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INFORMATION SERIES: Warm Air Heating

Warm air furnaces using gas, oil or electricity are the most common types of heating systems found in homes today.  Rather than relying on gravity to distribute the heated air throughout the house, most systems installed since the fifties are power driven by blower fans (see illustration).  Basically, the operation of a forced warm air system is quite simple. Circulating return air enters the furnace where a filter or electronic air cleaner traps air borne dirt. The blower forces the air up into a compartment called the heat exchanger which contains metal passageways heated to temperatures of several hundred degrees by rising combustion gases. The passageway exteriors heat the circulating air as the blower forces it pass and on into the network of ducts for distribution throughout the house. A fan control switches the blower on and of and shuts down the blower if the circulating air temperature rises too high.

Forced warm air systems heat uniformly with register temperatures in the 1100 to 125°F (43-52°C) range, thereby responding rapidly to changes in temperature. Contrary to common belief, turning up a thermostat does not raise the temperature of the air entering a room through the registers - it only signals the furnace to continue pumping the standard degree air into the room for a long period until the desired room temperature is reached. To conserve energy and improve home comfort, it is best to set a reasonable temperature on the thermostat and leave it alone rather than continuously adjusting it up and down on a whim.

Often the ductwork installed for a forced warm air system can serve for a central air conditioning system if the system has the proper air returns and a high volume blower fan. An air conditioning contractor can accurately determine the practicality of the dual use of the ductwork. A well designed system will have the ducts on the outside walls with both high and low adjustable supply registers and a return register in each room. In those homes with only a central return register, it is advisable to run the blower fan on continuous air circulation (CAC) to improve air circulation and reduce drafts.

The illustrated central heating system is operated by a thermostat and by automatic controls which keep the house at an even temperature and regulate the system on and of. If the house has two or more zones, there will be that many thermostats. Be certain the thermostats are properly positioned to suit your home's heating requirements efficiently.

Thermostats placed in cold hallways or hot kitchens will not provide balanced heat throughout the house. To conserve fuel, an automatic set-back (dual is best) thermostat should be considered to take the guesswork out of setting the thermostat. With gas systems, a flue damper can further
reduce energy use. The ultimate energy waster in a warm air system is a dirty air filter - the system must really labor to pump the air through such an obstruction.

Periodic maintenance of the system involves cleaning the burner and the heating surfaces, adjusting dampers, checking controls, oiling the blower motor and fan and adjusting the pulley belts. Many dealers and utility companies provide programs for such periodic (annual is preferred) system inspection and maintenance.

On a regular basis, you should check the condition of your electronic air cleaner (if you have one) and the system filters. Permanent, cleanable filters should be cleaned often during the heating system and disposable types replaced when dirty. Remember, the blower access door must be in place when the furnace is running.

Gas-fired furnaces have pilot lights which most utility companies suggest be left burning to prevent rust formation and condensation on heat exchanger surfaces. This practice also avoids delays in starting the system when chilly weather arrives. The furnace should also have a safety device to cut of the gas supply if the pilot light accidentally goes out. Newer furnaces use electronic ignition with no pilot light.

The HouseMaster Technical Staff advises that it is most important to maintain any humidifiers and any air conditioning condensate drains to prevent them from overflowing or leaking onto the heat exchanger causing rust and eventually failure. Some exchangers have failed due to this rusting after only 5 years, when the expected economic lifespan is 12 to 18 years. Since the heat exchanger is integral with many furnaces its replacement usually dictates the replacement of the entire furnace. An annual check by a heating professional will minimize the chances of this occurring. 

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