This is basic operation for a gas-fired furnace. Actual operation will vary with furnace age, efficiency rating, and brand;
  1. The Furnace is in standby mode .
  2. 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.
  3. The Inducer draft/combustion blower motor starts.
  4. The airflow switch closes and completes a circuit through t safeties as long as everything is okay.
  5. 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.
  6. The gas valve opens, and the burner ignites,
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  11. The furnace is again in standby mode waiting for another call for heat.