Combustion, Fuels, and Combustion Controls
Domain focus
Combustion questions test the fuel-air-ignition relationship, burner sequencing, flame supervision, draft, and efficiency. You must be able to explain why a burner locks out, why CO appears, and why more excess air is not always better.
Combustion fundamentals
Combustion requires fuel, oxygen, and ignition. Stoichiometric air is the exact amount needed for complete combustion; real burners use excess air to cover imperfect mixing. Too little air produces carbon monoxide and soot. Too much air lowers efficiency by heating nitrogen and carrying heat up the stack. Technicians measure oxygen or carbon dioxide in flue gas and stack temperature to tune the burner. Natural gas is lighter than air and rises; propane is heavier and can collect in low areas.
Burner sequence and safety controls
A typical sequence is: call for heat, prepurge the furnace and flue, prove airflow, ignition at low fire, prove pilot or main flame, then modulate to demand. The flame scanner must detect flame; if it does not, the safety shutoff valves close within the allowed timing. The high-limit control is manual reset. Gas trains include a regulator, manual valve, safety shutoff valves, pressure switches, and often a valve-proving system. Oil burners need filtration, preheat, atomization, and proper electrode setting.
Draft and efficiency
Natural draft uses a chimney; forced draft uses a fan upstream; induced draft uses a fan downstream. The furnace should run slightly negative to prevent hot gas leakage. A rising stack temperature at constant load suggests fouled heat-transfer surfaces or excess air. High CO suggests incomplete combustion, not simply "too little fuel."
Worked example
A gas burner locks out on startup. The operator resets it three times; it finally lights with a small puff. Reasoning: repeated resets without a successful purge can accumulate gas in the furnace. The correct response is to stop, investigate the flame scanner, ignition electrode, gas pressure, and air proving switch, then purge fully before one controlled retry. A puff is evidence of delayed ignition and must be treated as an unsafe condition.
Traps that cost marks
- Believing maximum excess air always means maximum safety.
- Adjusting gas pressure by feel instead of with a manometer.
- Jumpering a flame scanner or air pressure switch to "keep running."
- Ignoring CO while chasing CO2 readings.
- Using natural-gas orifices on propane without converting the burner.