Cold-Start Puffback After Burner Service
A technician tunes a residential oil burner firing a 0.85 gph, 80° A nozzle at 100 psi in a welded steel combustion chamber. The service includes a new nozzle, new oil filter, cleaned cad cell, and electrode adjustment. The electrodes are set to a 1/8 in. gap with tips 1/16 in. ahead of the nozzle face. The transformer is rated 10,000 V. The draft reading at the breach is -0.01 in. w.c., and the overfire draft is -0.005 in. w.c. The burner starts and runs normally after it is warm, but on the first cold start the next morning there is a muffled puffback, a light haze of smoke from the barometric damper, and a strong oil odor. The flame then establishes and the burner runs steadily. The homeowner reports the problem did not occur before service. The chimney is an exterior masonry chimney with a 6 in. liner, and outdoor temperature is 18°F.
1. Explain why a puffback can occur only on a cold start even though steady-state combustion readings are normal.
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A cold-start puffback is a timing and draft problem. At 18°F the exterior masonry chimney is cold and has little natural draft; the -0.01 in. w.c. breach reading is already weak. When oil is sprayed into a cold chamber, some oil can accumulate before a stable flame anchors. If ignition is delayed even briefly, the accumulated vapor ignites as a pulse. Once the chimney and chamber warm, draft improves and the same burner runs normally, so steady-state CO2 and smoke readings can look fine. The service likely changed something that affects ignition: electrode position, nozzle drip, or the cold-start draft. The puffback is not proof that the nozzle is wrong, but it is a warning that unburned oil is accumulating.
2. Give a diagnostic sequence to isolate the cause, including at least three specific measurements.
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Start cold and watch the first start. Measure draft at the breach and overfire before ignition, then observe spark at the electrode tips, time from oil-valve opening to flame, and nozzle drip after shutdown. Check electrode gap of 1/8 in. and that tips are 1/16 in. ahead of the nozzle face; verify the porcelain is not cracked and the transformer spark jumps the gap. Measure pump pressure and vacuum, and confirm nozzle size and angle. Inspect the chamber for oil pooling. If draft is weak only when cold, check chimney blockage, liner condition, barometric damper, and combustion air. Repeat the cold-start test after corrections.
3. What corrections are appropriate if the chimney has weak draft and the burner is otherwise correctly adjusted?
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If the chimney is the limiting factor, correct the venting rather than overfiring the burner. Verify the 6 in. liner is clean, correctly sized, and has adequate height; repair any blockage or broken liner. Confirm the barometric damper is set per the appliance listing, not wide open, and that combustion air is available. On a cold exterior chimney, a listed draft inducer or power vent may be needed if natural draft cannot be established. Do not solve a puffback by raising pump pressure or enlarging the nozzle. After any venting change, recheck cold-start draft, CO2, smoke, and stack temperature.