Combustion Numbers, Step by Step. Episode eighty-six of the Gas Service Animated series.

Your analyzer does this math in half a second. Today you do it once by hand — slowly, in order — because a technician who has walked the arithmetic trusts the instrument for the right reasons, and catches it when a probe or a sensor starts lying. Four readings in, one verdict out.

Annex G starts before the probe does. Confirm the appliance is venting: five minutes of main burner, then the spillage check at the draft hood relief with smoke or a mirror — then turn on every other air-consuming gas appliance in the space at full input and check again. And read the room itself: if ambient carbon monoxide is already over fifty p p m, stop — ventilate and treat the space, not the numbers.

Placement is the whole game: sample undiluted flue products. On a draft hood appliance, that means upstream of the hood — before dilution air pours in and flatters every reading. On fan-assisted equipment, the flue test port. Five minutes at steady state, then record five things: the fuel, oxygen, carbon monoxide, flue temperature, ambient temperature.

First calculation: correct the CO to air-free — the undiluted number the standards actually grade. The annex equation: measured CO, times twenty point nine, over twenty point nine minus the oxygen. Forty-five p p m at six percent oxygen: twenty point nine minus six is fourteen point nine; the ratio is about one point four; forty-five becomes sixty-three air-free. Judged on the annex scale: central furnaces and boilers, four hundred air-free. Water heaters, two hundred. And cooking appliances grade as-measured — two twenty-five at the oven, twenty-five per top burner — no air-free correction there.

Second: what the oxygen means. Excess air equals oxygen over twenty point nine minus oxygen, times one hundred — six percent oxygen is forty percent excess air. And carbon dioxide can be estimated from the same reading: C O two max, times one minus oxygen over twenty point nine — eleven point eight for natural gas gives eight point four percent. One caution for the sharp-eyed: the annex's own C O two equation uses a different constant pair — twelve point two and fourteen. Two pairs, two jobs. Keep them straight.

Third: the efficiency estimate. The classic stack-loss form: one hundred, minus K times the temperature rise over the C O two. K is about zero two one for natural gas in Fahrenheit. Flue three forty, ambient seventy — rise of two seventy. Zero two one times two seventy is fifty-seven; over eight point four, call it six point seven points of loss — roughly ninety-three percent steady-state. Say it the honest way: a sensible-heat training estimate. Your analyzer's algorithm, and any condensing credit, govern the number that goes on paper.

Fourth: the tune loop that ties it together. Trim the air in small steps. Oxygen falls, the stack cools, efficiency climbs — while CO stays flat and low. Then somewhere the curve breaks: CO jumps. That is the breakpoint — back off toward more air and leave margin. Below about three percent oxygen you are flying close to incomplete combustion; watch CO like it owes you money. Chasing the last point of efficiency past the breakpoint is how tuned becomes dangerous.

The calculator mirrors the worksheet exactly: fuel, oxygen, CO, flue, ambient in — excess air, C O two, air-free CO with its color flags, and the efficiency estimate out. Same equations you just ran, same annex thresholds. When the tab and your analyzer disagree, one of your inputs is lying — and now you know how to find which.

Combustion numbers in one breath. Prove venting first, mind the ambient fifty. Probe undiluted, five minutes, steady. Air-free the CO — twenty point nine over twenty point nine minus oxygen — judged four hundred furnace, two hundred water heater, ranges as-measured. Excess air and C O two from the oxygen. Efficiency as an estimate. Tune to the breakpoint, never past it. Next: altitude — the last variable, and the season closer.