F5
Glow-worm
Air pressure switch / flue proving fault
F5 error, furnace attempts to start but shuts down quickly, fan may run continuously
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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
Stuck or faulty air pressure switch, Blocked pressure sensing tubes, Incorrect flue installation on Glow-worm furnace, Condensate collecting in flue or pressure tubes
How to Fix / Troubleshooting
Safety: Power off and isolate gas. Flue and combustion safety checks must be done by a qualified engineer.
Homeowner actions:
- Check that the condensate drain is not visibly leaking or blocked at the outlet. A blocked condensate can affect flue pressure.
- Do not attempt to blow into or disconnect pressure tubes yourself.
Technician actions:
- Inspect silicone pressure tubes from the fan/combustion chamber to the air pressure switch for splits, kinks, or water ingress. Replace if damaged.
- Check the air pressure switch operation with a manometer and multimeter; ensure it switches at the correct pressure as per Glow-worm specifications.
- Verify flue integrity, length, and terminal position; correct any non-compliant installation issues.
- Drain any condensate trapped in the flue or pressure tubes and ensure correct fall on the flue.
- Replace the air pressure switch if it fails functional tests.
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Repair Difficulty
Professional Required
5/5
Required Part
Air Pressure Switch
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