A Residential Gas Furnace In Operation

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A residential gas furnace in operation.

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. An oil furnace generates heat using oil while gas furnaces are fueled by natural gas propane. The heat exchanger transfers its heat to the incoming air. A forced air furnace heats your home through a heating cycle that looks like this.

Flame sensor senses flame. The flames heat up a metal heat exchanger and exhaust out of the flue. Hot surface ignitor begins getting hot direct spark ignitor begins to spark. Heat call from the thermostat.

Finally the sequence of operation for the typical standing pilot gas furnace goes like this. The inducer draft combustion blower motor then starts. The thermostat reaches a set temperature and sends a signal for the furnace to turn on. Control board does a safety check limit switches roll out switches.

Natural gas or propane is ignited in the burner. The inducer motor begins to spin and creates a vacuum to insure the left over gasses go out the exhaust pipe. 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. While most furnaces will follow the below sequences there may be variations in models and manufacturers.

Variable speed furnace models using 50a51 505 506 507 controls. 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. Gas furnace sequence of operation. A gas furnace made in the early 1970s typically has an afue of about 65 percent.

The lowest efficiency allowed by law for new gas furnaces is 78 percent and some new models achieve 97 percent. A call for heat from the thermostat. Gas furnaces are typically more efficient and offer lower fuel costs but are more expensive than oil furnaces up front. Normally a hot air furnace oil or gas burner will turn on immediately when the thermostat calls for heat.

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. Inducer motor comes on. See furnace ecmtm2 motor operation. 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.