Combustion, Fuels, and Burner Systems
The three T's and the air-fuel ratio
Complete combustion needs time, temperature, and turbulence, plus enough air. Excess air beyond the stoichiometric requirement is unavoidable; too little makes CO and smoke, too much wastes heat up the stack. Natural gas is burned with roughly 10% excess air (about 3% O2 in the flue gas); fuel oil typically needs 15% or more because atomization is imperfect. Flue gas analysis - O2, CO2, CO, and smoke spot - is the operator's tool for judging air settings and burner condition. Stack temperature rising at constant load signals fouling or scale.
Draft may be natural, forced, induced, or balanced, and the furnace should normally run slightly negative to keep products of combustion in the gas path rather than the boiler room. Burner controls follow a fixed sequence: prepurge the furnace, prove the pilot or ignition, open the main fuel valve, prove main flame, then release to modulation. Never skip or shorten the purge, and never attempt an immediate relight after a flame failure - the fuel that accumulated must be purged first. A flame scanner must see only its own flame.
Traps
Furnace explosions almost always trace to unburned fuel plus an ignition source: a failed purge, a leaking fuel valve, repeated unsuccessful light-off attempts, or a short flame-failure response time.
Rule of thumb: if the burner does not light on the first or second try, stop, purge fully, and find out why before trying again.