I have had the dreaded Viessmann Vitodens 100 / Honeywell R8810 OT bridge combination. Sometimes the boiler grinds to a halt with the F:A9 error and we have to power the boiler off and wait. It usually works after a couple of resets – sometime for days on end.

Other times it appears to clear itself. Presumably there are times when we don’t notice this. When there is a problem I get three flashes of the red light on the R8810.

It would be good to have a reliable system, but at the moment the OT combination is not fit for purpose. In an old thread StephenC suggested “reversing the telemetry configuration on the Evohome controller” – I’m reasonably savvy but have no idea how I would do this. Does anyone have any suggestions as to how I can get these two bits of kit to work.


PS Not sure if it’s normal but even when the boiler is working it seems to run very hot. I thought the point of OpenTherm was to have the minimum temperature required.

PPS Happy to switch off the R8810 if it makes it more reliable, but it would be a shame