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

Tap to mark as ruled out

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

7 steps · ~45 min

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)
View source

More guides for Cleaver-Brooks CB / CBLE

How to reduce excessive fuel consumption on a Cleaver-Brooks CB boiler

Excess fuel use is almost always excess air in the combustion process or degraded heat transfer surfaces. Run a flue gas analysis to set the air-fuel ratio correctly, then check for scale and soot.

~60 min7 steps

How to fix low steam pressure on a Cleaver-Brooks CB boiler

Low steam pressure with the boiler at full fire means heat transfer is degraded, most often from fireside soot or waterside scale. Compare flue gas exit temperature to baseline: a rise of more than 10 degrees C per 100 hours indicates fouling.

~90 min7 steps

How to respond to a safety valve lifting on a Cleaver-Brooks CB boiler

A lifting safety valve means the pressure controls failed to cut the burner at the operating setpoint. Inspect the operating pressure control and high-limit control for proper setpoint and contact operation. Do not attempt to adjust or prevent the safety valve from lifting.

~60 min8 steps

How to eliminate scale buildup in a Cleaver-Brooks CB boiler

Scale is calcium and magnesium carbonate or sulfate deposited from untreated or undertreated feedwater. It insulates tubes and causes overheating. Chemical descaling removes it; fixing the water treatment program prevents recurrence.