Combustion analysis: the four readings. Episode forty-one of the Gas Service Animated series.

The carbon-monoxide season taught you to detect a bad burn. This one teaches you to measure it. A combustion analyzer reads four things from the flue — oxygen, carbon monoxide, carbon dioxide, and stack temperature — and from those four it tells you how completely the fuel is burning, how safe it is, and how much heat is going up the chimney. Detection keeps people alive. Analysis makes the appliance right.

It starts with a good sample. The probe goes into the undiluted flue gas — upstream of a draft hood or barometric damper, where dilution air hasn't mixed in — because a diluted sample lies low on everything. Let the appliance run to steady state, a few minutes, so the readings settle. A rushed sample is a wrong sample.

Oxygen is your excess-air gauge. Perfect combustion would use every bit of oxygen, but no real burner runs perfect, so you run a little extra air as a safety margin — and the leftover oxygen in the flue tells you how much. Too much and you're pumping heated room air up the flue, cooling the fire and wasting fuel. Too little and there isn't enough air to finish the burn — and carbon monoxide climbs. Excess air is oxygen divided by twenty point nine minus oxygen, times one hundred.

Carbon monoxide is the safety reading, and it's the payoff of the whole C-O season. As-measured C-O in a diluted sample understates the truth, so you correct it to air-free — carbon monoxide times twenty point nine, over twenty point nine minus the oxygen — the same correction from episode thirty. Compare that air-free number to the appliance limit: about four hundred parts per million air-free for a furnace, per the code's own table. A tuned appliance runs far below that — well under a hundred.

Carbon dioxide is oxygen's twin, read from the other side. As excess air goes down, oxygen falls and carbon dioxide rises toward its maximum — about eleven point eight percent for natural gas, close to fourteen for propane. High carbon dioxide means a tight, efficient burn; low carbon dioxide means too much air. You can measure it, or the analyzer computes it from the oxygen.

Stack temperature is where the heat you didn't keep goes. What matters is net stack temperature — the flue temperature minus the air temperature around the appliance. A high net stack temperature means a lot of heat is leaving up the vent instead of heating the house. But it can't go too low either, or the flue gases condense where they shouldn't. It's a window, not a target of zero.

Read together, the four tell one story. Oxygen and carbon dioxide fix the excess air. Carbon monoxide says whether the burn is safe. Stack temperature says how much heat escaped. A clean burn is a narrow window: enough excess air to keep carbon monoxide down, not so much that the fire runs cold, and a stack temperature high enough to vent but low enough to keep the heat. Miss any one and the other three will show it.

Put numbers on it. Natural gas, seven and a half percent oxygen: that's about fifty-six percent excess air, and carbon dioxide near seven and a half percent. Forty parts per million measured at that oxygen corrects to about sixty air-free — good. Net stack temperature two hundred eighty degrees. That's a healthy, safe burn — and next episode we turn those numbers into efficiency and a tune.

The four readings. Oxygen — your excess air. Carbon monoxide, corrected air-free — your safety. Carbon dioxide — the tightness of the burn. Stack temperature, net — the heat you lost. Sample undiluted, at steady state, and read all four together. Next: efficiency and tuning.