F20
Saunier Duval
Primary circuit overheat
F20 displayed, unit shuts down on high temperature alarm
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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 primary heat exchanger, Air in primary circuit, Failed circulation pump (if fitted), Faulty NTC giving low reading
How to Fix / Troubleshooting
Safety first: Turn off power and gas. Allow the heater to cool completely.
Steps:
- Bleed air: Use bleed points or automatic air vents to remove air from the primary circuit.
- Check pump operation: On models with a pump, listen for pump running and feel for vibration. Free a stuck pump as needed.
- Clean filters: Clean any primary circuit strainers or filters.
- Descale heat exchanger: If overheating persists, have the primary heat exchanger chemically descaled.
Warning: Overheating can crack the heat exchanger. Do not repeatedly reset without fixing the cause.
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Repair Difficulty
Hard
4/5
Required Part
Primary Heat Exchanger
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