Replacing honeywel cm907 with wireless cm927 and bdr91 relay

[DSTaylor22]

I want to replace a cm907 thermostat with a 927 and bdr91 relay. Removed 907 stat from wall to then wire in the bdr 91. 907 has only two wires - brown (live) to A and grey (neutral) to B. There is also an earth and a fourth black wire which are not connected. Bdr91 seems to requires L and N feed as well as A and B. Have had a few attempts based on wiring diagrams supplied and can get the relay to communicate with the wirelss stat but relay doesnt then communicate with boiler. Boiler is glowworm flexicom 24hx. Can anyone tell me how to wire this correctly? Thanks

 

[Hufty]

The relay unit usually goes quite close to the boiler. It needs mains power, which I would take from the same fused spur that supplies the boiler. It also needs to switch the heating on and off, which is what the wires that go to your existing 907 do. Either you need to cut and reroute them to the relay unit, or find where they go in the boiler and replace them entirely.

 

[MJN]

I want to replace a cm907 thermostat with a 927 and bdr91 relay. Removed 907 stat from wall to then wire in the bdr 91. 907 has only two wires - brown (live) to A and grey (neutral) to B. There is also an earth and a fourth black wire which are not connected.

Assuming that the brown wire connected to A is indeed the live then the grey wire connected to B is not a neutral otherwise when your 907 called for heat the supply fuse would pop (live/neutral short). The grey is actually connected to the boiler's Call-For-Heat (switched live) input. (These may be swapped around depending on who fitted it)

For your new relay: at the boiler end of the cable connect brown to Live, grey to Neutral, black to the Call-For-Heat input and earth to Earth. At the relay end (i.e. sited where the 907 was), connect brown to L, grey to N, black to B, link L and A with a short piece of brown wire and securely tuck the earth out of harms way (or locate it in whatever parking facility might be provided). As Hufty touched on you could locate the relay next/nearer to the boiler if that suits better (e.g. better radio range, improved aesthetics of the old 907 location by not having anything on the wall, etc).

You should wrap both ends of the black cable with either a piece of brown insulation stretched to fit or brown insulating tape - this will indicate for future reference that it is a switched live. Do likewise with the grey in blue to indicate the neutral. The earths should also be sleeved in green/yellow sleeving.

Shout if you need further clarification.

Mathew

 

[DSTaylor22]

Thanks for the feedback. I was hoping this would be as straight forward as simply replacing the 907 with the relay switch without needing to go back and change wiring at the boiler or re-siting the relay (the way the system has been installed I can't easily track back the wiring from the existing stat to the boiler) The system has some added complexity as there are currently two thermostats operating different parts of the system (on different floors) and I don't want to do anything which might interfere with that. Mathew if as you think the existing grey is the switched live and the brown the live, what is the currently unconnected black?

 

[MJN]

The black is probably not connected to anything. Of course, only you can verify this so you must never assume anything. Check at the boiler end.

Mathew