Combustion Performance Analysis, Flue Gas Testing and Carbon Monoxide Safety
This domain tests the use of a flue gas analyser and the judgement that follows: is combustion complete, is the appliance safe, and is CO reaching occupied space?
What to measure
A flue gas analyser reports oxygen, carbon monoxide, carbon dioxide, flue temperature and sometimes draught. From these you derive excess air, CO₂ against its maximum, CO/CO₂ ratio, and efficiency. Zero the analyser in fresh air, select the correct fuel, sample at the manufacturer's test point, and wait for stable readings before recording.
Interpreting results
For natural gas, a CO/CO₂ ratio around 0.004 or below indicates clean combustion; a rising ratio points to incomplete combustion from poor aeration, blocked flue, wrong injector or a failing heat exchanger. Low O₂ with high CO₂ means too little excess air; high O₂ with low CO₂ means too much. Efficiency falls as flue temperature rises, and condensing appliances need the appropriate calculation mode.
Ambient CO matters as much as flue CO. A low flue CO reading does not prove the room is safe if products are spilling from a draught diverter.
Worked example
A boiler reads O₂ 6%, CO 60 ppm, CO₂ 9.5%, flue temperature 55°C. CO/CO₂ = 60/9.5 ≈ 0.0063, which is above the clean-combustion benchmark. Investigate aeration, gas pressure and flue condition rather than adjusting the analyser or accepting the reading because 60 ppm looks small.
Traps
Sampling before steady state; using the wrong fuel setting; ignoring ambient CO and spillage; comparing CO ppm between appliances without their CO₂; failing to record readings and conditions.
Exam focus
Analyser technique, CO/CO₂ ratio, excess air interpretation, efficiency and the limits of a flue-only reading.