All HVAC systems require a method of bringing fresh air into the building for ventilation and the ability to move heated or cooled air within the building. In most buildings, this is accomplished with an air handling system. These systems are controlled, along with the return air supply from the building itself, to utilize a combination of return air and outside air that ensures appropriate levels of ventilation and heating or cooling comfort.
An air handling system consists of the fans, filters, dampers and ducts that deliver air throughout a building. In packaged heating/cooling systems, the required fans are usually included; air handling equipment is separately specified in engineer-designed, built-up systems.
Generally, air handling systems are either constant volume systems or variable air volume systems.
Constant Volume Systems
The constant volume, single zone system is characterized by a single space thermostat and a fan that delivers a constant volume of heated or cooled air (not a mixture of the two). Constant volume, multi-zone systems provide simultaneous heating and cooling to multiple temperature control zones from a single packaged unit. Individual temperature control zones receive air from two ducts, one for heating and the second for cooling. The thermostat calls for either heating or cooling as needed. Constant volume, multi-zone systems are very inefficient and expensive.
Another option, the constant-volume reheat system, provides a greater level of temperature and humidity control. In this system, each zone is served by a terminal heater (a reheat coil served by hot or chilled water in large, central systems or a direct expansion cooling coil and electric reheat in packaged systems), which reheats the air to a temperature compatible with the requirements of the zone.
Variable Air Volume (VAV) Systems
A well-designed, well-operated variable air volume system can be very efficient and allows for zone control that is not available with a constant volume, single zone system. With a variable volume system, the space can be divided into temperature control zones, each with its own thermostat. Each thermostat operates its zone’s automatic control damper, which opens and closes to allow either more or less conditioned air into the space. Adequate ventilation is provided by not allowing the damper to fully close.
Central System Air Handling System
Both constant and variable volume systems can employ economizers. Economizers bring in large volumes of outdoor air when it is sufficiently cooler than the indoor air, thus providing cooling at lower cost than operating the air conditioning. An economizer is generally an economical option in most climates. They are less effective in areas where the daily and seasonal temperature swings are very small.
Economizers use controls and supply and return air dampers to control outside air quantities. A number of methods exist for determining when the system changes over from the economizer to mechanical cooling, from simply setting a maximum outside temperature as the changeover point to “differential enthalpy” which compares the total heat (temperature and relative humidity) of the outside air to the total heat of the return air.
The air handling system contributes to the ventilation of a building and therefore to its indoor air quality. The standards for ventilation of commercial and industrial structures are set by ASHRAE in their Standard 62.1—Ventilation for Acceptable Indoor Air Quality and Standard 62.2—Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. (As of this writing, Standard 62.1-2007 and 62.2-2007 apply, but standards and addenda to those standards change periodically and should be referenced before HVAC system design.) The standards define requirements for ventilation and air-cleaning system design, installation, commissioning, operation and maintenance for all spaces intended for human occupancy except single-family houses, vehicles and aircraft.