Heating Systems and Combustion
The combustion triangle and gas pressures
Fuel, oxygen, and ignition, plus a path to vent products safely. Know typical pressures: natural gas manifold 3.5 in. w.c., LP manifold 10-11 in. w.c.; natural gas inlet 5-7 in. w.c., LP inlet 11-13 in. w.c. Set manifold pressure with the burner firing and the appliance in its normal configuration. Derate roughly 4 percent per 1,000 ft above 2,000 ft elevation.
What a combustion analyzer actually proves
You are looking at O2, CO, stack temperature, ambient temperature, excess air, and CO air-free. For natural gas, roughly 6-9 percent O2 (about 35-70 percent excess air) is a normal target; LP runs slightly higher O2. CO air-free is the measured CO corrected for dilution: CO air-free = measured CO × 20.9 ÷ (20.9 − measured O2). A clean appliance should screen under about 100 ppm air-free; rising CO as the blower runs is a heat exchanger red flag.
Traps
Blue flame does not equal safe. High CO2 with low O2 is not automatically "efficient" - it can mean overfiring or incomplete combustion. Draft measured with the burner compartment door off is meaningless. And CO2 is not CO.
Worked example
A furnace reads 60 ppm CO at 7.0 percent O2. Air-free CO = 60 × 20.9 ÷ (20.9 − 7.0) = 60 × 20.9 ÷ 13.9 = 90 ppm. That passes the 100 ppm screen but is close enough to justify inspecting the exchanger and checking gas pressure.