Warm-air systems

Air heated in a furnace travels via supply ducts to the rooms. The warm air enters the rooms and is discharged through wall or floor registers or ceiling diffusers. The cooler air in the room, being displaced by the heated air, travels through return ducts back to the furnace where it is reheated and recirculated. If the system does not have cold-air return ducts, the cold air travels back to the furnace via gravity. Usually the stairway allows the cold air to travel to the first floor, and grilles in the floor of the first level then allow it to recirculate to the furnace. Of course, doors must be open to permit the air movement. Where doors are closed, the heat rises in a very restricted manner.

The air in a warm-air system can be supplied to the furnace by recirculating heated air through ducts or by drawing in cool air from the basement. The latter method is no longer used in modern warm-air systems because of the potential danger of exposure to poisonous gas and because of the heating inefficiency. If the chimney is clogged, the exhaust gas, which contains poisonous carbon monoxide, backs up into the basement. It is then drawn into the furnace and distributed throughout the house. Also, since the temperature of the basement air is lower than the temperature of the recirculated air, more fuel will be required to heat it to the desired temperature. Although this method of supplying air to the system is for the most part no longer used, it will still be found in many older systems. You can easily spot it. Look for a large opening in the furnace casing.

There is only one condition under which a furnace or its heat exchanger must be replaced: when the walls in the heat exchanger that separate the circulating air from the hot exhaust gases deteriorate because of age, premature corrosion, or cracks and thus allow the exhaust gases to mix with the circulating air. Included in the exhaust gases is carbon monoxide which is poisonous. The mixture of air and exhaust gases circulating around the house is quite dangerous. The “life expectancy” of a furnace refers to the average number of years of usage that can be expected before the walls of the heat exchanger deteriorate. For many modern furnaces, the projected life is between fifteen and twenty years, although some older ones have been safely operational for well over thirty years. A number of manufacturers are now providing replacement heat exchangers so that the overall life expectancy of the furnace can be extended indefinitely.


Warm-air systems have an advantage over other types of heating systems in that the air in the house can be cleaned (dust particles removed by filtering) and humidified. Most systems use either inexpensive disposable filters or permanent-type filters that require periodic washing. Some systems utilize an electronic filter, which is very effective in removing dust and pollen from the air.

Not all warm-air furnaces are equipped with a humidifier for adding moisture to the circulating air. If you do not see one, you should consider installing one. The humidifier may be mounted in the main return or supply duct and is usually located near the furnace. Humidifiers such as the evaporative-plate or wick type add some moisture to the circulating air. However, they are not totally effective. A more positive introduction of moisture into the airstream can be achieved with a power spray humidifier that is controlled by a humidity-sensing device.

Additional advantages of a warm-air system:

  • Adaptability to a central air-conditioning system. (See chapter 17.) The distribution ducts and the furnace blower can be used to circulate the cool air. This results in considerable cost savings when installing a central cooling system.
  • There are no distribution pipes to freeze and burst. Consequently, if the heating system is not operational for several days during the winter, as would be the case in the event of an extended power failure, there would be no need to worry about the distribution system freezing (a condition that can occur with a hot-water heating system). Of course, regardless of the type of heating system, during an extended power failure, the domestic water pipes are vulnerable to freezing.
  • The replacement cost for a new warm-air furnace is less than the replacement cost for a new hot-water or steam boiler.


The major disadvantage of a warm-air system is that in the event of a faulty heat exchanger, the exhaust gases will mix with the circulating air and be distributed around the house.

In a multizoned warm-air heating system, the zones are not totally independent of one another. The zones are controlled by motorized dampers located in the ducts. When the dampers are closed, they block the airflow in their respective ducts, preventing that portion of the house from being heated. However, some air will always flow around the closed damper, decreasing the overall efficiency of operation. In quality-constructed homes, zone control is often obtained by using two separate furnaces with separate distribution systems rather than by using motorized dampers.

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