HVAC Excellence Oil Heat Certification Exam cheat sheet

Oil Heat Systems

• No. 2 heating oil: nominal 138,000–140,000 Btu/gal; usable heat = Btu/gal × AFUE (e.g., 0.80 AFUE ≈ 110,000–112,000 Btu delivered per gallon).

• Combustion air: roughly 10 ft³ of air per 1 ft³ of oil burned; a 1.0 gph nozzle at 12.5% CO₂ needs about 15–20 ft³ of excess air per gallon; target CO₂ 11.5–13.0% and smoke 0–1 (Bacharach scale) for retention-head burners.

• Draft: −0.02 to −0.04 in. w.c. overfire for conventional natural-draft oil furnaces/boilers; −0.02 to −0.04 in. w.c. at the breach is typical; positive pressure indicates a blocked or undersized flue.

• Nozzle sizing: gph = firing rate; 1 gph ≈ 140,000 Btu input; 0.75 gph ≈ 105,000 Btu; 1.25 gph ≈ 175,000 Btu. Nozzle spray angle: 70°–80° for round chambers, 45°–60° for long chambers; solid cone for retention-head burners, hollow cone for conventional.

• Electrode settings (typical retention head): 1/8–5/32 in. above nozzle centerline, 1/16–1/8 in. ahead of nozzle face, tips 5/32–3/16 in. apart; spark gap 1/8–3/16 in.; verify per burner manufacturer's label.

• Pump pressure: 100 psi standard for most residential oil burners; 140 psi for some high-static or two-pipe long-run systems; pressure too low causes poor atomization and delayed ignition; too high causes over-firing and noisy flame.

• Cad cell flame detector: dark resistance >50,000 Ω, light resistance 300–1,500 Ω; primary control locks out if no flame sensed within 15–45 s (varies by control); jumper the cad cell to test the control only briefly.

• Annual efficiency test sequence: clean heat exchanger and flue, replace nozzle and filter, set draft, adjust combustion air to target CO₂/smoke, measure stack temperature and net stack temperature (stack minus room), then compute steady-state efficiency with a combustion analyzer.