Furnaces – Packaged Heating & Cooling (Unitary Systems)
Typically buildings 100,000 square feet or less use one or multiple packaged or “unitary” heating and cooling systems. Most are roof-mounted units and incorporate a method of bringing in and conditioning outside air.
Packaged heating and cooling systems contain all the components needed to deliver heating and cooling, including a fan to move air, indoor cooling coil and outdoor condensing coil, compressors, a heating coil or furnace, air filters, dampers and controls. All commercial packaged units are air-cooled except water-cooled air conditioner/heat pump systems. These systems employ direct expansion (DX) air conditioning, whereby cooling is provided directly by the refrigerant via a direct expansion cooling coil rather than used to cool a stream of chilled water as in vapor compression chillers.
At about 10 tons of cooling, multiple compressors of different capacities are often included to allow for varying outputs and more efficient part load operation. Larger units can be designed for multi-zone, variable air volume applications.
Heating can be provided by a gas or oil furnace or electric heat. A gas or oil furnace is most common in smaller, single-zone, constant volume, roof-top units. Hot water or steam coils are alternative heating options, though not recommended for roof-mounted installation due to the possibility of freezing the coil or piping.
Although electric heat is generally more expensive than gas, if the electric option is chosen, it is suggested that first-source heating be provided by a heat pump, with electric resistance heating as a back-up.
Although systems vary, packaged systems are usually less costly and easier to maintain than central systems.
Split Systems or Indoor/Outdoor Systems
A variation of the packaged heating and cooling unit is the split system. This system consists of two factory-made assemblies—one positioned indoors and the second outside. For example, the fan, evaporative coil, filters and dampers may be in the indoor unit and the compressor and condenser coil in an outdoor cabinet. A furnace section can be included to provide heating. Separate, field-installed equipment is required to provide ventilation air.
Packaged Terminal Air Conditioners (PTACs)
For year round comfort with individual heating and cooling temperature control, PTACs are an option for hotels, motels, office buildings, schools, apartments, nursing homes, assisted living facilities and other multi-story buildings. The units combine electric air conditioning with gas-fired heat, electric resistance, electric heat pumps or hydronic heating. The PTAC is a self-contained, through-the-wall unit that generally fits an opening of 16-inches by 42-inches and is equipped with an attractive grille or faceplate to complement the building’s exterior appearance. The units slide in and out easily, and maintenance, repairs and replacement do not disrupt the entire building.
In the appropriate application, PTAC units offer many advantages over a central system. There is no need to run extensive ductwork or piping throughout the building and dedicated space for a central chiller or boiler plant is eliminated. Since the units fit into a wall opening, no floor space within suites is wasted. Individualized units mean that heating or air conditioning can be provided only to occupied/in use areas without conditioning the entire building. PTACs provide improved comfort for building occupants because each suite can be warmed or cooled to the occupant’s comfort level.
Although natural gas PTACs cost more to purchase than electric models, the savings in operating costs over the life of the unit more than compensate for the purchase premium and high efficiency models further enhance energy savings. Additionally, gas models are very quiet and there is no need to operate the compressor in heating mode.