by a person at rest in a comfortable environment; approximately 50 Btu per hour (Btu/h).

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Baseboard Radiator
A type of radiant heating system where the radiator is located along an exterior wall where the wall meets the floor.

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Batt/Blanket
A flexible roll or strip of insulating material in widths suited to standard spacings of building structural members (studs and joists). They are made from glass or rock wool fibers. Blankets are continuous rolls. Batts are pre-cut to four or eight foot lengths.

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Bearing Wall
A wall that carries ceiling rafters or roof trusses.

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Bimetal
Two metals of different coefficients of expansion welded together so that the piece will bend in one direction when heated, and in the other when cooled, and can be used to open or close electrical circuits, as in thermostats.

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Bin Method
A method of predicting heating and/or cooling loads using instantaneous load calculation at different outdoor dry-bulb temperatures, and multiplying the result by the number of hours of occurrence of each temperature.

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Blackbody
An ideal substance that absorbs all radiation falling on it, and reflecting nothing.

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Blower
The device in an air conditioner that distributes the filtered air from the return duct over the cooling coil/heat exchanger. This circulated air is cooled/heated and then sent through the supply duct, past dampers, and through supply diffusers to the living/working space.

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Blower Door
A device used by energy auditors to pressurize a building to locate places of air leakage and energy loss.

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Boiler
A vessel or tank where heat produced from the combustion of fuels such as natural gas, fuel oil, or coal is used to generate hot water or steam for applications ranging from building space heating to electric power production or industrial process heat.

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Boiler Feedwater
The water that is forced into a boiler to take the place of that which is evaporated in the generation of steam.

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Boiler Horsepower
A unit of rate of water evaporation equal to the evaporation per hour of 34.5 pounds of water at a temperature of 212 degrees Fahrenheit into steam at 212 degrees F.

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Boiler Pressure
The pressure of the steam or water in a boiler as measured; usually expressed in pounds per square inch gauge (psig).

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Boiler Rating
The heating capacity of a steam boiler; expressed in Btu per hour (Btu/h), or horsepower, or pounds of steam per hour.

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Booster Pump
A pump for circulating the heat transfer fluid in a hydronic heating system.

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Boot
In heating and cooling system distribution ductwork, the transformation pieces connecting horizontal round leaders to vertical rectangular stacks.

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Brine
Water saturated or strongly impregnated with salt.

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British Thermal Unit (Btu)
The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

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Building Energy Ratio
The space-conditioning load of a building.

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Building Envelope
The structural elements (walls, roof, floor, foundation) of a building that encloses conditioned space; the building shell.

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Building Heat-Loss Factor
A measure of the heating requirements of a building expressed in Btu per degree-day.

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Building Orientation
The relationship of a building to true south, as specified by the direction of its longest axis.

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Building Overall Energy Loss Coefficient-Area Product
The factor, when multiplied by the monthly degree-days, that yields the monthly space heating load.

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Building Overall Heat Loss Rate
The overall rate of heat loss from a building by means of transmission plus infiltration, expressed in Btu per hour, per degree temperature difference between the inside and outside.

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Burner Capacity
The maximum heat output (in Btu per hour) released by a burner with a stable flame and satisfactory combustion.

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Calorie
The amount of heat required to raise the temperature of a unit of water, at or near the temperature of maximum density, one degree Celsius (or Centigrade [C]); expressed as a "small calorie" (the amount of heat required to raise the temperature of 1 gram of water one degree C), or as a "large calorie" or "kilogram calorie" (the amount of heat required to raise one kilogram [1,000 grams] of water one degree C); capitalization of the word calorie indicates a kilogram-calorie.

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Calorific Value
The heat liberated by the combustion of a unit quantity of a fuel under specific conditions; measured in calories.

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Capacity (Condensing Unit)
The refrigerating effect in Btu/h produced by the difference in total enthalpy between a refrigerant liquid leaving the unit and the total enthalpy of the refrigerant vapor entering it. Generally measured in tons or Btu/h.
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Carbon Dioxide
A colorless, odorless noncombustible gas with the formula CO2 that is present in the atmosphere. It is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic matter in the soil.

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Carbon Monoxide
A colorless, odorless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal, petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass.

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Caulking
A material used to seal areas of potential air leakage into or out of a building envelope.

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Ceiling Fan
A mechanical device used for air circulation and to provide cooling.

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Cellulose Insulation
A type of insulation composed of waste newspaper, cardboard, or other forms of waste paper.

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Central Heating System
A system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.

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Chiller
A device for removing heat from a gas or liquid stream for air conditioning/cooling.

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Chimney
A masonry or metal stack that creates a draft to bring air to a fire and to carry the gaseous byproducts of combustion safely away.

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Chimney Effect
The tendency of heated air or gas to rise in a duct or other vertical passage, such as in a chimney, small enclosure, or building, due to its lower density compared to the surrounding air or gas.

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Chlorofluorocarbon (CFC)
A family of chemicals composed primarily of carbon, hydrogen, chlorine, and fluorine whose principal applications are as refrigerants and industrial cleansers and whose principal drawback is the tendency to destroy the Earth's protective ozone layer.

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Clerestory
A window located high in a wall near the eaves that allows daylight into a building interior, and may be used for ventilation and solar heat gain.

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Climate
The prevailing or average weather conditions of a geographic region.

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Closed Cycle
A system in which a working fluid is used over and over without introduction of new fluid, as in a hydronic heating system or mechanical refrigeration system.

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Closed-Loop Geothermal Heat Pump Systems
Closed-loop (also known as "indirect") systems circulate a solution of water and antifreeze through a series of sealed loops of piping. Once the heat has been transferred into or out of the solution, the solution is recirculated. The loops can be installed in the ground horizontally or vertically, or they can be placed in a body of water, such as a pond. See horizontal ground loop, vertical ground loop, slinky ground loop, and surface water loop for more information on the different types of closed-loop geothermal heat pump systems.

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Codes
Legal documents that regulate construction to protect the health, safety, and welfare of people. Codes establish minimum standards but do not guarantee efficiency or quality.

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Coefficient of Heat Transmission (U-Value)
A value that describes the ability of a material to conduct heat. The number of Btu that flow through 1 square foot of material, in one hour. It is the reciprocal of the R-Value (U-Value = 1/R-Value).

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Coefficient of Performance (COP)
A ratio of the work or useful energy output of a system versus the amount of work or energy inputted into the system as determined by using the same energy equivalents for energy in and out. Is used as a measure of the steady state performance or energy efficiency of heating, cooling, and refrigeration appliances. The COP is equal to the Energy Efficiency Ratio (EER) divided by 3.412. The higher the COP, the more efficient the device.

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Coil
As a component of a heating or cooling appliance, rows of tubing or pipe with fins attached through which a heat transfer fluid is circulated and to deliver heat or cooling energy to a building.

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Combustion
The process of burning; the oxidation of a material by applying heat, which unites oxygen with a material or fuel.

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Combustion Air
Air that provides the necessary oxygen for complete, clean combustion and maximum heating value.

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Combustion Chamber
Any wholly or partially enclosed space in which combustion takes place.

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Combustion Gases
The gaseous byproducts of the combustion of a fuel.

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Commercial Building
A building with more than 50 percent of its floor space used for commercial activities, which include stores, offices, schools, churches, libraries, museums, health care facilities, warehouses, and government buildings except those on military bases.

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Comfort Zone
A frequently used room or area that is maintained at a more comfortable level than the rest of the building; also known as a "warm room."

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Compression Chiller
A cooling device that uses mechanical energy to produce chilled water.

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Compressor
A device used to compress air for mechanical or electrical power production, and in air conditioners, heat pumps, and refrigerators to pressurize the refrigerant and enabling it to flow through the system.

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Condensate
The liquid resulting when water vapor contacts a cool surface; also the liquid resulting when a vaporized working fluid (such as a refrigerant) is cooled or depressurized.

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Condensation
The process by which water in air changes from a vapor to a liquid due to a change in temperature or pressure; occurs when water vapor reaches its dew point (condensation point); also used to express the existence of liquid water on a surface.

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Condenser
The device in an air conditioner or heat pump in which the refrigerant condenses from a gas to a liquid when it is depressurized or cooled.

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Condenser Coil
The device in an air conditioner or heat pump through which the refrigerant is circulated and releases heat to the surroundings when a fan blows outside air over the coils. This will return the hot vapor that entered the coil into a hot liquid upon exiting the coil.

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Condensing Furnace
A type of heating appliance that extracts so much of the available heat content from a combusted fuel that the moisture in the combustion gases condenses before it leaves the furnace. Also this furnace circulates a liquid to cool the furnace's heat exchanger. The heated liquid may either circulate through a liquid-to-air heat exchanger to warm room air, or it may circulate through a coil inside a separate indirect-fired water heater.

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Condensing Unit
The component of a central air conditioner that is designed to remove heat absorbed by the refrigerant and transfer it outside the conditioned space.

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Conditioned Space
The interior space of a building that is heated or cooled.

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Conduction
The transfer of heat through a material by the transfer of kinetic energy from particle to particle; the flow of heat between two materials of different temperatures that are in direct physical contact.

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Conductivity (Thermal)
This is a positive constant, k, that is a property of a substance and is used in the calculation of heat transfer rates for materials. It is the amount of heat that flows through a specified area and thickness of a material over a specified period of time when there is a temperature difference of one degree between the surfaces of the material.

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Connected Load
The sum of the ratings of the electricity consuming apparatus connected to a generating system.

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Consumption Charge
The part of a power provider's charge based on actual energy consumed by the customer; the product of the kilowatt-hour rate and the total kilowatt-hours consumed.

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Convection
The transfer of heat by means of air currents.

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Conventional Heat Pump
This type of heat pump is known as an air-to air system.

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Cooling Capacity
The quantity of heat that a cooling appliance is capable of removing from a room in one hour.

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Cooling Degree Day
A value used to estimate interior air cooling requirements (load) calculated as the number of degrees per day (over a specified period) that the daily average temperature is above 65 degrees Fahrenheit (or some other, specified base temperature). The daily average temperature is the mean of the maximum and minimum temperatures recorded for a specific location for a 24 hour period.

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Cooling Load
That amount of cooling energy to be supplied (or heat and humidity removed) based on the sensible and latent loads.

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Cooling Tower
A structure used to cool power plant water; water is pumped to the top of the tubular tower and sprayed out into the center, and is cooled by evaporation as it falls, and then is either recycled within the plant or is discharged.

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Counterflow Heat Exchanger
A heat exchanger in which two fluids flow in opposite directions for transfer heat energy from one to the other.

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Cubic Foot (of Natural Gas)
A unit of volume equal to 1 cubic foot at a pressure base of 14.73 pounds standard per square inch absolute and a temperature base of 60 degrees Fahrenheit.

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Customer Charge
An amount to be paid for energy periodically by a customer without regard to demand or energy consumption.

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Customer Class
Categories of energy consumers, as defined by consumption or demand levels, patterns, and conditions, and generally included residential, commercial, industrial, agricultural.

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Damper
A movable plate used to control air flow; in a wood stove or fireplace, used to control the amount and direction of air going to the fire.

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Degree Day
A unit for measuring the extent that the outdoor daily average temperature (the mean of the maximum and minimum daily dry-bulb temperatures) falls below (in the case of heating, see Heating Degree Day), or falls above (in the case of cooling, see Cooling Degree Day) an assumed base temperature, normally taken as 65 degrees Fahrenheit, unless otherwise stated. One degree day is counted for each degree below (for heating) or above (in the case of cooling) the base, for each calendar day on which the temperature goes below or above the base.

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Degree Hour
The product of 1 hour, and usually the number of degrees Fahrenheit the hourly mean temperature is above a base point (usually 65 degrees Fahrenheit); used in roughly estimating or measuring the cooling load in cases where processes heat, heat from building occupants, and humidity are relatively unimportant compared to the dry-bulb temperature.

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Dehumidifier
A device that cools air by removing moisture from it.

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Demand
The rate at which electricity is delivered to or by a system, part of a system, or piece of equipment expressed in kilowatts, kilovoltamperes, or other suitable unit, at a given instant or averaged over a specified period of time.

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Demand Charge
A charge for the maximum rate at which energy is used during peak hours of a billing period. That part of a power provider service charged for on the basis of the possible demand as distinguished from the energy actually consumed.

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Demand(ed) Factor
The ratio of the maximum demand on an electricity generating and distribution system to the total connected load on the system; usually expressed as a percentage.

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Demand Power
see Peak Power

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Demand-Side Management (DSM)
The process of managing the consumption of energy, generally to optimize available and planned generation resources.

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Department of Energy (DOE)
A federal government agency created in 1977, that is entrusted to contribute to the welfare of the United States by providing technical information, and a scientific and educational foundation for technology, policy and institutional leadership to achieve efficiency in energy use, diversity in energy sources, a more productive and competitive economy, improved environmental quality, and a secure national defense.

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Derating
The production of energy by a system or appliance at a level less than its design or nominal capacity.

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Desiccant
A material used to desiccate (dry) or dehumidify air.

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Desiccant Cooling
To condition/cool air by dessication.

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Desiccation
The process of removing moisture; involves evaporation.

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Design Cooling Load
The amount of conditioned air to be supplied by a cooling system; usually the maximum amount to be delivered based on a specified number of cooling degree days or design temperature.

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Design Heating Load
The amount of heated air, or heating capacity, to be supplied by a heating system; usually the maximum amount to be delivered based on a specified number of heating degree days or design outside temperature.
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Design Temperature
The temperature that a system is designed to maintain (inside) or operate against (outside) under the most extreme conditions.

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Desuperheater
An energy saving device in a heat pump that, during the cooling cycle, recycles some of the waste heat from the house to heat domestic water.

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Dewpoint
The temperature to which air must be cooled, at constant pressure and water vapor content, in order for saturation or condensation to occur; the temperature at which the saturation pressure is the same as the existing vapor pressure; also called saturation point.

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Direct Vent Heater
A type of combustion heating system in which combustion air is drawn directly from outside and the products of combustion are vented directly outside. These features are beneficial in tight, energy-efficient homes because they will not depressurize a home and cause air infiltration, and backdrafting of other combustion appliances.

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District Heating
A heating system in which steam or hot water for space heating or hot water is piped from a central boiler plant or electric power/heating plant to a cluster of buildings.

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Diversity Factor
The ratio of the sum of the noncoincidental maximum demands of two or more loads to their coincidental maximum demands for the same period.

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DOE-2.1
A computer software program that simulates energy consumption of commercial buildings; used for design and auditing purposes.

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Double-Pane or Glazed Window
A type of window having two layers (panes or glazing) of glass separated by an air space. Each layer of glass and surrounding air space reradiates and traps some of the heat that passes through thereby increasing the windows resistance to heat loss (R-value).

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Double Wall Heat Exchanger
A heat exchanger in a solar water heating system that has two distinct walls between the heat transfer fluid and the domestic water, to ensure that there is no mixing of the two.

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Draft Hood
A device built into or installed above a combustion appliance to assure the escape of combustion byproducts, to prevent backdrafting of the appliance, or to neutralize the effects of the stack action of the chimney or vent on the operation of the appliance.

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Dry Bulb Temperature
The temperature of the air as measured by a standard thermometer.

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Dual Duct System
An air conditioning system that has two ducts, one is heated and the other is cooled, so that air of the correct temperature is provided by mixing varying amounts of air from each duct.

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Duct(s)
The round or rectangular tube(s), generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, located within a wall, floor, and ceiling that distributes heated or cooled air in buildings.
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Duct Fan
An axial flow fan mounted in a section of duct to move conditioned air.

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Earth-Coupled Ground Source (Geothermal) Heat Pump
A type of heat pump that uses sealed horizontal or vertical pipes, buried in the ground, as heat exchangers through which a fluid is circulated to transfer heat.

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Economizer
A heat exchanger for recovering heat from flue gases for heating water or air.

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Efficiency
Under the First Law of Thermodynamics, efficiency is the ratio of work or energy output to work or energy input, and cannot exceed 100 percent. Efficiency under the Second Law of Thermodynamics is determined by the ratio of the theoretical minimum energy that is required to accomplish a task relative to the energy actually consumed to accomplish the task. Generally, the measured efficiency of a device, as defined by the First Law, will be higher than that defined by the Second Law.

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Efficiency (Appliance) Ratings
A measure of the efficiency of an appliance's energy efficiency.

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Electrical Charge
A condition that results from an imbalance between the number of protons and the number of electrons in a substance.

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Electric Furnace
An air heater in which air is blown over electric resistance heating coils.

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Electric Rate
The unit price and quantity to which it applies as specified in a rate schedule or contract.

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Electric Rate Schedule
A statement of the electric rate(s), terms, and conditions for electricity sale or supply.

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Electric Resistance Heating
A type of heating system where heat, resulting when electric current flows through an "element" or conductor, such as Nichrome, which has a high resistance, is radiated to a room.

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Emissivity
The ratio of the radiant energy (heat) leaving (being emitted by) a surface to that of a black body at the same temperature and with the same area; expressed as a number between 0 and 1.

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Endothermic
A heat absorbing reaction or a reaction that requires heat.

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Energy Audit
A survey that shows how much energy you use in a building structure. It will help you find ways to use less energy.

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Energy Charge
That part of an electricity bill that is based on the amount of electrical energy consumed or supplied.

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Energy Efficiency Ratio (EER)
The measure of the instantaneous energy efficiency of room air conditioners; the cooling capacity in Btu/hr divided by the watts of power consumed at a specific outdoor temperature (usually 95 degrees Fahrenheit).
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Energy Guide Labels
The labels placed on appliances to enable consumers to compare appliance energy efficiency and energy consumption under specified test conditions as required by the Federal Trade Commission.

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Enthalpy
A thermodynamic property of a substance, defined as the sum of its internal energy plus the pressure of the substance times its volume, divided by the mechanical equivalent of heat. The total heat content of air; the sum of the enthalpies of dry air and water vapor, per unit weight of dry air; measured in Btu per pound (or calories per kilogram).

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Entropy
A measure of the unavailable or unusable energy in a system; energy that cannot be converted to another form.

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Evaporation
The conversion of a liquid to a vapor (gas), usually by means of heat.

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Evaporative Cooling
The physical process by which a liquid or solid is transformed into the gaseous state. For this process a mechanical device uses the outside air's heat to evaporate water that is held by pads inside the cooler. The heat is drawn out of the air through this process and the cooled air is blown into the home by the cooler's fan.

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Evaporator Coil
The inner coil in a heat pump that, during the cooling mode, absorbs heat from the inside air and boils the liquid refrigerant to a vapor, which cools the house.

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Expansion Tank
A tank used in a closed-loop solar heating system that provides space for the expansion of the heat transfer fluid in the pressurized collector loop.

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Expansion Valve
The device that reduces the pressure of liquid refrigerant thereby cooling it before it enters the evaporator coil in a heat pump.

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Fan
A device that moves and/or circulates air and provides ventilation for a room or a building.

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Fan Coil
A heat exchanger coil in which a fluid such as water is circulated and a fan blows air over the coil to distribute heat or cool air to the different rooms.

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Fan Velocity Pressure
The pressure corresponding to the outlet velocity of a fan; the kinetic energy per unit volume of flowing air.

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Fiberglass Insulation
A type of insulation, composed of small diameter pink, yellow, or white glass fibers, formed into blankets or batts, or used in loose-fill and blown-in applications.

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Filter (air)
A device that removes contaminants, by mechanical filtration, from the fresh air stream before the air enters the living space. Filters can be installed as part of a heating/cooling system through which air flows for the purpose of removing particulates before or after the air enters the mechanical components.

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Fin
A thin sheet of material (metal) of a heat exchanger that conducts heat to a fluid.

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Firing Rate
The amount of BTUs/hour or kWs produced by a heating system from the burning of a fuel.

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Flashing
Metal, usually galvanized sheet metal, used to provide protection against infiltration of precipitation into a roof or exterior wall; usually placed around roof penetrations such as chimneys.

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Flue
The structure (in a residential heating appliance, industrial furnace, or power plant) into which combustion gases flow and are contained until they are emitted to the atmosphere.

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Flue Gas
The gas resulting from the combustion of a fuel that is emitted to the flue.

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Foam (Insulation)
A high R-value insulation product usually made from urethane that can be injected into wall cavities, or sprayed onto roofs or floors, where it expands and sets quickly.

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Foam Board
A plastic foam insulation product, pressed or extruded into board-like forms, used as sheathing and insulation for interior basement or crawl space walls or beneath a basement slab; can also be used for exterior applications inside or outside foundations, crawl spaces, and slab-on-grade foundation walls.

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Foam Core Panels
A type of structural, insulated product with foam insulation contained between two facings of drywall, or structural wood composition boards such as plywood, waferboard, and oriented strand board.

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Forced Air System or Furnace
A type of heating system in which heated air is blown by a fan through air channels or ducts to rooms.

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Forced Ventilation
A type of building ventilation system that uses fans or blowers to provide fresh air to rooms when the forces of air pressure and gravity are not enough to circulate air through a building.

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Freon
A registered trademark for a cholorfluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.

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Fuel Efficiency
The ratio of heat produced by a fuel for doing work to the available heat in the fuel.

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Fuel Oil
Any liquid petroleum product burned for the generation of heat in a furnace or firebox, or for the generation of power in an engine. Domestic (residential) heating fuels are classed as Nos. 1, 2, 3; Industrial fuels as Nos. 4, 5, and 6.

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Full Sun
The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).

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Furnace
A combustion heating appliance in which heat is captured from the burning of a fuel for distribution, comprised mainly of a combustion chamber and heat exchanger.

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Geothermal Energy
Energy produced by the internal heat of the earth; geothermal heat sources include: hydrothermal convective systems; pressurized water reservoirs; hot dry rocks; manual gradients; and magma. Geothermal energy can be used directly for heating or to produce electric power.

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Geothermal Heat Pump
A type of heat pump that uses the ground, ground water, or ponds as a heat source and heat sink, rather than outside air. Ground or water temperatures are more constant and are warmer in winter and cooler in summer than air temperatures. Geothermal heat pumps operate more efficiently than "conventional" or "air source" heat pumps.

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Greenhouse Effect
A popular term used to describe the heating effect due to the trapping of long wave (length) radiation by greenhouse gases produced from natural and human sources.

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Greenhouse Gases
Those gases, such as water vapor, carbon dioxide, tropospheric ozone, methane, and low level ozone that are transparent to solar radiation, but opaque to long wave radiation, and which contribute to the greenhouse effect.

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Ground-Source Heat Pump
(See Geothermal Heat Pump)

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Head
A unit of pressure for a fluid, commonly used in water pumping and hydro power to express height a pump must lift water, or the distance water falls. Total head accounts for friction head losses, etc.

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Heat Balance
Energy output from a system that equals energy input.

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Heat Content
The amount of heat in a quantity of matter at a specific temperature and pressure.

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Heat Exchanger
A device used to transfer heat from a fluid (liquid or gas) to another fluid where the two fluids are physically separated.

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Heat Gain
The amount of heat introduced to a space from all heat producing sources, such as building occupants, lights, appliances, and from the environment, mainly solar energy.

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Heating Capacity (Also specific heat)
The quantity of heat necessary to raise the temperature of a specific mass of a substance by one degree.

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Heating Degree Day(s) (HDD)
The number of degrees per day that the daily average temperature (the mean of the maximum and minimum recorded temperatures) is below a base temperature, usually 65 degrees Fahrenheit, unless otherwise specified; used to determine indoor space heating requirements and heating system sizing. Total HDD is the cumulative total for the year/heating season. The higher the HDD for a location, the colder the daily average temperature(s).

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Heating Load
The rate of heat flow required to maintain a specific indoor temperature; usually measured in Btu per hour.

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Heating Season
The coldest months of the year; months where average daily temperatures fall below 65 degrees Fahrenheit creating demand for indoor space heating.

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Heating Seasonal Performance Factor (HSPF)
The measure of seasonal or annual efficiency of a heat pump operating in the heating mode. It takes into account the variations in temperature that can occur within a season and is the average number of Btu of heat delivered for every watt-hour of electricity used by the heat pump over a heating season.

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Heating Value
The amount of heat produced from the complete combustion of a unit of fuel. The higher (or gross) heating value is that when all products of combustion are cooled to the pre-combustion temperature, water vapor formed during combustion is condensed, and necessary corrections have been made. Lower (or net) heating value is obtained by subtracting from the gross heating value the latent heat of vaporization of the water vapor formed by the combustion of the hydrogen in the fuel.

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Heating, Ventilation, and Air-Conditioning (HVAC) System
All the components of the appliance used to condition interior air of a building.

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Heat Loss
The heat that flows from the building interior, through the building envelope to the outside environment.

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Heat Pump
An electricity powered device that extracts available heat from one area (the heat source) and transfers it to another (the heat sink) to either heat or cool an interior space or to extract heat energy from a fluid.

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Heat Recovery Ventilator
A device that captures the heat from the exhaust air from a building and transfers it to the supply/fresh air entering the building to preheat the air and increase overall heating efficiency.

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Heat Register
The grilled opening into a room by which the amount of warm air from a furnace can be directed or controlled; may include a damper.

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Heat Transfer
The flow of heat from one area to another by conduction, convection, and/or radiation. Heat flows naturally from a warmer to a cooler material or space.

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Heat Transfer Fluid
A gas or liquid used to move heat energy from one place to another; a refrigerant.

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Heat Transmission Coefficient
Any coefficient used to calculate heat transmission by conduction, convection, or radiation through materials or structures.

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Higher Heating Value (HHV)
The maximum heating value of a fuel sample, which includes the calorific value of the fuel (bone dry) and the latent heat of vaporization of the water in the fuel. (See moisture content and net (lower) heating value, below.)

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Horizontal Ground Loop
In this type of closed-loop geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are laid out in a plane parallel to the ground surface. The most common layouts either use two pipes, one buried at six feet, and the other at four feet, or two pipes placed side-by-side at five feet in the ground in a two-foot wide trench. The trenches must be at least four feet deep. Horizontal ground loops are generally most cost-effective for residential installations, particularly for new construction where sufficient land is available. Also see closed-loop geothermal heat pump systems.

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Hot Air Furnace
A heating unit where heat is distributed by means of convection or fans.

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Hot Water Heating Systems
(See Hydronic)

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Humidifier
A device used to maintain a specified humidity in a conditioned space.

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Humidity
A measure of the moisture content of air; may be expressed as absolute, mixing ratio, saturation deficit, relative, or specific.

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Hydronic Heating Systems
A type of heating system where water is heated in a boiler and either moves by natural convection or is pumped to heat exchangers or radiators in rooms; radiant floor systems have a grid of tubing laid out in the floor for distributing heat. The temperature in each room is controlled by regulating the flow of hot water through the radiators or tubing.

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Ignite
To heat a gaseous mixture to the temperature at which combustion takes place.

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Ignition Point
The minimum temperature at which combustion of a solid or fluid can occur.

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Infrared Radiation
Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.

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Insulation
Materials that prevent or slow down the movement of heat.

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Integrated Heating Systems
A type of heating appliance that performs more than one function, for example space and water heating.

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Internal Gain
The heat produced by sources of heat in a building (occupants, appliances, lighting, etc).

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Ion
An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.

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Ionizer
A device that removes airborne particles from breathable air. Negative ions are produced and give up their negative charge to the particles. These new negative particles are then attracted to the positive particles surrounding them. This accumulation process continues until the particles become heavy enough to fall to the ground.

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Jacket
The enclosure on a water heater, furnace, or boiler.

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Joule
A metric unit of energy or work; the energy produced by a force of one Newton operating through a distance of one meter; 1 Joule per second equals 1 Watt or 0.737 foot-pounds; 1 Btu equals 1,055 Joules.

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Kilovolt-Ampere (kVa)
A unit of apparent power, equal to 1,000 volt-amperes; the mathematical product of the volts and amperes in an electrical circuit.

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Kilowatt (kW)
A standard unit of electrical power equal to one thousand watts, or to the energy consumption at a rate of 1000 Joules per second.

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Kilowatt-hour
A unit or measure of electricity supply or consumption of 1,000 Watts over the period of one hour; equivalent to 3,412 Btu.

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Latent Cooling Load
The load created by moisture in the air, including from outside air infiltration and that from indoor sources such as occupants, plants, cooking, showering, etc.

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Latent Heat
The change in heat content that occurs with a change in phase and without change in temperature.

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Latent Heat of Vaporization
The quantity of heat produced to change a unit weight of a liquid to vapor with no change in temperature.

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Life Cycle Cost
The sum of all the costs both recurring and nonrecurring, related to a product, structure, system, or service during its life span or specified time period.

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Liquid-To-Air Heat Exchanger
A heat exchanger that transfers the heat contained in a liquid heat transfer fluid to air.

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Liquid-To-Liquid Heat Exchanger
A heat exchanger that transfers heat contained in a liquid heat transfer fluid to another liquid.

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Live Steam
Steam available directly from a boiler under full pressure.

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Long-Wave Radiation
Infrared or radiant heat.

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Loose Fill Insulation
Insulation made from rockwool fibers, fiberglass, cellulose fiber, vermiculite or perlite minerals, and composed of loose fibers or granules can be applied by pouring directly from the bag or with a blower.

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Losses (Energy)
A general term applied to the energy that is converted to a form that can not be effectively used (lost) during the operation of an energy producing, conducting, or consuming system.

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Lower (Net) Heating Value
The lower or net heat of combustion for a fuel that assumes that all products of combustion are in a gaseous state. (See Net Heating Value below.)

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Make-Up Air
Air brought into a building from outside to replace exhaust air.

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MCF
An abbreviation for one thousand cubic feet of natural gas with a heat content of 1,000,000 Btus, or 10 therms.

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Mechanical Systems
Those elements of building used to control the interior climate.

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Megawatt
One thousand kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

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Megawatt-hour
One thousand kilowatt-hours or 1 million watt-hours.

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Mixing Valve
A valve operated by a thermostat that can be installed in a water heating systems to mix cold water with water to maintain a safe water temperature.

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Modified Degree-Day Method
A method used to estimate building heating loads by assuming that heat loss and gain is proportional to the equivalent heat-loss coefficient for the building envelope.

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Multi-Zone System
A building heating, ventilation, and/or air conditioning system that distributes conditioned air to individual zones or rooms.

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Name Plate
A metal tag attached to a machine or appliance that contains information such as brand name, serial number, voltage, power ratings under specified conditions, and other manufacturer supplied data.

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National Electrical Code (NEC)
The NEC is a set of regulations that have contributed to making the electrical systems in the United States one of the safest in the world. The intent of the NEC is to ensure safe electrical systems are designed and installed. The National Fire Protection Association has sponsored the NEC since 1911. The NEC changes as technology evolves and component sophistication increases. The NEC is updated every three years. Following the NEC is required in most locations.

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Natural Cooling
Space cooling achieved by shading, natural (unassisted, as opposed to forced) ventilation, conduction control, radiation, and evaporation; also called passive cooling.

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Natural Draft
Draft that is caused by temperature differences in the air.

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Natural Ventilation
Ventilation that is created by the differences in the distribution of air pressures around a building. Air moves from areas of high pressure to areas of low pressure with gravity and wind pressure affecting the airflow. The placement and control of doors and windows alters natural ventilation patterns.

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Net Energy Production (or Balance)
The amount of useful energy produced by a system less the amount of energy required to produce the fuel.
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Net Present Value
The value of a personal portfolio, product, or investment after depreciation and interest on debt capital are subtracted from operating income. It can also be thought of as the equivalent worth of all cash flows relative to a base point called the present.

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Nitrogen Dioxide
This compound of nitrogen and oxygen is formed by the oxidation of nitric oxide (NO) which is produced by the combustion of solid fuels.

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Nitrogen Oxides (NOx)
The products of all combustion processes formed by the combination of nitrogen and oxygen.

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Nocturnal Cooling
The effect of cooling by the radiation of heat from a building to the night sky.

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Occupancy Sensor
An optical, ultrasonic, or infrared sensor that turns room lights on when they detect a person's presence and off after the space is vacated.

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Occupied Space
The space within a building or structure that is normally occupied by people, and that may be conditioned (heated, cooled and/or ventilated).

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Off-Peak
The period of low energy demand, as opposed to maximum, or peak, demand.

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On-Peak Energy
Energy supplied during periods of relatively high system demands as specified by the supplier.

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Open-Loop Geothermal Heat Pump System
Open-loop (also known as "direct") systems circulate water drawn from a ground or surface water source. Once the heat has been transferred into or out of the water, the water is returned to a well or surface discharge (instead of being recirculated through the system). This option is practical where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.

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Orientation
The alignment of a building along a given axis to face a specific geographical direction. The alignment of a solar collector, in number of degrees east or west of true south.

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Outside Air
Air that is taken from the outdoors.

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Outside Coil
The heat-transfer (exchanger) component of a heat pump, located outdoors, from which heat is collected in the heating mode, or expelled in the cooling mode.

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Overhang
A building element that shades windows, walls, and doors from direct solar radiation and protects these elements from precipitation.

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Passive/Natural Cooling
To allow or augment the natural movement of cooler air from exterior, shaded areas of a building through or around a building.

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Passive Solar (Building) Design
A building design that uses structural elements of a building to heat and cool a building, without the use of mechanical equipment, which requires careful consideration of the local climate and solar energy resource, building orientation, and landscape features, to name a few. The principal elements include proper building orientation, proper window sizing and placement and design of window overhangs to reduce summer heat gain and ensure winter heat gain, and proper sizing of thermal energy storage mass (for example a Trombe wall or masonry tiles). The heat is distributed primarily by natural convection and radiation, though fans can also be used to circulate room air or ensure proper ventilation.

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Payback Period
The amount of time required before the savings resulting from your system equal the system cost.

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Peak Clipping/Shaving
The process of implementing measures to reduce peak power demands on a system.

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Peak Demand/Load
The maximum energy demand or load in a specified time period.

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Peak Power
Power generated that operates at a very low capacity factor; generally used to meet short-lived and variable high demand periods.

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Peak Shifting
The process of moving existing loads to off-peak periods.

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Perimeter Heating
A term applied to warm-air heating systems that deliver heated air to rooms by means of registers or baseboards located along exterior walls.

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Permeance
A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4 F (23 C). A perm, short for permeance, is the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury.

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Phase Change
The process of changing from one physical state (solid, liquid, or gas) to another, with a necessary or coincidental input or release of energy.

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Plenum
The space between a hanging ceiling and the floor above or roof; usually contains HVAC ducts, electrical wiring, fire suppression system piping, etc.

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Pound of Steam
One pound of water in vapor phase; is NOT steam pressure, which is expressed as pounds per square inch (psi).

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Pound Per Square Inch Absolute (psia)
A unit of pressure [hydraulic (liquid) or pneumatic (gas)] that does not include atmospheric pressure.

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Power Factor (PF)
The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

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Present Value
The amount of money required to secure a specified cash flow at a future date at a specified return.

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Pressure Drop
The loss in static pressure of a fluid (liquid or gas) in a system due to friction from obstructions in pipes, from valves, fittings, regulators, burners, etc, or by a breech or rupture of the system.

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Pressurization Testing
A technique used by energy auditors, using a blower door, to locate areas of air infiltration by exaggerating the defects in the building shell. This test only measures air infiltration at the time of the test. It does not take into account changes in atmospheric pressure, weather, wind velocity, or any activities the occupants conduct that may affect air infiltration rates over a period of time.

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Primary Air
The air that is supplied to the combustion chamber of a furnace.

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Products of Combustion
The elements and compounds that result from the combustion of a fuel.

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Programmable Thermostat
A type of thermostat that allows the user to program into the devices' memory a pre-set schedule of times (when certain temperatures occur) to turn on HVAC equipment.

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Psi
Pounds of pressure per square inch.

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Psia
Pounds/force per square inch absolute.

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Psig
Pounds/force per square inch gauge.

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Psychrometer
An instrument for measuring relative humidity by means of wet and dry-bulb temperatures.

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Psychrometrics
The analysis of atmospheric conditions, particularly moisture in the air.

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Quad
One quadrillion Btu. (1,000,000,000,000,000 Btu)

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Radiant Barrier
A thin, reflective foil sheet that exhibits low radiant energy transmission and under certain conditions can block radiant heat transfer; installed in attics to reduce heat flow through a roof assembly into the living space.

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Radiant Ceiling Panels
Ceiling panels that contain electric resistance heating elements embedded within them to provide radiant heat to a room.

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Radiant Energy
Energy that transmits away from its source in all directions.

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Radiant Floor
A type of radiant heating system where the building floor contains channels or tubes through which hot fluids such as air or water are circulated. The whole floor is evenly heated. Thus, the room heats from the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced air heating systems.
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Radiant Heating System
A heating system where heat is supplied (radiated) into a room by means of heated surfaces, such as electric resistance elements, hot water (hydronic) radiators, etc.

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Radiation
The transfer of heat through matter or space by means of electromagnetic waves.

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Radiator
A room heat delivery (or exchanger) component of a hydronic (hot water or steam) heating system; hot water or steam is delivered to it by natural convection or by a pump from a boiler.

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Recirculated Air
Air that is returned from a heated or cooled space, reconditioned and/or cleaned, and returned to the space.
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Refrigerant
The compound (working fluid) used in air conditioners, heat pumps, and refrigerators to transfer heat into or out of an interior space. This fluid boils at a very low temperature enabling it to evaporate and absorb heat.

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Refrigeration
The process of the absorption of heat from one location and its transfer to another for rejection or recuperation.

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Refrigeration Capacity
A measure of the effective cooling capacity of a refrigerator, expressed in Btu per hour or in tons, where one (1) ton of capacity is equal to the heat required to melt 2,000 pounds of ice in 24 hours or 12,000 Btu per hour.

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Refrigeration Cycle
The complete cycle of stages (evaporation and condensation) of refrigeration or of the refrigerant.

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Relative Humidity
A measure of the percent of moisture actually in the air compared with what would be in it if it were fully saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.

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Resistance Heating
A type of heating system that provides heat from the resistance of an electrical current flowing through a conductor.

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Return Air
Air that is returned to a heating or cooling appliance from a heated or cooled space.

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Return Duct
The central heating or cooling system contains a fan that gets its air supply through these ducts, which ideally should be installed in every room of the house. The air from a room will move towards the lower pressure of the return duct.

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Reversing Valve
A component of a heat pump that reverses the refrigerant's direction of flow, allowing the heat pump to switch from cooling to heating or heating to cooling.

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R-Factor
See R-Value.

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Rock Wool
A type of insulation made from virgin basalt, an igneous rock, and spun into loose fill or a batt. It is fire resistant and helps with soundproofing.

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Roof Ventilator
A stationary or rotating vent used to ventilate attics or cathedral ceilings; usually made of galvanized steel, or polypropylene.

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R-Value
A measure of the capacity of a material to resist heat transfer. The R-Value is the reciprocal of the conductivity of a material (U-Value). The larger the R-Value of a material, the greater its insulating properties.
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Sealed Combustion Heating System
A heating system that uses only outside air for combustion and vents combustion gases directly to the outdoors. These systems are less likely to backdraft and to negatively affect indoor air quality.

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Seasonal Energy Efficiency Ratio (SEER)
A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of Btu of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season.

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Seasonal Performance Factor (SPF)
Ratio of useful energy output of a device to the energy input, averaged over an entire heating season.

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Sensible Cooling Effect
The difference between the total cooling effect and the dehumidifying effect.

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Sensible Cooling Load
The interior heat gain due to heat conduction, convection, and radiation from the exterior into the interior, and from occupants and appliances.

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Sensible Heat
The heat absorbed or released when a substance undergoes a change in temperature.

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Setback Thermostat
A thermostat that can be set to automatically lower temperatures in an unoccupied house and raise them again before the occupant returns.

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Simple CS (Caulk and Seal)
A technique for insulating and sealing exterior walls that reduces vapor diffusion through air leakage points by installing pre-cut blocks of rigid foam insulation over floor joists, sheet subfloor, and top plates before drywall is installed.

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Single-Package System
A year 'round heating and air conditioning system that has all the components completely encased in one unit outside the home. Proper matching of components can mean more energy-efficient operation compared to components purchased separately.

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Single-Phase
A generator with a single armature coil, which may have many turns and the alternating current output consists of a succession of cycles.

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SlinkyTM Ground Loop
In this type of closed-loop, horizontal geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are coiled like a SlinkyTM to allow more pipe in a shorter trench. This type of installation cuts down on installation costs and makes horizontal installation possible in areas it would not be with conventional horizontal applications. Also see closed-loop geothermal heat pump systems.

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Solenoid Valve
An automatic valve that is opened or closed by an electromagnet.

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Space Heater
A movable or fixed heater used to heat individual rooms.

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Specific Heat
The amount of heat required to raise a unit mass of a substance through one degree, expressed as a ratio of the amount of heat required to raise an equal mass of water through the same range.

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Specific Heat Capacity
The quantity of heat required to change the temperature of one unit weight of a material by one degree.

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Specific Humidity
The weight of water vapor, per unit weight of dry air.

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Split System Air Conditioner
An air conditioning system that comes in two to five pieces: one piece contains the compressor, condenser, and a fan; the others have an evaporator and a fan. The condenser, installed outside the house, connects to several evaporators, one in each room to be cooled, mounted inside the house. Each evaporator is individually controlled, allowing different rooms or zones to be cooled to varying degrees.

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Squirrel Cage Motor
This is another name for an induction motor. The motors consist of a rotor inside a stator. The rotor has laminated, thin flat steel discs, stacked with channels along the length. If the casting composed of bars and attached end rings were viewed without the laminations the casting would appear similar to a squirrel cage.
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Stack
A smokestack or flue for exhausting the products of combustion from a combustion appliance.

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Stack (Heat) Loss
Sensible and latent heat contained in combustion gases and vapor emitted to the atmosphere.

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Standard Air
Air with a weight of 0.075 pounds per cubic foot with an equivalent density of dry air at a temperature of 86 degrees Fahrenheit and standard barometric pressure of 29.92 inches of mercury.

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Standard Conditions
In refrigeration, an evaporating temperature of 5 degrees Fahrenheit (F), a condensing temperature of 86 degrees F., liquid temperature before expansion of 77 degrees F., and suction temperature of 12 degrees F.
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Standard Cubic Foot
A column of gas at standard conditions of temperature and pressure (32 degrees Fahrenheit and one atmosphere).

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Static Pressure
The force per unit area acting on the surface of a solid boundary parallel to the flow.

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Steam
Water in vapor form; used as the working fluid in steam turbines and heating systems.

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Steam Boiler
A type of furnace in which fuel is burned and the heat is used to produce steam.

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Supplementary Heat
A heat source, such as a space heater, used to provide more heat than that provided by a primary heating source.

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Supply Duct
The duct(s) of a forced air heating/cooling system through which heated or cooled air is supplied to rooms by the action of the fan of the central heating or cooling unit.

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Surface Water Loop
In this type of closed-loop geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are coiled into circles and submerged at least eight feet below the surface of a body of surface water, such as a pond or lake. The coils should only be placed in a water source that meets minimum volume, depth, and quality criteria. Also see closed-loop geothermal heat pump systems.

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Swamp Cooler
A popular term used for an evaporative cooling device.

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Temperature Humidity Index
An index that combines sensible temperature and air humidity to arrive at a number that closely responds to the effective temperature; used to relate temperature and humidity to levels of comfort.

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Temperature/Pressure Relief Valve
A component of a water heating system that opens at a designated temperature or pressure to prevent a possible tank, radiator, or delivery pipe rupture.

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Temperature Zones
Individual rooms or zones in a building where temperature is controlled separately from other rooms or zones.

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Tempering Valve
A valve used to mix heated water with cold in a heating system to provide a desired water temperature for end use.

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Therm
A unit of heat containing 100,000 British thermal units (Btu).

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Thermal Efficiency
A measure of the efficiency of converting a fuel to energy and useful work; useful work and energy output divided by higher heating value of input fuel times 100 (for percent).

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Thermal Resistance (R-Value)
This designates the resistance of a material to heat conduction. The greater the R-value the larger the number.

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Thermodynamic Cycle
An idealized process in which a working fluid (water, air, ammonia, etc) successively changes its state (from a liquid to a gas and back to a liquid) for the purpose of producing useful work or energy, or transferring energy.

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Thermography
A building energy auditing technique for locating areas of low insulation in a building envelope by means of a thermographic scanner.

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Thermostat
A device used to control temperatures; used to control the operation of heating and cooling devices by turning the device on or off when a specified temperature is reached.

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Three-phase Current
Alternating current in which three separate pulses are present, identical in frequency and voltage, but separated 120 degrees in phase.

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Time-of-Use (TOU) Rates
The pricing of electricity based on the estimated cost of electricity during a particular time block. Time-of-use rates are usually divided into three or four time blocks per twenty-four hour period (on-peak, mid-peak, off-peak and sometimes super off-peak) and by seasons of the year (summer and winter). Real-time pricing differs from TOU rates in that it is based on actual (as opposed to forecasted) prices which may fluctuate many times a day and are weather-sensitive, rather than varying with a fixed schedule.

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Ton (of Air Conditioning)
A unit of air cooling capacity; 12,000 Btu per hour.

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Total Heat
The sum of the sensible and latent heat in a substance or fluid above a base point, usually 32 degrees Fahrenheit.

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Turn Down Ratio
The ratio of a boiler's or gasifier's maximum output to its minimum output.

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Unitary Air Conditioner
An air conditioner consisting of one or more assemblies that move, clean, cool, and dehumidify air.

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Unvented Heater
A combustion heating appliance that vents the combustion by-products directly into the heated space. The latest models have oxygen-sensors that shut off the unit when the oxygen level in the room falls below a safe level.

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U-Value (see Coefficient of Heat Transmission)
The reciprocal of R-Value. The lower the number, the greater the heat transfer resistance (insulating) characteristics of the material.

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Vent
A component of a heating or ventilation appliance used to conduct fresh air into, or waste air or combustion gases out of, an appliance or interior space.

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Vent Damper
A device mounted in the vent connector that closes the vent when the heating unit is not firing. This traps heat inside the heating system and house rather than letting it draft up and out the vent system.

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Vented Heater
A type of combustion heating appliance in which the combustion gases are vented to the outside, either with a fan (forced) or by natural convection.

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Ventilation
The process of moving air (changing) into and out of an interior space either by natural or mechanically induced (forced) means.

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Ventilation Air
That portion of supply air that is drawn from outside, plus any recirculated air that has been treated to maintain a desired air quality.

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Vent Pipe
A tube in which combustion gases from a combustion appliance are vented out of the appliance to the outdoors.

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Vertical Ground Loop
In this type of closed-loop geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are laid out in a plane perpendicular to the ground surface. For a vertical system, holes (approximately four inches in diameter) are drilled about 20 feet apart and 100 to 400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building. Large commercial buildings and schools often use vertical systems because the land area required for horizontal ground loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching, or for existing buildings, as they minimize the disturbance to landscaping. Also see closed-loop geothermal heat pump systems.

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Water Jacket
A heat exchanger element enclosed in a boiler. Water is circulated with a pump through the jacket where it picks up heat from the combustion chamber after which the heated water circulates to heat distribution devices. A water jacket is also an enclosed water-filled chamber in a tankless coiled water heater. When a faucet is turned on water flows into the water heater heat exchanger. The water in the chamber is heated and transfers heat to the cooler water in the heat exchanger and is sent through the hot water outlet to the appropriate faucet.

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Water Source Heat Pump
A type of (geothermal) heat pump that uses well (ground) or surface water as a heat source. Water has a more stable seasonal temperature than air thus making for a more efficient heat source.

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Watt
The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of Voltage and Current (amperage).

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Watt-hour
A unit of electricity consumption of one Watt over the period of one hour.

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Wavelength
The distance between similar points on successive waves.

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Working Fluid
A fluid used to absorb and transfer heat energy.

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Zone
An area within the interior space of a building, such as an individual room(s), to be cooled, heated, or ventilated. A zone has its own thermostat to control the flow of conditioned air into the space.

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Zoning
The combining of rooms in a structure according to similar heating and cooling patterns. Zoning requires using more than one thermostat to control heating, cooling, and ventilation equipment.
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