F20
Protherm
Primary Circuit Overheat / Circulation Fault
F20 error, appliance shuts down with overheat indication, may occur during long draws of hot water
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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
Insufficient water flow through heat exchanger, Limescale buildup, Partially closed isolation valves, Air lock in primary circuit
How to Fix / Troubleshooting
Safety first: Isolate power and gas. Allow the unit to cool before touching pipes or the heat exchanger.
- Check inlet filters: Clean the cold water inlet filter and any strainers to ensure adequate flow.
- Verify valves: Confirm that all service valves on the heater are fully open.
- Descale heat exchanger: If the unit is in a hard water area and F20 recurs, perform a chemical descaling of the primary or plate heat exchanger.
- Bleed air: On models with a primary circuit, bleed air from vents and high points to restore circulation.
- Reset and test: After corrective actions, reset the fault and run the heater while monitoring temperature rise.
Warning: Persistent overheating can permanently damage the heat exchanger; do not ignore repeated F20 faults.
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Repair Difficulty
Medium
3/5
Required Part
Plate Heat Exchanger
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