Gas-Fired Boiler and Burner Components
Domain 2, Gas-Fired Boiler and Burner Components, tests identification and function of the gas train, burner assembly, heat exchanger, and safety devices. The exam favors sequence-of-operation questions and component-function matching.
Core rules
- The gas train normally includes a manual shutoff valve, a redundant gas valve with regulator, a manifold, and burner orifices. The gas valve opens only after the ignition source is proven or energized.
- Ignition may be direct spark, hot surface, or intermittent pilot. Flame is proven by flame rectification (flame rod) or a thermocouple/thermopile on standing-pilot systems.
- An inducer or draft fan proves airflow through a pressure switch before the gas valve opens. A blocked flue, condensate trap, or failed switch prevents firing.
- Safety devices include high limit, low-water cutoff, rollout switch, blocked-vent or spill switch, relief valve, and expansion tank. Each has a distinct trip condition.
Worked example
A boiler receives a call for heat, the inducer runs, but the gas valve never opens. The pressure switch does not close. Reason: the sequence requires proof of draft before ignition. Check the pressure switch tubing, condensate trap, flue termination, and inducer. Do not jumper the switch; prove the cause.
Traps
- Confusing a flame rod with a thermocouple; they use different sensing principles.
- Assuming a high limit and an aquastat are interchangeable.
- Ignoring low-water cutoff on a hot-water boiler.
- Replacing the gas valve when the real fault is a blocked pressure-switch port.