Room thermostat wiring

6 Nov 2013
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United Kingdom
Hi all, I hope you can help.

I'm trying to replace an old room thermostat with a honeywell CM900. I've got a Worcester 35cdi combi boiler fitted with a timeswitch and there are 3 cables (I ssume they are Neutral, live and switched live) attached to the old thermostat. The wiring diagrams for the new Honeywell thermostat only show 2 wires to be connected for a combi boiler. Is there a simple way to connect the 3 wires to the new thermostat? I don't really want to change the wiring in the boiler.

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There is always going to be a 'live' and a 'switched live' probably a red wire and a yellow wire.
The neutral wire (blue) in the old stat would have been needed for the anticipator which is obviously no longer required.

No doubt inside the combi, there will be two terminals which was 'linked' when fitted.
The link will have been removed and replaced with the stat live wire and switched live wire.
So it seems the options are to use the same terminals, or the wires coming from them.
The blue neutral could be removed or made safe.
Thanks very much Mandate

I think I understand. Does it matter which terminal in the room thermostat I connect the switched and unswitched live to?

Can't really say, it may not, but why consider it, when Its so easy to identify which is the 'live', Do you not have a multi meter, or non contact voltage tester.
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Looked at wiring for BDR91 on combi and no mention of 'live' but on other diagrams it appears live goes in to 'A'
Thanks Mandate
I don't think it matters which way they are wired, but I'll put the live in terminal A.

Thanks very much for your help.

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