Sanity check on boiler wiring

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Hiya,

I'm thinking of replacing a wireless myson thermostat linked to my logic 35 combi boiler with a honeywell wireless cmt927

The current wiring looks like its been done with 3 core + earth cable and has:

  • Brown constant live (not currently used as battery operated)
  • Earth (marked with yellow/green sleeve)
  • Grey wire (marked as brown connected to thermostat switch)
  • Black wire (marked as brown connected to thermostat switch)

On the new thermostat i'm thinking

  • Brown constant live goes to either live
  • Earth is unused
  • Grey wire to A terminal
  • Black wire to B terminal
  • No neutral needed

Does this sound sensible?

Docs are:
old: http://www.myson.co.uk/MPRT_Hard_Wired_Instalation_Instructions.pdf
new: http://www.honeywelluk.com/Documents/Installation-Guide/pdf/1069.pdf
 
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Please excuse my ignorance, I read a lot of info last night and one of the posts on here said it didn't need a neutral. After rereading they later correct themselves as they were thinking about the battery thermostat cm907 so my mistake for not reading more. I was wondering how this would work.

So, its sounding like this may not be possible with the wires I have available by the thermostat?
 
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Sorry but looking at the manual, it just seems to show how to connect it via 2 wires for the stat. But surely I still need a live/neutral to power the actual wireless relay for the thermostat? My problem being I don't seem to have a neutral, only the 2 cables for the stat and a constant live. Sorry if I'm being dense.
 
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A bit.



Why not dump the old wire and use a short piece (or two) of new wire to power the L, N, E,

And assuming the boiler is switching 230V internally - Earth with, the two switch wires across the boiler stat connections and A & B.
 
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Thanks for the replies. The main reason is the thermostat isn't anywhere near the boiler and all the wiring is behind the walls so is a bit more involved. I could of course ditch the wiring at the boiler and fit the relay in the boiler cupboard, blanking off the old thermostat hole. I was just Hoping for a straightforward swap like at our last house :( oh well
 
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You will definitely need a neutral (the L &N terminals power the receiver). As there is no neutral at the present thermostat location, you will have to locate the receiver somewhere else. Near the boiler is the easiest place.

You will need a four core heat resistant cable connected as follows:

Receiver - Boiler
L ------------ L (Brown)
N ----------- N (Blue)
A ----------- 2 (Room stat terminal next to L)
B ------------1 (2nd room stat terminal)

If the cable also has an earth wire it should be connected to the earth terminal of the boiler. The other end shuld be made safe.
 
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You do not have to locate the receiver somewhere else, if you don't want to.
You have 3 cores + earth coming from the boiler to your current stat.
Look at the drawing I referred to on page 30.
Only 3 cores + Earth required. :D
You take live and neutral from the boiler to L & N in the receiver, link live to A within the receiver and B goes back to the input in the boiler which is shown with a connection to it.
Simple?
 
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