Combustion, Fuels, Burners and Draft
The combustion triangle
Fuel plus oxygen plus heat gives CO2, water vapor and heat. Theoretical (stoichiometric) air is the exact amount for complete combustion; in practice excess air is supplied to avoid CO and smoke. Typical targets are roughly 2 to 3 percent oxygen in flue gas for natural gas and higher for oil. Too little air yields CO, soot and possibly unburned fuel in the furnace; too much air wastes heat up the stack and lowers efficiency. Questions often reverse the logic - high O2 is not a safety margin, it is an efficiency loss.
Burners and fuels
Know atmospheric, power gas, pressure-atomizing oil, air-atomizing and rotary-cup burners, plus turndown ratio. Fuel oils are graded (No. 2 light, No. 4 blended, No. 6 heavy); heavier oils need preheating to control viscosity, and each has a flash point and pumping temperature. Gas trains include regulators, strainers, high and low gas pressure switches, and double block-and-bleed valve arrangements.
Draft
Draft is a pressure difference, measured in inches of water column. Natural draft comes from stack effect; mechanical draft is forced, induced or balanced. A balanced-draft unit uses both an FD fan and an ID fan with the furnace held slightly negative. Measure draft at the specified points, not in the middle of a flue, and remember that a dirty economizer or breached breaching changes draft.
Rule of thumb
Purge the furnace with four to five air changes before every light-off, and tune to roughly 3 percent O2 with CO near zero for gas firing.