F.76
Vaillant
Primary Heat Exchanger Overtemperature / Thermal Fuse Trip
Boiler shows F.76, immediate lockout, possible smell of overheating
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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
Severe blockage in primary heat exchanger, Pump failure, Dry firing due to no water, Blown thermal fuse on heat exchanger
How to Fix / Troubleshooting
Safety: This is a serious overheat fault. Switch off power and gas immediately. Do not attempt to reset repeatedly.
- Allow to cool: Let the boiler cool completely before any inspection.
- Check system pressure and leaks: Ensure the system is full and not leaking. If pressure is zero, do not refill until a leak is found and repaired.
- Engineer intervention: The thermal fuse on the primary heat exchanger may have blown and is not resettable. A Gas Safe engineer must inspect, flush or replace the heat exchanger, and replace the thermal fuse assembly as per Vaillant instructions.
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Repair Difficulty
Professional Required
5/5
Required Part
Primary Heat Exchanger Thermal Fuse
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