Thermostat not switching off boiler

[BelfastGuy82]

Perhaps they have it wired, so that either stat can call for heating?

As I explained in post #6.

 

[Johntheo5]

Assuming two new stats, if you turn one off on its on/off switch as shown, and the other (the one you say isn't working normally) doesn't switch off the boiler off "normally" and you then switch that off also, see does it shut down the boiler. If it does, leave it switched off, turn on the other stat and turn it up to make the boiler fire up, then turn it back down, see does that one switch off the boilder without having to reduce it to 14C, if not switch it off as well, see if boiler stops firing.

You may not have the on/off switch, thats on the CM3S, also check that the switched Live out is connected to terminal 2.

 

[CountryFan]

I'm not in the know enough to understand what that really means, or how it relates to my problem...?

My apologies, just re-read and it's the stat misbehaving. I'm which case, get them back, could have been wired wrong

 

[ericmark]

Perhaps they have it wired, so that either stat can call for heating?

Very good point, that is how mine are configured.

My first guess would be a contractor upselling a more expensive boiler to an unknowing housing association.

Yes seems likely.

I'm not in the know enough to understand what that really means, or how it relates to my problem...?

@Harry Bloomfield has raised a good point, so @CountryFan may be in error here. My house is too large for a single thermostat to work, following guidelines, we should not fit a thermostat in a room with alternative heating, or doors to the outside, as with alternative heating it can result with other rooms too cool, and with doors to the outside it could result in other rooms getting too hot, using TRV heads stops overheating, and using multi-thermostats in parallel stops getting too cool, by time one has messed around setting up multi-thermostatic control, often easier to just fit a smart control system.