(The following is taken from SMACNA's Residential Comfort
System Installation Manual.)
Most residences are comfort conditioned with filtered and heated air. A large percentage also have mechanical or evaporate cooling. In areas requiring cooling, almost all new construction contains the complete package, and older structures are continually being retrofitted with add-on equipment and systems. The following sections briefly describe each general category of residential heating and cooling systems, and their recommended application.
Heating systemsSince many system variations are available for residential comfort heating, the appropriate system should be selected based on the climate, the type of structure, energy costs and availability, and consumer preferences. Forced-air heating systems find their greatest popularity in regions that use unitary air conditioners for cooling.
Hydronic systems are more frequently used in northern climates. The hot water or steam is usually supplied from individual boilers located on the site. In high-density population areas, hot water or steam may be supplied by a utility. Hydronic systems are also used frequently in high-rise apartment buildings in all climates.
Types and locationsThe source of energy is a major consideration in equipment selection. Gas, oil and electricity are the principal energy sources used for forced-air residential heating. The energy source must be selected in relation to the present and expected future availability of the energy form, the present and projected annual heating costs, the initial equipment costs, installation costs, maintenance costs and the expected life of the equipment. The best selection is one that provides the heating function at the lowest total owning and operating cost to the consumer.
Forced-air heating systemsThe most widely used form of residential heating is the forced-air system. This system includes a central heat source such as a furnace or an air-to-air or hydronic (water-source) heat pump. The heated air is distributed throughout the residence by a system of ducts or pipes. These air-distribution systems may have metal, plastic or fiberglass construction, with flexible ducts and pipes allowed in many applications. Cooling, humidifying and air-filtering functions can readily be included with this type of system.
Forced-air heating equipment is available in many variations to accommodate equipment-location preferences and ducting practices. These variations are classified by component configuration and airflow path using such terms as horizontal flow, upflow and downflow.
When supply ductwork is located above the equipment, an upflow system is usually selected. Horizontal equipment is best suited for installations in attics and crawl spaces. Downflow (sometimes referred to as counterflow in fossil-fuel furnaces) equipment is usually installed in utility rooms, garages or closets or structures built on concrete-slab floors or crawl spaces. The supply plenum extends down into ducts in the slab or crawl space.
If the heating equipment and duct system are located in the conditioned space, no allowances need to be made for jacket loss or duct loss in the sizing of the furnace. However, if the furnace or ducts, or both, are located in an unconditioned space, the calculated heat loss of the structure must be increased. Ductwork in unconditioned spaces should be insulated to minimize these losses.
Air distributionTo obtain comfortable winter conditioning, it is essential to select the proper air-distribution system, supply outlets, and supply outlet and return inlet locations. Supply outlets fall into four general groups as defined by their air-discharge pattern:
- Horizontal high
- Vertical non-spreading
- Vertical spreading
- Horizontal low
In general, the best outlet types for heating are those that provide a vertical-spreading air jet located in the floor at outside walls, preferably under windows, to blanket the cold areas and to counteract the cold drafts. This method of introducing warm air into a structure is called a perimeter system and is preferred for heating, particularly in cold regions. In milder regions, perimeter-floor and low-sidewall outlets are acceptable for heating. However, if summer cooling is contemplated, ceiling diffusers are also satisfactory.
In split-level homes and open two-story houses, comfort heating and air balancing are complicated by gravity circulation. The upper levels tend to overheat because of the tendency for the warm air in the lower level to rise to the upper level. Extra outlets in the lower levels are adequate return openings in the upper levels, particularly at the top of stairways. Together, with continuous air, circulation tend to overcome the gravitycirculation problem. The use of more than one unit or zoning should be considered for split-level houses.
Hydronic heating systemsAnother widely used form of residential heating is the hydronic system. The heart of the system is a central hot water or steam boiler. The heated fluid is distributed throughout the residence by a system of piping to baseboard convectors or fan-coil units located in each room.
(For information on ordering the Sheet Metal and Air Conditioning Contractors' National Association's Residential Comfort System Installation Manual, write SMACNA, 4201 Lafayette Center Drive, Chantilly, VA 20151-1209; call (703) 803-2989; see www.smacna.org on the Internet.)