Warm Air Furnace – Oil and Gas - Exton, Wilmington, King of Prussia



How a Forced Warm Air Furnace Operates

A forced-air furnace is a very efficient way to heat a home in Southeastern PA, Delaware and Eastern Maryland. Although this type of system is simple in its operation, few people know exactly how a warm air furnace works. Knowing how the system generally works can help you troubleshoot problems and better communicate with your ServiceMark service technician in case something is wrong and a qualified NATE-certified technician is required.

The Main Components of a Warm Air Furnace:


This control is placed in the indoor area to be heated and tells the furnace when to turn on and off. When the air temperature around the thermostat dips about two degrees below the desired setting, the burner will come on, and then go off once the temperature is reached.

Gas or Oil Burner

This component is responsible for creating the heat that is required, and is controlled by the thermostat or by the fan limit control located in the plenum chamber that has a pre-set air temperature limit. If the burner seems to turn on and off too frequently it may be due to a dirty air filter or blocked fan. If changing the air filter does not resolve the issue, then your ServiceMark service professional should be contacted.

Heat Exchanger

The hot combustion gases that result by burning natural gas, propane or heating oil circulate through a heat exchanger releasing heat, which then is circulated by the system’s blower. The flue gas then exits the system by using a flue vent, carrying the gas outside of the home. Annual preventive maintenance can help keep the heat exchanger from cracking, which usually requires a complete system replacement.


The blower assembly is located between the cold air return and the main furnace. The blower pulls air from your home and sends it through the furnace and heat exchanger, which picks up the heat from the hot combustion gases. Then it is circulated throughout the home by using the home’s duct system. The air filter is located in the blower compartment, and regular-style filters should be replaced or cleaned on a monthly schedule.

Air Ducts and Registers

There are two types of air ducts in the home: cool air (return) ducts and warm air (supply) ducts. Warm air is produced by the furnace and pushed through the warm air ducts, entering each room through the warm air register. Remember to NEVER close off more than 20% of the warm air ducts, because the impeded airflow could damage the heat exchanger by overheating!

Combination Fan and Limit Control

The fan limit switch, which has only three settings, controls the blower: FAN ON, FAN OFF and LIMIT. The FAN ON setting ensures that the blower will not operate until there is warm air in the plenum, which prevents non-heated air from blowing to the home’s rooms. The FAN OFF setting keeps the blower operating even after the burner shuts down making sure all the hot air produced is circulated from the furnace. The LIMIT setting, a safety control, turns the burner off if the air in the warm air plenum reaches an unsafe temperature. This rarely happens, but may occur in the event of blocked ducts or improperly operating system controls.

Annual Maintenance

An Annual Preventative Maintenance Service, completed by your ServiceMark HVAC professional, is essential for keeping your furnace operating safely and efficiently, helping to eliminate emergency service calls, and prolonging equipment life.

Please call your local heating experts at ServiceMark Heating, Cooling and Plumbing for preventative maintenance, or in the event repair service or system replacement is required. ServiceMark has earned a 97% customer satisfaction rating, as measured by an independent third party, and has provided superior customer service for more than 60 years.