Electrical and Control Systems for Gas-Fired Equipment
This domain tests the electrical side of gas equipment: safe supply and bonding, hazardous locations, 24 V control circuits, the operating sequence, flame-safety devices and meter-based diagnosis.
Electrical installation
Follow the Canadian Electrical Code: correct supply and overcurrent protection, a disconnecting means within sight, Class 2 24 V control circuits from a listed transformer, and bonding of gas piping and CSST. Around propane equipment and in pits, classify the location (Class I Division 1 or 2) and use sealed or intrinsically safe equipment as required.
Sequence of operation
Thermostat call → inducer or combustion air blower → air pressure switch proves → ignition source energizes → gas valve opens → flame proven → main blower. Safety limits - high limit, rollout switch, blocked-flue and pressure switches - sit in series in the 24 V circuit, so any one opening stops the sequence.
Flame safety
- Thermocouples and powerpiles generate millivolts; a drooping pilot flame gives low output.
- Flame rectification produces microamps and depends on a clean sensor, correct sensor position in the flame, burner ground and correct polarity.
- Commercial burners use UV or IR scanners with a self-check shutter.
- Hot-surface igniters are checked by resistance; spark systems by gap and electrode condition.
Diagnosis
Read the fault code, then verify with a meter: voltage present, continuity through limits, microamp signal, transformer output and polarity. Do not bypass safety devices to "prove" a fault.
Worked example
A furnace lights, runs about 20 s and locks out. A series microamp meter reads 0.3 µA where the module requires at least 0.5 µA. Reasoning: a steady but low signal means flame is present but rectification is weak - check sensor contamination, sensor position, burner ground and moisture, not the gas valve. Cleaning and repositioning the sensor restores 1.8 µA and the lockout stops. Replacing the module would have changed nothing.
Traps that cost marks
Measuring microamps in parallel or with a low-impedance meter; jumping a limit to diagnose; assuming 24 V at the valve proves the operator is good; reversed polarity causing no flame sense; and fitting a non-listed replacement igniter or module.