Gas Furnace Components and Sequence of Operation
Gas Furnace Components and Sequence of Operation
This domain tests whether you can name the parts of a modern gas furnace and follow the exact order in which they operate. The control board is the sequence manager; every safety must prove before the next step is allowed.
Key components include the thermostat, 24 V transformer, control board, induced-draft blower, pressure switch, hot-surface igniter (HSI) or spark electrode, gas valve, burner manifold, flame sensor, limit switch, rollout switch, blower motor, and condensate system on condensing models.
Normal sequence
- Thermostat calls for heat (W terminal energized).
- Induced-draft blower starts; pressure switch closes when draft is proven.
- Igniter warms (HSI typically 17–40 seconds) or spark ignites the pilot or main burner.
- Gas valve opens; burner lights.
- Flame sensor proves flame within a few seconds; if not, the valve closes and the board retries or locks out.
- After a blower-on delay, the circulating blower starts.
- When the thermostat is satisfied, the gas valve closes, the inducer post-purges, and the blower runs through its off-delay.
Worked example
A furnace lights, runs about 4 seconds, then the burner drops out. It repeats three times and locks out. The pressure switch proved and the igniter glowed, so the failure is in flame proving, not ignition. A flame sensor reading of 0.4 µA is below the usual 2–6 µA range. Clean the sensor with steel wool, verify it is in the flame, check the burner ground and polarity, and retest. If microamps return to 4 µA and the burner stays lit, the sensor circuit was the cause.
Traps
Do not expect the blower to start before flame is proven. Do not treat a manual-reset rollout switch like an automatic limit. Do not jump a pressure switch to test a furnace. Remember that a dirty flame sensor often works when cold and fails when hot.