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This is basic operation for a gas-fired furnace. Actual operation will vary with furnace age, efficiency rating, and brand;
- The Furnace is in standby mode .
- The Thermostat calls for heat by closing a switch inside the thermostat and sending 24 volts to the W (Heat) Terminal on the control board.
- The Inducer draft/combustion blower motor starts.
- The airflow switch closes and completes a circuit through t safeties as long as everything is okay.
- Depending on the furnace type, either the Hot Surface Igniter (HSI) or Spark Igniter activates unless the furnace is older and has a standing pilot.
- The gas valve opens, and the burner ignites,
- Once the flame sensor senses flame it shuts off the ignition source, if it doesn’t sense a flame it shuts the gas valve off and then tries again usually 3 times before it goes into lockout mode.
- Next the indoor fan/blower motor starts. Some start immediately but others will be delayed by a timer or thermostat in order to allow the heat exchanger to build up heat. This way the furnace doesn't start by blowing cold air through the home.
- Once the home reaches the high limit temperature, thermostat is satisfied and opens the switch, cutting off the 24 Volt signal to the control board.
- The Gas valve shuts off , and then immediately or after a delay , shuts off the Inducer draft/combustion blower motor. The delay blows the last of the hot air out of the ducts.
- The furnace is again in standby mode waiting for another call for heat.