Help with Thermostat wiring

Frisbee54

Hi
This is my first post so be gentle!
I currently have an old Honeywell dial thermostat which I'm hoping to replace with a new Honeywell CM / CMT 907. I've searched everywhere for information on how to wire up the new stat but can't find any information which matches the current wiring. My current stat is wired as follows :-

Earth - unsheathed copper wire currently connected.
Terminal 2 - Blue wire connected
Terminal 3 - Yellow wire connected
Terminal 4 - No wire connected
No Number - Red wire connected
Terminal 5 - Not used
Terminal 6 - Not used

I understand that the CM / CMT 907 is a simple two wire installation - I presume the red and yellow wires. My question is what do I do with the earth and blue wires?
Any help would be much appreciated.
Thanks.

penrhyn

If its a battery operated stat, then just insulate the unused wires and push them to the back out of the way.

Frisbee54

Thanks Penrhyn
My question perhaps should have been WHICH WIRES ARE THE UNUSED WIRES?
By the way, the new stat is battery powered.

gas4you

If your existing one is wired as standard, then you will need to make the red wire a permanent live from the wiring centre and wire it to L on the 907. The blue wire to N, then link L to A on the 907, then yellow to B on the 907.

The above assumes that everything is switching at 240VAC.

Scrounger

Frisbee54 wrote: »

My current stat is wired as follows :-

Earth - unsheathed copper wire currently connected.
Terminal 2 - Blue wire connected
Terminal 3 - Yellow wire connected
Terminal 4 - No wire connected
No Number - Red wire connected
Terminal 5 - Not used
Terminal 6 - Not used

Connect the red wire to "A", the yellow wire to "B" and the blue wire is unused.

More info here

[NB The blue is a neutral return for the 'accelerator']

Scrounger

C_Mababejive

It really depends upon what sort of system your existing thermostat is connected to.

Is it voltage free or is it wired into a 240Vac control circuit?

What sort of boiler do you have?

I would suggest you obtan the installer notes for the existing roomstat so that you can work out what the connections do.

In a 240Vac control ciruit there will be a live in,a switched live out and a neutral for the heating anticipator circuit. The earth may be connected to any metallic backplate.

Assuming all is well,i'd suggest you use the red and yellow and insulate the blue and green..but dont assume..always check.

Pincher

Voltage free, what a nightmare.

I bought a Glowworm Climastat, which is voltage free and designed to work with the Smart wiring Centre.

The zone valves are AC, so should have gone through the Smart Wiring Centre, but they wired it S-Plan.

The old wall thermostat was 240V AC, which was connected to the old boiler demand line. Guess what, the (then) CORGI registered plumber, who went on the Glowworm training course, didn't wire it. His plumber's mate promptly wired the Climastat into the new boiler's AC connections. The boiler came on, and we agreed to leave it running for a few days. I went away for Christmas for about TEN days, and came back to find the whole house was 28 degrees, and it cost me about £100 in gas. The Climastat died under the AC load, and the boiler was on for ten days! I was lucky the house didn't burn down while I was away.

They never heard of Weather Compensation, so the external thermostat was connected to AC as well!

In the end, I designed a hybrid control system, incorporating a Climapro, Smart Wiring Centre, and four zone valves, so that the under floor heating can be switched on/off, via a separate Honeywell timer.

So, sit down with the circuit diagrams and understand what you are doing.
If you don't, nobody else does, either.

matelodave

I would have to agree with Pincher - just trying out a couple of wires here or there without actually knowing what they do is a recipe for disaster or even electrocution.

If you don't know check, don't rely on someone else who has no idea on how your system is wired - their guess is as good as yours.

You cannot rely on the wire colours being connected properly or on some of the info given above - you must check on their function using a multimeter or check where they are connected at the other end or, at the very least, get information on your existing thermostat - it's probably got a makers name and part number on it which should enable to find out how the terminals are wired and configured.

C_Mababejive

Indeed..a lot of people dont understand the concept of voltage free switching which is now almost always the norm in modern boilers..