F20
Viessmann
Primary Circuit Overtemperature
Boiler displays F20, burner shuts down, pump may continue
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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 or scaled primary heat exchanger, failed pump, closed valves, air lock in system, incorrect pump speed setting
How to Fix / Troubleshooting
Safety: Overheating can cause boiling and noise. Allow the boiler to cool before working on it.
- Check all valves: Ensure flow and return isolation valves under the boiler are fully open.
- System pressure and air: Confirm correct pressure and bleed radiators and high points to remove air.
- Check pump: Listen for the internal pump. If noisy or not running, it may be seized or air-locked. Some pumps have a vent screw that can be opened briefly to release air (have a cloth ready).
- Clean system (technician): If F20 recurs, the primary heat exchanger may be restricted by sludge or limescale. A professional should power-flush or chemically clean the system and, if necessary, replace the heat exchanger.
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Repair Difficulty
Hard
4/5
Required Part
Primary Heat Exchanger
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