I recently spoke with a customer who stated that his furnace was running but the temperature in the house was dropping. He said his thermostat was set at 70°F but the room temperature was 63°F. It was obvious that there was a problem with the heating cycle even though his blower was running.
The sequence of operation on all newer furnaces is:
- Thermostat has a call for heat which sends a 24-volt signal to the control board.
- This sends a signal to the inducer motor which makes sure the flue is not obstructed.
- The igniter is then activated which will either heat up the HSI (Hot Surface Igniter) or the spark igniter.
- The gas valve will then open and the burners will ignite.
- After heating up the heat exchanger the blower will start and shortly after that you will feel warm air coming out of the registers.
- Once the thermostat is satisfied, the gas valve and inducer will be shut down and the burners will be off.
- The blower motor will remain on for a short time to cool off the heat exchanger.