Oil Heat Systems: Combustion, Burner Setup, and Flame Proving
This domain tests the service technician's grasp of an oil heating system from tank to flue: how No. 2 fuel oil is stored and delivered, how the burner atomizes it, ignites it and proves flame, and how combustion analysis and draft readings prove the appliance is safe and efficient. Expect questions on combustion targets, draft control, pump and nozzle relationships, cad cell flame proving, and the ignition faults that cause puff-back and delayed ignition.
Combustion and draft targets
- No. 2 heating oil delivers roughly 140,000 Btu per gallon; theoretical CO2 maximum is about 15%, and a well-tuned appliance typically runs 11–13% CO2 with 4–6% O2.
- Smoke should be a trace (0–1) - never trade smoke for CO2. Net stack temperature equals gross flue temperature minus room air temperature; lower net stack means higher steady-state efficiency.
- Overfire draft is normally about −0.02 in. w.c., held there by the barometric damper; the damper also stabilizes breech draft against chimney fluctuations.
Burner hardware and settings
- Standard pump pressure is 100 psi; electrodes, nozzle depth and air settings follow the burner manufacturer's spec, not a guess. Soot, delayed ignition and rumbling almost always trace back to electrodes, a partially plugged nozzle, or wrong air shutter/static plate settings.
- Suction vacuum limits are about 6 in. Hg for a one-pipe lift and up to 12–15 in. Hg on a two-pipe system; high vacuum means a restricted strainer, filter, or line.
- The cad cell proves flame by seeing light: low resistance (well under ~1,600 ohms) means flame is present, while very high resistance signals no flame and the primary control locks out.
Worked example
The pump is raised from 100 psi to 140 psi on a nozzle stamped 0.85 gph. Nozzle flow varies with the square root of pressure:
- Pressure ratio = 140 ÷ 100 = 1.4
- √1.4 = 1.183
- New flow = 0.85 × 1.183 = 1.01 gph
The firing rate is now over 1 gph, so the appliance must be checked against its rating plate and combustion re-verified - you cannot change pump pressure without changing the effective nozzle size.
Traps that cost marks
Assuming nozzle flow changes linearly with pressure rather than with its square root; adjusting the air shutter to chase a CO2 number while smoke rises; setting draft with a cold chimney; and condemning a primary control when the real fault is a dirty cad cell lens or a failed transformer.