Ignition, Safety Controls, and Electrical Interface
Domain 4, Ignition, Safety Controls, and Electrical Interface, tests ignition systems, flame sensing, safety switches, the heating sequence, and 24 V troubleshooting.
Ignition types
A standing pilot uses a thermocouple, producing about 25–30 mV open-circuit and 10–15 mV under load. Intermittent pilot, hot-surface ignition (HSI), and direct spark are common on modern furnaces. HSI igniters may be 120 V or 24 V; the module proves the igniter is powered before opening the gas valve. Spark gaps are typically 1/8–3/16 in.
Flame rectification and safeties
A flame sensor uses flame rectification: the module applies an AC signal, and the flame conducts enough to produce a small DC current, usually above 1 µA and often 2–6 µA. Clean the sensor with steel wool, not sandpaper. Safety devices include the high limit, rollout switch, pressure switch, and blocked-vent switch. A rollout switch is manual-reset and must not be bypassed.
Sequence
Thermostat calls for heat → inducer starts → pressure switch closes → igniter energizes → gas valve opens → flame is sensed → blower starts after a delay → blower stops after the call ends. Any failed step should stop or lock out the sequence.
Worked example
A furnace lights and shuts off after 5 seconds. The flame sensor reads 0.3 µA. Check sensor position, cleanliness, and burner ground. Low current tells the module the flame is unproven, so it closes the gas valve. Clean the sensor and verify at least 1 µA; if current stays low, inspect the flame and grounding.
Traps
Do not jumper a safety switch to 'test' it. Do not assume 24 V at the thermostat means 24 V at the gas valve; check under load. Do not replace a pressure switch before checking its tubing, the inducer, and the flue for blockage.