Carbon Monoxide Safety, Diagnostics, and Efficiency Tuning
CO fundamentals
CO is colorless, odorless, and binds hemoglobin far more readily than oxygen. It comes from incomplete combustion: rich mixture, flame impingement, blocked or undersized flue, backdrafting, overfiring, or a failed heat exchanger.
Limits to quote
- Ambient: 9 ppm over 8 hours (EPA/ASHRAE guidance); OSHA permissible exposure limit 50 ppm over 8 hours.
- Flue: ANSI limits vented appliances to 400 ppm air-free CO; many commissioning specs target under 100 ppm.
- Worst-case depressurization in the combustion appliance zone: beyond about -5 Pa, natural-draft appliances are at risk of spillage.
Diagnostics and tuning
- Measure before and after every adjustment; record O2, CO, CO2, stack and ambient temperature.
- Natural gas target: roughly 3-6% O2, about 8.5-9.5% CO2, with CO stable and low.
- Net stack temperature drives sensible efficiency in non-condensing equipment. Condensing units should run 100-130F flue temperature, below the roughly 130F water-vapor dew point where latent heat is recovered.
Worked example
A furnace runs 8.5% O2 and a 350F net stack. After trimming gas pressure and air, it reads 4.5% O2, so excess air falls from about 68% to 27%, and the net stack drops to 300F, worth roughly 2 points of efficiency. Air-free CO stays at 40 ppm, so the change is safe as well as economical.
Traps
- "The alarm didn't sound, so it's safe."
- Tuning for maximum CO2 without watching CO.
- Testing only in mild weather, when draft is easiest.
- Using O2 targets on oil; oil is judged on CO2 and smoke number.
- Skipping the worst-case depressurization test after a tighten-up.