Raw CO readings drop as more air dilutes the flue gas, which can make a bad burner look clean. Air-free CO scales the reading as if no excess air were present, using the measured O2. Appliance standards and many manufacturers treat 400 ppm air-free as the upper limit, and a well-tuned gas boiler usually reads well under 100 ppm.
Air-free CO is 620 at high fire, that burner's not going back in service until I find where the air's short.
Also heard as
- CO air-free
- COAF
- CO corrected to 0 percent O2
Related terms
Combustion analysis
Testing a burner's flue gas for oxygen, CO, CO2 and temperature to set it up safely and efficiently.
Excess air
Combustion air supplied beyond the exact amount needed to burn all the fuel, expressed as a percentage.
Running rich
Burning with too much fuel for the air available, so combustion is incomplete.
Spillage
Flue gas escaping into the room from a draft hood or damper instead of going up the chimney.