TL;DR
Most flame failure lockouts are a contaminated flame detector (UV cell or flame rod), a failed igniter, or a fuel supply problem. Clean the flame detector first. It fixes the fault in the majority of cases.
What you might see
- flame failure lockout alarm on burner control panel
- burner goes through trial-for-ignition and shuts out
- boiler restarts but trips again within minutes
- intermittent operation with frequent resets needed
Likely causes
Contaminated UV scanner or flame rod unable to detect the established flame
Failed or fouled igniter electrode not initiating flame during trial-for-ignition
Low fuel gas pressure or interrupted fuel supply during the firing sequence
Air-fuel ratio out of adjustment producing a flame the detector cannot see reliably
Required tools
- LOTO kit
- Lint-free cloth for detector cleaning
- Manometer or gas pressure gauge
- Replacement UV scanner or flame rod (OEM part number)
- Multimeter for igniter coil and proving switch continuity checks
Safety first
- Never manually override a burner safety lockout to allow firing without diagnosing the fault.
- Confirm the gas supply manual valve is closed before touching the igniter or removing the flame detector.
- Do not leave a boiler unattended during post-maintenance startup until flame has been stable for at least 15 minutes.
Procedure
- 1
Reset the burner lockout from the control panel. Observe the startup sequence and note at which stage the lockout occurs: during trial-for-ignition or after flame is established.[1]
- 2
Lock out the burner. Locate the UV scanner or flame rod in the burner housing. Remove it and clean the detector window with a dry lint-free cloth. Contaminated quartz glass is the most common cause of nuisance flame failure.[1]
- 3
Inspect the igniter electrode for fouling, cracking, or incorrect gap. The electrode tip should be clean and the gap should match the Cleaver-Brooks service specification.
Warning: Confirm the burner control is in lockout and the gas supply valve is closed before touching the igniter. - 4
Verify gas supply pressure at the burner train inlet with a manometer or pressure gauge. Confirm it is within the specified range at the rated firing rate.
- 5
Check the combustion air proving switch. If the air proving switch does not close, the burner control will not allow ignition.
- 6
After cleaning the detector, restore the gas supply, clear the lockout, and observe the startup sequence. Confirm a clean trial-for-ignition and stable flame establishment.
- 7
If lockouts persist after cleaning the detector and confirming fuel supply, replace the UV scanner with a new OEM cell. Do not substitute a different cell model without verifying the burner control compatibility.
Sources
Cleaver-Brooks CB Firetube Boiler Operation, Service, and Parts Manual, Cleaver-Brooks
Cleaver-Brooks CB Boiler Operation, Maintenance and Parts Manual, burner flame failure diagnostics (general)
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