Intermittent Oil Burner Lockout After Long Off Cycles
An oil-fired boiler has a two-pipe oil line. The tank is 8 ft below the burner and 25 ft away; the line is 3/8 in. OD copper with a two-stage pump and the bypass plug installed. The burner locks out after sitting off overnight, but after the homeowner presses reset two or three times it starts and runs normally all day. The nozzle, electrodes, oil filter, and pump strainer are new or clean, and the transformer tests good. The cad cell reads about 1,200 ohms when the flame is running and goes high after shutdown. The primary control has a standard trial for ignition. There is a slight oil weep at the flare fitting at the tank, and the tank has a vent pipe that terminates outside. The homeowner says the problem began after the last oil delivery. The burner starts with a puff after a lockout but then settles down.
1. What is the most likely cause of the overnight lockout, and why does resetting sometimes restore operation?
Read the worked answer
A two-pipe system with the tank below the burner must lift oil, so any air leak on the suction side can let oil drain back or allow air into the pump during long off periods. The slight weep at the tank flare fitting is a strong clue: a fitting that weeps oil can also admit air when the pump is off. Overnight, the line loses prime or accumulates air. On start-up the pump cannot deliver oil immediately, ignition is delayed, and the primary control locks out. Repeated resets run the pump long enough to pull oil and clear the air, so the burner then runs all day. The puff is unburned oil igniting after the delayed start. The recent oil delivery may have disturbed the tank fitting or created a temporary air slug.
2. Outline a systematic troubleshooting sequence, including what to do before another reset.
Read the worked answer
Do not keep resetting the burner; each reset can add oil to the chamber. First inspect and tighten or remake the suction flare at the tank and any other suction-side fittings. Check the tank for a working check valve or anti-siphon device, and verify the bypass plug is correct for two-pipe operation. Install a vacuum gauge on the pump suction and observe vacuum during operation; a high or fluctuating vacuum suggests a restriction, while a vacuum that bleeds down when the pump stops suggests an air leak. Pressure-test or isolate the suction line if needed. Bleed the pump according to the manufacturer’s procedure. Then fire the burner and verify pump pressure, flame, smoke, CO2, and draft. Finally, let the burner sit through a long off cycle and recheck for prime loss before declaring the repair complete.
3. What combustion and safety checks should follow the repair?
Read the worked answer
After restoring oil delivery, verify pump pressure and nozzle flow rate, then check draft overfire and at the breach. Run a combustion test and confirm smoke is near zero, CO2 is within the manufacturer’s range, and flue temperature is normal. Check for after-drip at the nozzle and for any oil accumulation in the combustion chamber. Verify the cad cell sees flame promptly and that the primary control’s trial-for-ignition timing is correct. Inspect the vent pipe and chimney for blockage, and confirm combustion air is adequate. Clean any soot from the previous delayed ignitions, check the heat exchanger for cracks, and make sure the relief valve and high limit are functional. Document the vacuum reading and the final combustion numbers so a repeat lockout can be compared against a known baseline.