F10
Ideal
Primary Flow Overheat (Thermal Runaway)
Display shows F10, heater may run briefly then shut down, often with noisy boiling sounds.
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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
Severely restricted primary flow, Pump failure, Blocked plate heat exchanger, Heavy limescale on primary heat exchanger
How to Fix / Troubleshooting
Safety: Isolate power and allow the appliance to cool. Do not continue to reset if the unit repeatedly overheats.
- Check system valves: Ensure all isolation valves around the heater and cylinder coil (if used) are fully open.
- Bleed air from high points: Air in the primary circuit can cause local boiling. Bleed radiators, cylinder coil, and any manual air vents near the heater.
- Listen to pump: When restarted, listen for the pump. If it hums but does not circulate, it may be airlocked or seized. A professional can free or replace it.
- Descaling: In hard water areas, internal scaling can cause F10. A qualified engineer can chemically descale the primary or plate heat exchanger.
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Repair Difficulty
Hard
4/5
Required Part
Primary Circulation Pump
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