Combustion Safety and Indoor Air Quality
The combustion recipe
Methane plus oxygen yields carbon dioxide, water vapor, and heat. Stoichiometric combustion needs about 10 cubic feet of air per cubic foot of natural gas; real appliances run with excess air for safety. Too little oxygen, flame impingement, a cold flue, or recirculated flue gases produce carbon monoxide. Draft is the pressure difference that moves flue gases out; natural-draft appliances rely on a warm chimney and a draft hood, which is also a spillage point.
The combustion appliance zone (CAZ) is the test envelope. Worst-case depressurization means closing doors, turning on exhaust fans and dryers, and operating the appliance to see whether draft reverses or spillage occurs.
Traps
- Testing draft with a cold appliance, or testing only in the as-found condition rather than worst case.
- Reading a draft hood as if it were a flue collar, or sampling flue gas before steady-state operation.
- Reporting measured CO instead of air-free CO, which allows comparison across dilution levels.
- Forgetting that ambient CO in the CAZ matters as much as flue CO.
Action levels are version-specific: BPI-style programs typically flag ambient CO near 35 ppm and air-free flue CO near 400 ppm, with draft and depressurization thresholds that must be verified against the current standard and local code. When in doubt, shut down, tag, and document.
Worked example
Air-free CO = measured CO × 20.9 ÷ (20.9 − O2). A flue reading of 100 ppm CO at 5.0% O2 becomes 100 × 20.9 ÷ 15.9 ≈ 131 ppm air-free. Rule of thumb: any spillage, backdrafting, or CO above the program threshold means the appliance stays off until repaired and retested.