Combustion, Fuels, and Burner Systems
Candidates must understand the combustion triangle-fuel, oxygen, and heat-plus the need for proper mixing and time. Know stoichiometric air, excess air, and the symptoms of too little or too much air: insufficient air causes smoke and carbon monoxide; excessive air wastes heat up the stack. Learn the three T's: temperature, turbulence, and time. Fuel oil needs atomization and preheating to burn cleanly; natural gas needs stable pressure and proper air-fuel ratio. Burner systems include the fuel train (strainers, regulators, valves, pressure switches), the ignition system, the flame scanner, and the draft system.
Understand draft: natural draft uses stack effect; forced draft pushes air; induced draft pulls gas; balanced draft uses both and maintains slight negative furnace pressure. Know that purging the furnace before ignition is essential to prevent explosions from accumulated fuel. Learn the correct light-off sequence: purge, prove airflow, establish ignition, open fuel valve, prove flame, then modulate.
Traps: assuming more excess air is always safer (it lowers efficiency and can cause flame instability); forgetting that a furnace must be purged before every light-off; confusing forced and induced draft; and overlooking that low fuel pressure or high fuel pressure can both cause unsafe firing. Questions may ask what happens if the flame scanner fails-fuel must shut off immediately.
Rule of thumb: aim for the lowest excess air that still gives clean combustion; oxygen readings and stack temperature tell you whether you are wasting heat.