F20
Viessmann
Safety interlock / limit circuit open
Safety chain open, boiler will not start, safety loop error
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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
Open high-limit thermostat, Tripped flue gas thermostat, Open safety pressure switch, Loose wiring in safety loop
How to Fix / Troubleshooting
Safety: Do not bypass any safety devices. Identify and correct the root cause.
- Visual check of safety devices: Locate the high-limit thermostat, flue gas thermostat, and any external safety switches wired in series. Check for manual reset buttons and press once if tripped.
- Inspect wiring: Follow the safety loop wiring and ensure all spade connectors are tight and not corroded.
- Check system conditions: A tripped safety often indicates overheating or flue issues. Verify pumps are running and flue is clear.
- Reset and test: After addressing the cause, reset the boiler. If F20 returns, a technician should test each safety device individually.
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Repair Difficulty
Hard
4/5
Required Part
High-Limit Thermostat
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