Electrical Controls, Ignition, and Safety Devices
Domain 6 covers electrical controls, ignition, and safety devices. The exam tests control voltage, flame sensing, gas valve operation, limits, pressure switches, and the electrical code requirements that keep the appliance safe.
Control sequence
A call for heat from the thermostat energizes the control board. The board proves safety switches (limit, rollout, pressure), starts the inducer, proves airflow, then opens the gas valve and ignites. Flame is proven by a thermocouple, thermopile, or flame rectification. If proof fails, the valve closes and the board locks out or retries.
Electrical basics
Low-voltage controls are commonly 24 VAC from a transformer. Polarity and grounding matter: flame rectification depends on the burner being grounded and the flame conducting current to the sensing electrode. The Canadian Electrical Code and local rules govern wiring methods, bonding, and disconnects.
Worked example
A furnace ignites, then shuts off after 5 seconds. The flame sensor reads 0.8 microamps; the specification is at least 1.5 microamps. You clean the sensor, check the burner ground, verify polarity at the receptacle, and confirm the sensor is in the flame. The reading rises to 3.0 microamps and the furnace runs. The fix was grounding and sensor condition, not the gas valve.
Traps
- Replacing a gas valve before checking flame signal and ground.
- Ignoring reversed polarity or a poor ground.
- Jumping out a limit or rollout switch to test it.
- Using the wrong transformer voltage or fuse rating.
- Failing to confirm the safety lockout works.