Honeywell CM927 question


31 May 2006
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United Kingdom

I'm thinking about replacing my current room thermostat (an ACL Model TS142) with an RF unit, and the Honeywell CM927 seems to get a few positive comments on this forum.

I have a query about the electrical wiring for this unit. Does the RF unit need a permanent mains supply, or can I wire it into the Common and Neutral wires from my old unit? As I understand, the Common wire is switched from the CH timer so is not always live.

I have looked at the Honeywell manual for the product and the wiring for using the CM927 as a zone valve controller (image below) and it looks like it needs a permament supply. Is this right, or can I put it on the Common / Neutral supply?

If this unit won't work, can anyone recommend an RF unit that will work?

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I suggest you read through the Honeywell Application Guides. One of them should answer your question and save me the time of asking you a lot of questions ;)

The simple answer to your question is that the CM927 is fed from after the main switch which turns the whole system off. So it is only permanent while the switch is on (if that makes sense) :!:

Don't forget that, unless you have a combi boiler, you will need to retain your old time switch for the HW side.

It's all explained in the Application Guides.
That's a very helpful link - thank you!
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You will need four wires, permanent live and neutral, plus your switch cables to the valve.

You can use three if you do not have a programmer. Dropping the live cable (ie: making it switched) will lose the radio connection. Effectively you will be rebooting the link, which will cause problems.

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