A Residential Gas Furnace In Operation

Natural gas or propane is ignited in the burner.

A residential gas furnace in operation.

A call for heat from the thermostat. Gas furnace sequence of operation. Gas furnaces are typically more efficient and offer lower fuel costs but are more expensive than oil furnaces up front. The inducer motor begins to spin and creates a vacuum to insure the left over gasses go out the exhaust pipe.

Carrier gas furnaces with greenspeed intelligence monitor temperature and humidity inside and out then automatically vary furnace operation as needed to provide an ideal combination of comfort and energy efficiency. Normally a hot air furnace oil or gas burner will turn on immediately when the thermostat calls for heat. With greenspeed intelligence and variable speed components you can avoid wasting money for heating capacity you don t need. But for the most part manufacturers have all gone to a hot surface or direct spark ignition the sequence of operation is as follows.

The white rodgers integrated furnace control i f c is an automatic ignition control module that uses microcomputer based circuitry to continuously monitor analyze and control the proper operation of the gas. Flame sensor senses flame. Finally the sequence of operation for the typical standing pilot gas furnace goes like this. A gas furnace made in the early 1970s typically has an afue of about 65 percent.

An oil furnace generates heat using oil while gas furnaces are fueled by natural gas propane. The inducer draft combustion blower motor then starts. While most furnaces will follow the below sequences there may be variations in models and manufacturers. However the blower or fan unit that delivers warm air to the living area will not turn on until the fan limit switch in the hot air supply plenum or furnace top senses that the air in that area is warm.

Heat call from the thermostat. Inducer motor comes on. Oil furnaces are typically priced lower and provide more heat per btu but are less efficient. The flames heat up a metal heat exchanger and exhaust out of the flue.

The lowest efficiency allowed by law for new gas furnaces is 78 percent and some new models achieve 97 percent. The gas side of the standing pilot gas furnace consists of a gas valve a pilot assembly the pilot and main burners and the flue which exhausts the gases after the combustion process is complete. The thermostat reaches a set temperature and sends a signal for the furnace to turn on. The heat exchanger transfers its heat to the incoming air.

Pressure switch must close to go to the next step. Hot surface ignitor begins getting hot direct spark ignitor begins to spark. Variable speed furnace models using 50a51 505 506 507 controls. Control board does a safety check limit switches roll out switches.