I've just had to replace the room thermostat on MIL's CH system, and have installed a Honeywell CM927 programmable wireless stat.
The old stat had live, switched live, neutral and earth connections, so I just linked the live to one side of the switch in the Relay Box, with the switched live on the otherside of the switch. All works ok except........
The live to the stat is actually switched by the old mechanical timer, so the power to the relay box gets disconnected when the timer goes off. The wireless stat seems to resync ok, but after a delay, and it might lead to some unreliability later on.
As a temporary work around I've left the mechanical timer permanently on, and relying on the HW cylinder stat to switch off the hot water when it gets up to around 45C.
It's going to be quite a job getting a permanent live to the relay box, even if I move it nearer to the airing cupboard where all the rest of the gubbins is, and the distance (plus walls) from it to the new room stat might affect the wireless ability.
An option I'd considered is to remove the live feed to the relay box from the mechanical timer and connect it to the switched live from the FCU for the system, thereby bypassing the timer. This would mean that the hot water only is controlled by the timer, and the CH is only controlled by the new stat (which of course has its own timer anyway). I'd put a note somewhere prominent to warn of this.
Anyone see any problems with this?