Gas Heating Fundamentals and Combustion
Gas Heating Fundamentals and Combustion
This domain tests the physics and chemistry behind safe gas heating: fuel Btu content, the combustion equation, excess air, draft, and the difference between complete and incomplete combustion. Expect questions that ask you to interpret a combustion analyzer or calculate excess air.
Core rules
- Natural gas is roughly 1,000 Btu per cubic foot; propane vapor is roughly 2,500 Btu per cubic foot. These values change with altitude and gas composition, so use the local heating value when clocking a meter.
- Methane burns as CH4 + 2O2 → CO2 + 2H2O. Because air is only about 21% oxygen, stoichiometric combustion needs about 10 cubic feet of air per cubic foot of natural gas. Field settings typically run 25–50% excess air.
- Excess air from measured oxygen: EA% = O2 ÷ (20.9 − O2) × 100. Measure O2 on a dry basis.
- CO2 is a product of complete combustion; CO is a product of incomplete combustion. Air-free CO corrects for dilution and is the number to compare against limits.
- Draft is the pressure difference that moves flue gases. It must overcome the vent and provide safe exit without spilling products into the space.
Worked example
A flue sample reads 6.0% O2. EA = 6.0 ÷ (20.9 − 6.0) × 100 = 6.0 ÷ 14.9 × 100 ≈ 40%. That is acceptable excess air. If the same burner showed 9% O2, EA = 9 ÷ 11.9 × 100 ≈ 76%, indicating too much dilution and likely lower efficiency. If O2 fell to 2%, EA ≈ 10.6%, close to the edge where CO can rise.
Traps
Do not confuse CO2 with CO. Do not use room-air oxygen as the flue value. Do not assume more excess air is always better: it lowers efficiency and can cool the flame. Remember that a clean blue flame is not proof of correct combustion; only analysis confirms it.