F3
Glow-worm
Primary heat exchanger overheat / high limit thermostat trip
F3 displayed, furnace overheats and shuts down, may be noisy before lockout
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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
Failed overheat thermostat on Glow-worm heat exchanger, Blocked primary heat exchanger waterways, Pump not circulating, Airlock in heating circuit
How to Fix / Troubleshooting
Safety: Turn off power and allow the furnace to cool fully before touching any components. Hot surfaces can cause burns.
Homeowner checks:
- Check system pressure on the gauge (if sealed system). It should typically be around 1.0–1.5 bar when cold. Top up via the filling loop if low, following the user manual.
- Bleed radiators to remove trapped air, starting with the highest radiators.
- Ensure all radiator valves and zone valves are open and that thermostats are calling for heat.
Technician checks:
- Verify operation of the system pump (Glow-worm compatible circulation pump). Check that it runs during a heat demand and that the impeller is not seized.
- Inspect and test the overheat thermostat/NTC sensor on the primary heat exchanger; replace if faulty.
- Check for sludge or scale blockage in the primary heat exchanger and system pipework; powerflush or chemically clean if necessary.
- Confirm correct wiring and operation of any external 2-port or 3-port motorised valves that may be restricting flow.
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Repair Difficulty
Hard
4/5
Required Part
Primary Overheat Thermostat / NTC Sensor
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