Delayed Ignition and Puffback After Nozzle and Electrode Replacement
A service technician replaces the nozzle and electrodes on a Beckett AFG burner firing a cast-iron boiler. The nameplate specifies 0.75 GPH at 100 psig, 80°B pattern. The technician installs a 0.85 GPH 80°A nozzle, sets pump pressure to 140 psig, and sets electrode gap to 5/32 in. with tips 5/16 in. ahead of the nozzle face. On the first call for heat, the burner rumbles for 2–3 seconds before ignition and occasionally produces a light puffback; the boiler front shows fresh soot. Instruments read: smoke 2, CO2 11.2%, O2 5.8%, draft −0.03 in. w.c., flue temperature 510°F, ambient 68°F. The oil line is a one-pipe gravity feed from a tank 12 ft away; vacuum at the pump inlet is 0 in. Hg. The homeowner reports the problem began immediately after the tune-up.
1. Identify the most likely causes of the delayed ignition and puffback, and explain how each setup detail contributes.
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Three details point to an overfired, incorrectly atomized flame:
- Actual firing rate: a 0.85 GPH nozzle at 140 psig flows 0.85 × √(140/100) = 1.01 GPH, about 34% above the 0.75 GPH nameplate rate. The extra oil floods the combustion chamber before the spark can light it, producing the 2–3 second rumble and puffback.
- Pattern: 80°A is a hollow-cone pattern; the nameplate calls for 80°B (solid). A hollow cone can be harder to ignite and can mismatch the burner head’s air pattern.
- Air/fuel: smoke 2 at CO2 11.2% confirms incomplete combustion and a rich condition. The soot on the front is unburned carbon from delayed ignition.
Electrode gap and 5/16 in. tip position are nominal, but they should still be checked for parallel alignment; a bent tip can weaken the spark. The one-pipe gravity feed and 0 in. Hg vacuum rule out oil supply as the cause.
2. What should the technician do first, and what final readings would confirm a correct setup?
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First, restore the nameplate configuration: install a 0.75 GPH 80°B nozzle and set pump pressure to 100 psig. Do not try to tune around an overfired nozzle; the chamber and heat exchanger are designed for the rated input. Next, verify electrodes are parallel, gap 5/32 in., tips 5/16 in. ahead of the nozzle face, and clean.
Then adjust the burner air shutter for a trace of smoke (0) and recheck:
- CO2 should land near 11.5–12.5% (O2 about 4–5%) after smoke is clean.
- Draft should be −0.02 to −0.04 in. w.c. overfire, with a stable flame.
- Flue temperature should fall from 510°F because the firing rate is back to design.
Run at least 10 minutes and cycle the burner several times. Confirmation is no rumble, no puffback, smoke 0, and steady flame. If delayed ignition persists at correct nozzle/pressure, inspect the transformer spark and burner head position.