Combustion, Fuels, and Draft
Core concepts
Combustion needs fuel, air, and ignition, and completes only with the three T's: temperature high enough to sustain ignition, turbulence to mix fuel and air, and time in the furnace. Theoretical (stoichiometric) air is the exact quantity for complete combustion; in practice you fire excess air - roughly 5 to 15% for gas, 10 to 20% for oil, more for coal. Too little air produces CO and smoke; too much wastes heat up the stack.
Flue gas analysis: O2 and CO2 move in opposite directions. Rising O2 means more excess air and lower efficiency. Rising CO at steady O2 points to poor mixing, a dirty burner, or overfiring.
Draft is the pressure difference that moves gases. Natural draft comes from stack effect; mechanical draft is forced (FD fan upstream), induced (ID fan downstream), or balanced (both). Furnaces normally run slightly negative, a fraction of an inch of water column, read on a draft gauge.
Traps
Writers invert the O2 and CO2 trends, call an ID fan a forced draft fan, or imply maximum excess air is best. Excess air beyond combustion needs only raises stack loss.
Worked example
Measured O2 = 3%. Approximate excess air = O2 / (21 - O2) x 100 = 3 / 18 x 100 = about 17%. As a rule of thumb, each 40 F reduction in stack temperature yields roughly 1% fuel savings.