Electrical Controls, Ignition Systems, and Safeties
Electrical Controls, Ignition Systems, and Safeties
This domain tests low-voltage control logic, ignition systems, and the safety devices that must stop gas flow. The 24 V circuit typically includes the transformer, thermostat, control board, gas valve, and safety switches in series. The board sequences the inducer, ignition, gas valve, and blower.
Common ignition types:
- Standing pilot: continuous pilot with thermocouple or thermopile.
- Intermittent pilot: pilot lights only on a call, proved by flame rectification.
- Direct spark: spark at the main burner.
- Hot-surface ignition: a silicon carbide or nitride igniter heats and lights the burner.
Flame rectification uses the flame as a conductor. The board applies an AC voltage to the sensor; the flame rectifies it to a small DC current, typically 2–6 µA. The sensor must be in the flame, clean, and properly grounded. Polarity and ground matter.
Safeties
The limit switch opens on high temperature and usually resets automatically. The rollout switch is manual-reset and indicates flame rollout. The pressure switch proves inducer draft. A blocked flue, dirty filter, or failed blower can open a safety; never bypass it.
Worked example
An HSI glows brightly, the gas valve clicks, but no flame appears. Check gas supply and manifold pressure first, then inspect the igniter position and burner alignment. If gas pressure is 3.5 in. w.c. at the valve inlet but zero at the manifold, the valve is not opening or the control is not sending voltage. If the burner lights but drops out at 3 seconds, move to flame-sensor microamp testing rather than replacing the igniter.
Traps
Do not clean a flame sensor with sandpaper. Do not assume a glowing igniter is good; check current and position. Do not replace a limit switch without finding why it opened. Do not overlook a bad ground or reversed polarity on flame-sense systems.