Hi, wondering if anyone can give any thoughts as to what is going on here.

We have a Baxi Duotec boiler, with a wireless Siemens thermostat attached, from memory I think that's the RDJ10 RF. They worked fine together for a number of years.

In the past few months we have had a few occasions where the heating will not come on, although hot water is being supplied on demand as normal. What I see is that when we turn the thermostat to the daytime setting, and the temp is low enough that the thermostat calls for heat (we see a little triangle on the display to show that it is calling the boiler), the boiler does not respond.

There are no error codes on the boiler. It doesn't have a manual switch so unfortunately I can't press that to try to get the boiler to produce heat without the thermostat. The first time this happened, we waited several hours (we were out of the house so left the heating as it was), and then tried to put the thermostat on again, and the boiler fired up as normal.

The only way I seem to be able to get the heating to work again when it stops, is to turn the thermostat to the Off setting and wait 2-3 hours. It then usually comes back on.

What I've tried so far:
I've turned the boiler off and reset it completely, to no effect.
I've reset the thermostat to factory settings and re-paired it with the boiler, to no effect. When the boiler refuses to work, I've checked the cconnection to the wireless receiver from the thermostat and the little light shows that it is connected correctly. We get no errors on the thermostat, the display is working fine, so i don't think it's a battery issue on that either but I have just replaced the batteries and the problem still occurs.

We have called an engineer out several times, but due to the waiting times, by the time they come out all they have to do is flick the thermostat and it comes on. They then announce that they can't do anything if it's working, and leave. It's getting quite annoying now as the problem is becoming more frequent, at it would seem that we aren't going to get any resolution at all with an engineer until whatever is at fault packs up completely.

I'm finding it hard to determine if it's the thermostat itself at fault, the connection from the receiver to the boiler, or the boiler itself. The only thing I can think to try is to get the thermostat replaced first as I guess that's the cheapest thing to try, but I would rather get to the bottom of the issue first if possible.

Any help would be greatly appreciated, cheers!