Combustion Theory and Gas Fundamentals
Domain 1, Combustion Theory and Gas Fundamentals, tests how fuel and air become heat, how gas properties change orifice and pressure settings, and how flame quality and flue gases reveal combustion problems.
Stoichiometry and gas properties
Natural gas is mostly methane. Complete combustion follows CH4 + 2O2 → CO2 + 2H2O. Stoichiometric air is about 9.5 ft3 per ft3 of natural gas; real appliances add excess air, commonly 25–50%, leaving roughly 4–9% O2 in the flue. Natural gas is about 1,000–1,050 BTU/ft3 with specific gravity near 0.60. Propane is about 2,500 BTU/ft3 with specific gravity near 1.5. Propane therefore needs smaller orifices and a different manifold pressure: about 3.5 in. w.c. for natural gas and 10–11 in. w.c. for propane.
Flame behavior
Primary air makes the inner cone; secondary air completes combustion at the flame edge. A sharp blue inner cone with a stable flame indicates good air/fuel mixing. A lazy yellow flame can mean insufficient primary air, a dirty burner, a wrong orifice, or excessive manifold pressure. Flame impingement on a cold surface quenches combustion and raises CO.
Worked example
A flue reading shows 7% O2 and 90 ppm CO. Air-free CO = 90 × 20.9/(20.9 − 7) = 90 × 1.503 = 135 ppm. Dilution lowers the measured value, so air-free CO puts every reading on the same basis. 135 ppm is below the common 400 ppm air-free action level, but a rising trend still warrants investigation.
Traps
Never judge CO from the raw reading alone. Do not call a yellow flame normal or adjust only the air shutter before checking orifice, pressure, and burner condition. Excess air raises O2 and lowers CO2; it does not guarantee low CO.