Combustion and Fuel Systems
Domain 5 - Combustion and Fuel Systems. This domain tests air/fuel ratio, draft, purge and light-off sequence, and the gas and oil trains that feed the burner.
Combustion fundamentals
Combustion requires fuel, air, and ignition in the right proportions. Stoichiometric air is the exact amount for complete burning; in practice excess air is supplied to avoid CO and smoke. Too much excess air lowers efficiency by carrying heat up the stack; too little produces CO, soot, and possibly unburned fuel in the furnace. Draft may be natural, forced, induced, or balanced. A furnace under positive pressure can leak hot gas; excessive negative pressure pulls in cold air and raises excess air.
Burner sequence
Pre-purge the furnace with air (typically about five furnace volume changes) before every light-off. Prove low fire, prove pilot flame, open the main safety shutoff valves, and prove main flame within the trial-for-ignition time; otherwise lock out. The gas train includes regulator, strainers, two safety shutoff valves with a leak test, low and high gas pressure switches, and a vent valve. Oil systems add a pump, heater, strainer, and atomizer - viscosity control is what makes atomization work.
Worked example
A flue gas analyzer reads 4 percent O2. Approximate excess air = O2 ÷ (21 − O2) × 100 = 4 ÷ 17 × 100 = 23.5 percent. If the burner was set at 15 percent, the extra air is unnecessary and raises stack loss. Reduce excess air cautiously and recheck CO and smoke - the goal is the lowest excess air that still burns cleanly.
Traps
Lighting off without a completed purge; adding fuel before air when increasing load; assuming a high gas pressure switch protects against low pressure; tuning to O2 alone while ignoring CO or air infiltration.