Repeated Burner Lockouts with a Weak UV Flame Signal
A 100 hp, 15 psig gas-fired steam boiler has a parallel-positioning burner, a UV flame scanner, a 30-second pre-purge, a 4-second trial for ignition, and a 15-second post-purge. On a call for steam, the pre-purge completes and the pilot ignites, but the flame signal reads only 1.2 VDC; the main gas valve opens and the burner locks out after 4 seconds. The operator has reset the control three times; each time the same sequence occurs. The gas supply pressure is 7 in. wc at the regulator outlet and 4 in. wc at the burner manifold; the combustion air proving switch is closed; the UV scanner sight tube is sooted. The boiler room is at 45°F. The operator wants to jumper the flame scanner to keep the burner running because the plant needs steam. The flame safeguard is a programmed control with a manual reset lockout.
1. What is the correct response to the repeated lockouts and the jumper request?
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Repeated lockouts are a stop signal, not a reset problem. I would not jumper the UV scanner; that defeats flame-failure protection and could allow gas to enter the furnace if the flame goes out. The sequence shows pilot ignition and a 1.2 VDC signal, which is likely below the amplifier’s reliable threshold.
I would check the UV scanner sight tube (sooted, as noted), scanner alignment and cooling, pilot flame quality, pilot gas pressure, and the flame amplifier and grounding. The 4 in. wc manifold pressure should be checked against the burner data; low gas pressure can give a weak pilot. After cleaning and correcting, reset once and observe the full trial-for-ignition. If the signal is still low, call a qualified burner technician. Log the lockouts and corrective action.
2. How would you distinguish a flame-sensing problem from a fuel/air problem?
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Observe the pilot through the sight port and measure pilot gas pressure while watching the flame signal. A healthy-looking pilot with a low, erratic UV signal points to the scanner, sight tube, amplifier, or grounding. A weak, lifting, or yellow pilot with low gas pressure points to fuel/air adjustment.
Check that the UV scanner is aimed at the pilot and main flame and that the sight tube is clean and not blocked. If the signal is good during pilot but drops when the main valve opens, the scanner may be seeing only the pilot, or the main flame may be lifting off. A UV test lamp can confirm the scanner/amplifier response, but it does not prove the flame. Do not use a jumper to fake a flame signal.
3. What tests and records are required before returning the boiler to automatic operation?
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After repair, the flame safeguard must be proven by a flame-failure test: with the burner running, remove the flame signal or close the pilot gas valve and confirm the main gas valve closes and lockout occurs within the 4-second trial-for-ignition time. Also verify the 30-second pre-purge, the 15-second post-purge, and the manual-reset lockout.
Record the date, technician, test method, response time, scanner signal, and any parts replaced. The jumper request should be documented as refused, with the reason. The boiler should not be returned to automatic operation until the flame signal is stable at the manufacturer’s recommended level and the failure test passes. This record is important for insurance and inspection.