Gas Furnace

A gas furnace burns natural gas or propane inside a sealed heat exchanger and uses a blower to move building air across the outside of that heat exchanger. The products of combustion never mix with the air delivered to the building - that separation is the entire safety premise of the appliance.

1. 1The thermostat closes a call for heat to the control board.
2. 2The inducer motor starts and establishes a draft through the heat exchanger and vent.
3. 3The pressure switch proves that draft and closes.
4. 4The ignition source energizes - a hot surface ignitor warms up, or a spark and pilot are established.
5. 5The gas valve opens and the burners light.
6. 6The flame sensor proves flame within the trial for ignition period.
7. 7After a fixed delay the circulating blower starts and delivers heated air.
8. 8When the call ends, the gas valve closes, and the inducer and blower run off over a timed period.

• Control board and transformer
• Inducer motor and pressure switch
• Hot surface ignitor or spark ignition system
• Gas valve and burner assembly
• Flame sensor
• Heat exchanger
• Limit switches and rollout switches
• Circulating blower and motor

• A weak flame signal causes the furnace to light and then shut down after a few seconds.
• A blocked vent or condensate trap keeps the pressure switch from proving draft.
• A dirty filter or closed registers cause the limit switch to open on high temperature.
• A cracked or corroded heat exchanger is a combustion safety failure and requires the appliance to be taken out of service.