Delayed Ignition and Rising Smoke After an Oil Burner Tune-Up
A technician tunes a 1.10 gph, 80°B nozzle oil burner in a cast-iron boiler. After replacing the nozzle and cleaning the heat exchanger, the burner starts with a slight puff and settles at 10.5% CO2, 1 smoke, −0.02 in. w.c. draft, and 480°F flue temperature. Pump pressure is 120 psi and the electrodes are set to the manufacturer’s 5/32 in. gap. Two weeks later the homeowner reports oil odor in the basement, soot on the boiler-room wall, and a rumble on start-up. A repeat test shows 8.5% CO2, 3 smoke, −0.01 in. w.c. draft, 560°F flue temperature, and delayed flame. The tank is indoors and feeds the burner by single-pipe gravity through a new filter. The chimney is masonry with a 6 in. liner. The homeowner has not changed anything, but the boiler room door is now kept closed.
1. What do the changed combustion readings indicate, and what is the most likely cause?
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The smoke rose from 1 to 3 while CO2 fell from 10.5% to 8.5%, draft weakened from −0.02 to −0.01 in. w.c., and flue temperature rose 80°F. That pattern points to incomplete combustion with poor air supply or poor oil atomization, not a normal tune drift. The delayed ignition and rumble suggest oil is accumulating in the combustion chamber before it lights. Likely causes are a partially restricted nozzle strainer or nozzle seat leak causing after-drip, a shifted electrode or cracked insulator, or reduced combustion air from the now-closed boiler-room door. The weaker draft also matters: without adequate draft, the burner cannot pull enough secondary air and flue gases linger. Do not simply open the air shutter; find why oil delivery or draft changed.
2. List a logical test sequence to isolate the cause before replacing parts.
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First verify draft with a manometer overfire and at the breach, and inspect the barometric damper and chimney for blockage. Confirm the boiler-room combustion-air opening is adequate with the door closed. Then observe the flame shape and check pump pressure at 120 psi. Remove the nozzle and inspect the strainer and seat for debris or leakage; check for after-drip by watching the nozzle after shutdown. Check electrode gap, position, and insulator condition, and test the transformer. Verify the oil filter and single-pipe line for air or restriction. Only after oil delivery, ignition, and draft are correct should you adjust the air shutter and recheck CO2, smoke, and flue temperature. A combustion analyzer reading taken before these checks can hide the real cause.
3. Explain how draft and combustion air interact with smoke and CO2 in this scenario.
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Draft performs two jobs: it supplies combustion air and removes flue gases. When draft falls from −0.02 to −0.01 in. w.c., the burner may still fire but with less excess air and poorer mixing. Incomplete combustion raises smoke and carbon monoxide, and CO2 can fall because the air/fuel mixture is not uniform and some fuel is not burning. A closed boiler-room door can starve the burner even if the chimney is fine, and a blocked barometric damper or liner can do the same. High flue temperature with high smoke suggests fuel burning late in the chamber or after the heat exchanger. Restoring proper draft and combustion air, then correcting oil delivery, should bring smoke back near zero and CO2 back into the manufacturer’s range.