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Sequence of Operation Gas Furnaces


Sequence of operation.

Natural draft furnace.
24 vollts is always present to the thermostat R terminal.
1.   When the thermostat closes its switch to call for heat, 24 volts is sent out of the thermostat on the W terminal.
2.   This 24 volts goes back into the furnace, then typically through 1 or 2 safety devices to the gas valve.
3.   If the standing pilot is lit. The gas valve opens, and the gas to the burners is then ignited by the pilot light.
4.   The heat exchanger is heated untill a device tells the blower to come on, usually a fan and limit control. Also see camstat fan and limit control.
5.   When the thermostat is satisfied, the switch in the thermostat opens, the gas valve closes, stopping fuel and the fan continues to blow untill the fan and limit cools, turning off the blower.

High efficiency operation.
24 vollts is always present to the thermostat R terminal.
1.   Thermostat sends 24 volts through white wire to furnace.
2.   Furnace checks pressure switch, and safety switch's for proper positions (open/closed).
3.   Draft inducer blower motor, starts to run.
4.   If vent and vacuum hoses are clear the pressure switch, closes. (only a few pressure switch's open to prove venting)
5.   After inducer plower purges the heat exchanger, either a pilot lights, a hot surface ignitor (HSI), or a spark begins.
6.   If the furnace is an intermittant pilot model, once the pilot lights, current is sent through the flame by a flame sensor, if current is present through the flame the main gas valve opens and the burners ignite.
7.   If the furnace is direct ignition, The HSI or spark ignites the flame. A flame sensor passes current through the flame, if current is present the main gas valve stays open and heat exchanger begins to warm up.
8.   Most high efficiency furnaces then use an electronic timer on the circuit board to start the blower after the burners are proved.
9.   Thermostat opens when desired temperature is reached.
10.   The gas valve closes, burners extinguish, and the timer starts again to turn off the fan.


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