A Residential Gas Furnace In Operation

Gas furnace sequence of operation.

A residential gas furnace in operation.

A gas furnace made in the early 1970s typically has an afue of about 65 percent. Control board does a safety check limit switches roll out switches. A forced air furnace heats your home through a heating cycle that looks like this. The inducer motor begins to spin and creates a vacuum to insure the left over gasses go out the exhaust pipe.

But for the most part manufacturers have all gone to a hot surface or direct spark ignition the sequence of operation is as follows. With greenspeed intelligence and variable speed components you can avoid wasting money for heating capacity you don t need. 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. Inducer motor comes on.

The inducer draft combustion blower motor then starts. Finally the sequence of operation for the typical standing pilot gas furnace goes like this. Variable speed furnace models using 50a51 505 506 507 controls. Natural gas or propane is ignited in the burner.

The lowest efficiency allowed by law for new gas furnaces is 78 percent and some new models achieve 97 percent. Hot surface ignitor begins getting hot direct spark ignitor begins to spark. The heat exchanger transfers its heat to the incoming air. A call for heat from the thermostat.

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. See furnace ecmtm2 motor operation. Flame sensor senses flame. The flames heat up a metal heat exchanger and exhaust out of the flue.

Normally a hot air furnace oil or gas burner will turn on immediately when the thermostat calls for heat. Oil furnaces are typically priced lower and provide more heat per btu but are less efficient. Heat call from the thermostat. Gas furnaces are typically more efficient and offer lower fuel costs but are more expensive than oil furnaces up front.

The furnace is in standby mode waiting for a call for heat the thermostat has a call for heat which closes a switch inside the thermostat and sends 24 volts to the w heat terminal on the control board. Pressure switch must close to go to the next step. 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. The thermostat reaches a set temperature and sends a signal for the furnace to turn on.