F20
Glow-worm
Primary circuit flow restriction / overheat
F20 displayed, furnace overheats and locks out frequently
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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 plate heat exchanger (on combi units), Sludged primary heat exchanger, Closed isolation valves under Glow-worm furnace, Faulty or stuck motorised valve
How to Fix / Troubleshooting
Safety: Allow the system to cool fully. Surfaces and water may be very hot.
User checks:
- Ensure all radiator valves are open.
- Check that isolation valves under the appliance are fully open (if accessible and clearly marked).
Technician actions:
- Inspect and clean or replace the plate heat exchanger if blocked with scale or sludge.
- Check the primary heat exchanger for flow and consider powerflushing the system.
- Verify operation of any external motorised valves controlling heating circuits.
- Ensure the pump is correctly sized and functioning.
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Repair Difficulty
Hard
4/5
Required Part
Plate Heat Exchanger
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