Help fitting replacement CH room thermostat

[claire79]

Posted this in the plumbing/heating forum but realised it should be here...

Can anyone give me some advise on changing my 20 year old Drayton thermostat to a new Honeywell CM907 digital programmable thermostat.

The problem I have is none of the connections on the old thermostat correspond with those on the new, I guess things have moved on a little over the past two decades!

Here is a wiring diagram of my current thermostat with a few notes:

And this is the wiring diagram for the new thermostat:

It's also worth noting that I have a combi boiler - a Thorn Olympic 38/50B.

The old thermostat has 6 wires and the diagram above suggests only two are required on the new unit?

HELP!

Thanks,

Claire.
___________________________
lynda, moderator

I have removed your post from plumbing and heating

 

[Pensdown]

As a direct replacement,

Old 3 = new A
Old 1 = new C
Old 2 = new B

The cables that connect to N are no longer required.

I'm a bit confused as to why you have six cables? Have you got 2 stats?

Also, the wiring diagram is for a CM900 and you say you have a CM907, are they the same?

 

[claire79]

As a direct replacement,

Old 3 = new A
Old 1 = new C
Old 2 = new B

The cables that connect to N are no longer required.

I'm a bit confused as to why you have six cables? Have you got 2 stats?

Many thanks for that, I only have one room thermostat and of course there is a thermostat on the boiler.

N only has one cable.

Why does 2 have 2 cables on it though? Are you saying move both of these to 'B' on the new stat?

 

[claire79]

Also, the wiring diagram is for a CM900 and you say you have a CM907, are they the same?

Yes, according to this document:

http://www.honeywell.be/DocsAdobePD..._EN.pdf#search="honeywell cm907 installation"

Thanks again

 

[Pensdown]

I'm not sure why term 2 has 2 cables but yes, move them both onto B. The cable that was in N needs to be put into a piece of terminal block and left in the back box or any spare space in the stat as it's no longer required.

There are standard ways to wire heating systems but often these are not followed. If your system is working with 2 cables in T2 it will still work with 2 cables in B.

 

[Pensdown]

Claire, I've just the data sheet and your CM907 is a bit more than just a thermostat, it's a blumin optimiser!

What time control do you already have on your heating system because the DM907 is designed to replace it?

 

[claire79]

For the timer there is just a manual dial on the boiler with 2 ON and 2 OFF sliders.

 

[Pensdown]

OK. Once you have fitted your CM907 from what I've read you should set your manual dials to permanently ON. Your on/off times should be programmed into your 907.

I'm not sure if you know what you've brought but an optimiser works in a different way to your old dial time switch.

On your old time switch you would set a start time say 2 hours before you get up so the house is warm.

With an optimiser you set it for the time you get up and it works out how long it will take to warm the house up to the temperature you have set. It then starts the boiler accordingly.

I'm not quite sure how the 907 does this because normally optimiser's learn the required on time by comparing the outside temp to the inside temp over a set period.

There is an option for an outside sensor but according to the data sheet it's an option so I assume by that it's not required for normal operation.

Softus, (or anybody else) Have you come across one of these?

 

[claire79]

Ah that might be a problem, if I leave it on constant (from the boiler) it turns the hot water on too. There is no option to just have CH on by itself.

 

[slippyr4]

Ah that might be a problem, if I leave it on constant (from the boiler) it turns the hot water on too. There is no option to just have CH on by itself.

If, as you said, it's a combi boiler, then the hot water being on all the time won't matter because it won't heat it until you turn a tap on.