Every fall maintenance I do gets a combustion analysis. Not a flame-looks-blue check — an analyzer in the flue with the readings written down. Here is how I read the screen.

Probe placement and steady state

Drill the test port where the manufacturer says, usually in the vent connector before any draft hood or dilution air. Sample too far downstream and you are reading diluted gas that makes everything look better than it is. Let the appliance reach steady state before you trust a number — watch stack temperature level off, usually several minutes into the cycle.

CO as measured vs. CO air-free

Your analyzer shows CO in ppm, but excess air dilutes that reading. CO air-free corrects for it: CO air-free = CO measured × 20.9 ÷ (20.9 − O2%). That is the number to compare against limits. A commonly cited limit for central furnaces is 400 ppm air-free (ANSI Z21.47), with lower limits for some other appliance types. Know the limit that applies to the appliance in front of you, and remember a number under the limit does not make a rising trend acceptable.

Oxygen and excess air

O2 tells you how much excess air is going through the burner. Too little and CO climbs. Too much and you are heating the outdoors and lowering efficiency. The right range is in the manufacturer’s literature for that burner — use it instead of a number from memory.

Stack temperature and efficiency

Stack temperature together with O2 drives the calculated efficiency. A stack temp well above what that appliance normally runs can point to overfiring or a fouled heat exchanger; too low on a non-condensing appliance risks condensation in the vent.

Clock the meter

Before you adjust anything, verify input. Clock the gas meter and compare the firing rate with the nameplate. An overfired furnace makes every other reading suspect. (See my note on clocking the meter.)

When to stop

Rising CO air-free, CO showing up in the supply air, spillage at the draft hood, or ambient CO in the mechanical room means you stop tuning and start investigating — heat exchanger, venting, combustion air, flame impingement. Write down what you found, explain it plainly, and do not let the customer leave the conversation unsure whether their home is safe.

Every reading goes in the customer’s report. Numbers end arguments.