Flame Failure on a Cold Start with Gas Odor at the Burner Front
A packaged gas/oil boiler on a cold start. The operator selects gas; the panel runs its pre-purge, the igniter energizes, the pilot proves, the main flame valve opens, and about 6 seconds later the flame scanner drops out and the burner trips on 'flame failure' before the main flame was well established. There is a faint gas odor near the burner front. The operator's instinct is to reset the panel and try again immediately, saying 'it does this every time when it's cold, it catches on the third or fourth try.' It is 5°F outside and the boiler room door has been standing open.
1. What are the hazards, and what is the correct immediate action?
Read the worked answer
Hazard: furnace explosion. Main gas flowed for several seconds without a proven flame. That unburned gas is sitting in the furnace and breeching. A reset-and-retry puts an ignition source into it. The faint gas odor at the burner front raises a second possibility - gas leaking past a safety shutoff valve that is not seating, which is worse, because it means gas is present even when the burner is off.
Immediate actions, in order:
- Leave the burner off. No reset, no retry, no 'third or fourth try.'
- Close the manual gas shutoff valve at the burner train.
- Ventilate - keep the door open, start the combustion air fan at purge airflow only if doing so is safe and does not introduce an ignition source, and get people out of the immediate area.
- Eliminate ignition sources - no smoking, no switching, no sparking tools, no vehicle traffic. Do not operate electrical switches in the area of the odor.
- Notify supervision and the gas supplier if the odor persists or gas is detected in the room.
- Investigate the leak before any relight - leak-test the safety shutoff valves, or rely on the valve-proving system if one is fitted, and confirm it is working.
'It does this every time when it's cold' is the most dangerous sentence in the boiler room: it means the startup problem has been normalized for months.
2. What is the correct relight procedure, and what must be proven before each attempt?
Read the worked answer
Prerequisites before any relight:
- Furnace and breeching ventilated and free of gas - odor gone and, where available, a combustible-gas reading near zero.
- Leak test/valve proving confirms the main safety shutoff valves are tight.
- All doors, peep sights, and access openings closed.
Then the full sequence - start from the beginning, not from where it tripped:
- Pre-purge at the specified airflow for the manufacturer's required duration. The common minimum is five minutes at purge airflow, and it must be the manufacturer's/NFPA 85 purge, never shortened because it is cold or the plant needs steam. The purge exists to sweep the furnace of exactly what you just put in it.
- Prove airflow with the switch or differential pressure device, not by watching the fan spin.
- Prove igniter, then pilot, then main flame - each proven by its own device before the next step is permitted.
- Watch the flame on light-off through the sight port, and confirm the scanner signal is steady, not just present.
Do not: bypass or jumper the flame safeguard, hold the reset button in, defeat the purge timer, or light off with a torch. Each of those has produced a fatal furnace explosion somewhere.
3. The burner fails three more times with the same symptom. What do you check, and what do you suspect given the weather?
Read the worked answer
The symptom - pilot proves, main valve opens, flame lost within seconds - points to the main flame failing to establish or the scanner losing sight of it, not to the pilot circuit.
Check:
- Flame scanner and sight tube - a dirty, scaled, or misaligned sight tube sees the pilot but not the main flame. Check signal strength, not just the on/off state.
- Gas supply - pressure at the train at 5°F outside. Frozen regulators, hydrates in the line, and low street pressure are common winter failures; low pressure gives a weak, detached flame that lifts off and drops out. Compare against the nameplate range.
- Igniter and pilot - spark gap, pilot orifice, position relative to the main flame.
- Combustion air - cold, dense air plus an open outside door changes the air/fuel balance, and a strong stack effect can pull the flame away at light-off.
- Low-fire position - the burner must light off at minimum fire; a linkage that drifted can put it too high.
- Valve proving and valve seating - connects back to the original gas odor.
- Draft at the breach and furnace pressure at light-off.
Given the weather and the open door, the leading suspects are a gas pressure/regulator problem and an air/fuel imbalance at light-off caused by cold combustion air. Neither is fixed by persistence. If nothing is found, lock the burner out, get the burner servicer in, and put the boiler on the outage list; the flame safeguard is telling you something real.
Note the scanner dropout timing - 6 seconds - in the log. That interval is diagnostic, and the next person needs it.