Combustion analysis: efficiency and tuning. Episode forty-two of the Gas Service Animated series.

You've got the four readings. Now make them mean something. Steady-state efficiency turns stack temperature and carbon dioxide into a single number — how much of the fuel's heat the appliance actually kept — and tuning is the craft of nudging the burner until that number is as high as it can safely go.

Efficiency starts as its opposite: stack loss, the heat that left up the flue. The bigger the net stack temperature, the more heat escaped; the higher the carbon dioxide, the tighter the burn carrying it. A classic estimate multiplies the net stack temperature by a fuel constant and divides by the carbon dioxide — for natural gas the constant is about zero point two one in Fahrenheit. Efficiency is a hundred minus that loss. It's an estimate — your analyzer's own algorithm and any condensing credit will differ.

Run last episode's numbers. Net stack temperature two hundred eighty, carbon dioxide seven and a half percent, natural gas. Zero point two one times two hundred eighty, divided by seven and a half, is about seven point eight percent of the heat lost up the flue. Efficiency: about ninety-two percent. Change one reading and watch it move.

Here's the tension every tune lives in. Add excess air and carbon monoxide drops — but the extra air cools the fire and carries heat up the flue, so efficiency falls. Cut the air and efficiency climbs — until there isn't enough to finish the burn and carbon monoxide spikes. The sweet spot sits just to the safe side of that cliff: the lowest excess air that still keeps carbon monoxide low and steady.

So what are you aiming for on a typical atmospheric appliance? Oxygen somewhere around four to nine percent, carbon dioxide in the eight-and-a-half to ten-and-a-half range, carbon monoxide as low as it'll go and always well under the four-hundred air-free limit, and a stack temperature hot enough to vent cleanly but no hotter than it needs. Targets vary by appliance — the manufacturer's spec always wins over a rule of thumb.

Tuning itself is small, deliberate moves. On adjustable equipment you change the fuel-air mix — the gas pressure at the manifold to the nameplate value, and the primary air where the burner allows — then you let it settle and read again. Change one thing, wait, re-read. Chase the readings toward the targets: trim excess air down while you watch carbon monoxide, and stop the moment carbon monoxide starts to climb. Never tune by eye alone — the analyzer is the truth.

One more twist: condensing appliances. A high-efficiency furnace is built to pull the stack temperature so low that the water vapor in the flue gas condenses, and it harvests that hidden latent heat — which is why it can beat ninety percent and needs a condensate drain and plastic venting. The simple stack-loss estimate doesn't credit that latent heat, so on a condensing unit, trust the appliance's own rating and analyzer, not the back-of-the-envelope number.

The whole job as one loop. Sample undiluted at steady state. Read the four. Correct carbon monoxide to air-free and check it against the limit. Estimate efficiency. Compare to the manufacturer's targets. Make one small adjustment, let it settle, and read again — until it's safe, efficient, and stable. Then document it. That's combustion analysis.

Efficiency and tuning. Efficiency is a hundred minus the stack loss — net stack temperature and carbon dioxide, an estimate. Every tune trades excess air against carbon monoxide; live just to the safe side of the cliff. Aim at the manufacturer's targets, adjust in small steps, and let carbon monoxide set your limit. Analysis makes the appliance right.