Combustion Theory and Fuel Gas Fundamentals
What the exam tests
This domain covers the chemistry and physics of burning fuel gas: stoichiometry, Btu content, air requirements, flame characteristics, and the byproducts that determine safety and efficiency. Expect calculations and interpretation of CO/CO2 readings.
Core rules
- Natural gas is mostly methane, with a heating value near 1,000 Btu per cubic foot (HHV); propane near 2,500 Btu per cubic foot and 91,500 Btu per gallon.
- Complete combustion needs roughly 10 cubic feet of air per cubic foot of natural gas at stoichiometric ratio; appliances are set with 25–50% excess air for safety.
- Products: CO2, water vapor, nitrogen, and small amounts of CO and NOx. Water-vapor dew point is about 130°F, which is why condensing appliances recover latent heat.
- Target air-free CO below 100 ppm; CO2 is typically 8–9.5% for natural gas. A yellow flame, soot, or rising CO signals incomplete combustion.
Worked example
An 80,000 Btu/hr natural-gas furnace at 80% efficiency delivers 64,000 Btu/hr. Using 1,000 Btu/ft³, it burns about 80 ft³/hr. At 10:1 stoichiometric air, that is 800 ft³/hr of theoretical air; at 30% excess air, actual air is about 1,040 ft³/hr. This is why a tight closet needs combustion-air openings.
Traps
Do not confuse HHV and LHV; do not assume propane and natural-gas orifices interchange; do not read CO before the analyzer is zeroed. A blue flame can still produce high CO if the heat exchanger is dirty or the burner is misaligned.