Gas Furnace Sequence of Operation

The sequence of operation is the fixed order of events a furnace follows from a call for heat to shutdown. Each step is a permission granted by a control or a safety. Knowing the sequence lets you narrow a no heat call to a single step instead of guessing.

1. 1The thermostat closes the heat circuit and sends 24 volts to the control board.
2. 2The board checks that the pressure switch is open, proving it is not stuck closed, then starts the inducer.
3. 3The inducer creates negative pressure and the pressure switch closes, proving draft.
4. 4On a hot surface ignition system the igniter warms for its programmed time. On a spark or pilot system the appropriate ignition source is energized.
5. 5The gas valve opens and the burners light.
6. 6The flame sensor proves flame within the trial for ignition period, typically a few seconds.
7. 7After the blower on delay, the indoor blower starts and heat reaches the space.
8. 8When the thermostat is satisfied the gas valve closes, the inducer runs a post purge and the blower runs an off delay before stopping.

Every step has a proving device. If the furnace stops at a given point, the device that should grant the next permission is the place to look. A furnace that runs the inducer and then quits is almost always a pressure switch or draft problem. A furnace that lights and drops out after a few seconds is almost always a flame sensing or grounding problem.

Venting, combustion air and gas piping requirements vary by jurisdiction and by the manufacturer instructions for the specific appliance.