F3
Peerless
Blocked Flue / Vent Safety Lockout
Boiler displays F3 and locks out; may occur during venting issues
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blocked chimney or vent, failed blocked vent switch, condensate backup in vent, improper vent slope
How to Fix / Troubleshooting
Safety first: Turn off power and gas. Do not operate the boiler until the venting is verified safe. Carbon monoxide hazard.
Steps:
- Check vent termination: Inspect the outside termination for obstructions (snow, ice, nests, vegetation).
- Inspect vent piping: Follow the vent run and look for sags, low spots filled with condensate, or disconnections. Correct slope per Peerless instructions.
- Check blocked vent switch: On atmospheric Peerless boilers, locate the blocked vent switch near the draft hood. Ensure it is not manually tripped and that wiring is intact.
- Clear condensate: Clean the condensate trap and hoses to ensure free drainage.
- Reset safety: Some blocked vent switches are manual reset. Press the reset button if present after resolving the cause.
- If F3 returns: Have a professional inspect the chimney/vent and test draft and combustion.
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Repair Difficulty
Hard
4/5
Required Part
Blocked vent safety switch
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