Workshop: combustion numbers. Episode seventy-five of the Gas Service Animated series.
Two appliances, two verdicts, all pencil. The formulas from the combustion season: excess air from oxygen, the air-free correction, carbon dioxide from oxygen, and the stack-loss efficiency estimate. Board's clean â let's dirty it.
Problem one: a natural-gas furnace at steady state. Oxygen seven-and-a-half percent. Carbon monoxide forty parts per million as measured. Stack three hundred fifty, ambient seventy. Verdict?
Excess air: seven-point-five over twenty-point-nine minus seven-point-five â times a hundred â is about fifty-six percent. Carbon dioxide from oxygen: eleven-point-eight times one minus seven-point-five over twenty-point-nine â about seven-point-six percent. Both say the same thing: a healthy, slightly generous air supply.
Air-free the carbon monoxide: forty times twenty-point-nine over thirteen-point-four â about sixty-two parts per million air-free. Well under the four hundred limit. Efficiency: net stack two-eighty; loss is point-two-one times two-eighty over seven-point-six â about seven-point-seven; call it ninety-two percent steady-state. Verdict: tuned, safe, done. Write the numbers on the ticket.
Problem two: a propane furnace, freshly converted from natural gas â the episode fifty-three scenario. Readings: oxygen five percent. Carbon monoxide ninety as measured. Stack three seventy, ambient seventy. Is this conversion finished?
Propane changes the constants, not the method. Air-free: ninety times twenty-point-nine over fifteen-point-nine â about one-eighteen air-free. Under four hundred, but triple problem one â noted. Carbon dioxide: propane's max is thirteen-point-eight, so thirteen-point-eight times fifteen-point-nine over twenty-point-nine â about ten-and-a-half percent. Tight burn, modest excess air â about thirty-one percent.
Efficiency with propane's constant: point-two-three times net three hundred over ten-point-five â about six-point-six percent loss; call it ninety-three percent. Efficient â but that carbon monoxide trend says the primary air wants a nudge. Open the shutter slightly, re-read: seventy as measured, air-free about ninety-two. Better. That's the tune-then-verify loop, with propane's numbers.
The workshop habits: always correct to air-free before judging. Always use the fuel's own constants â eleven-eight versus thirteen-eight, point-two-one versus point-two-three. And a passing number with a bad trend is not a pass â the direction of carbon monoxide outranks its snapshot.
Combustion numbers in one breath. Oxygen sets excess air and carbon dioxide. Air-free before judgment. Fuel constants matter. Estimate the efficiency, then let the trend and the manufacturer's targets rule. Two furnaces, two verdicts, one method â and that's the workshop season: the tables and the math, in your hands.