F8
Saunier Duval
OpenTherm Protocol Error
Thermostat shows F8 and cannot control the boiler via OpenTherm mode
Advertisements
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.
Advertisements
Possible Causes
Boiler not configured for OpenTherm mode, Incompatible boiler (non-OpenTherm) connected to OT terminals, Faulty OpenTherm interface on boiler PCB, Damaged OpenTherm cable between thermostat and boiler
How to Fix / Troubleshooting
Safety: Isolate mains power to the boiler before touching internal terminals.
- Check boiler manual: Confirm that your Saunier Duval boiler supports OpenTherm and that the correct terminals are used.
- Boiler configuration: In the boiler installer menu, ensure the control type is set to OpenTherm/eBus as required by the thermostat.
- Inspect cable: Verify that a dedicated two-core low-voltage cable is used for OpenTherm and is not shared with 230 V circuits.
- Switch mode: If your thermostat supports both on/off and OpenTherm, temporarily set it to on/off relay mode to restore basic heating while you diagnose the OT fault.
If F8 persists with a compatible boiler and correct wiring, the OpenTherm interface on either the boiler or thermostat may be defective.
Advertisements
Repair Difficulty
Hard
4/5
Required Part
Saunier Duval OpenTherm Interface / Communication Cable
Check Price on Amazon
We may earn a commission from links on this page.
Advertisements