Thermostat Problem

[Avo Mk8]

Thank you for your reply.

We have always thought the flame symbol showed the thermostat was communicating with the boiler. The fact that we have not seen it at all since this started was very confusing.

'So in normal operation, when the temperature is at 13 degrees first thing in the morning and you up the target temperature to say 20 degrees then you should see a radio signal briefly flash up and then the flame symbol should remain on. When 20 degrees is reached, the flame symbol will go off (and the green light on the relay box should go off as well).' That is exactly what happened before but as described above we are seeing no flame symbol now.

To be honest I have not noticed if the green light was always on before but if pressed I would say it wasn't on all the time.

So to summarise:
The green light on the BDR91 is on all the time. That is not normal behaviour.
The green light (which indicates demand for CH) should only come on when the thermostat demands heat (showing the flame symbol) and should otherwise be off.

My understanding is that there should be two way communication between BDR91 and DT92, ie that both receive and transmit, when one transmits an instruction the other acknowledges it.

So one hypothesis is that the BDR91 is 'stuck' with the light on. And is not acknowledging a command, so the DT92 is showing a communication failure. You might try power cycling the BDR91 (turning off, wait a minute, turn on).
If that doesn't do anything, changing the BDR91module would prove if that's faulty. But that is stooping to the 'swapping parts' method of fault finding, which I don't encourage.

All I can suggest is:
Power cycle the BDR91. See if you can get the green light to turn off, and if you succeed, check that the CH stops accordingly.
Take the DT92 up to the BDR91 location and see if you can get the two of them talking to each other, by pairing or factory reset etc.
If that proves impossible, replace the BDR91 module.
If that doesn't fix it, replace both the BDR and DT pair with another solution!

All this assumes the boiler is not at fault in any way!