Updating Honeywell T40 Thermostat - too many wires?

[Larkey106]

Hi,

I’m trying to update my old manual Honeywell T40 thermostat to a programmable T40 on a combi boiler.

the man in the shop said nice easy swap but once I removed old I found there’s too many wires.

I think the T4 only needs 2 but the old T40 has 4.

Is it a simple problem to solve or do I need a professional?

Photos attached for info.

Thanks
Paul

 

[JohnD]

the pic is badly lit and difficult to see

the green and yellow is the Earth, and is not concerned with switching.

Look at the cable that comes out of the wall. How many cores does it have, apart from the earth, and what colour are they?

I can see one brown core, so I would expect one blue core in the cable as well, or, in your case, I think you have brown, black and grey.

Are there others? or is the "extra" cable just a loop from one terminal to another?

Cut the power off, then if you lift the G&Y out of the way, and use a small dry paintbrush to clean away the dirt, it will become clearer.

It should be
1 - Live
2 - Neutral
3 - SW Live

I expect the extra is for an "accelerator" which you do not need. The two you need are Live and Switched Live. Mark the wires in terminal 1 and 3, as they are the two you need in your new stat. The Neutral must be parked safely, so it can't touch anything, and you may have a "spare" terminal in your new stat for it. If not, a chocblock or other safe termination is needed.

 

[JohnD]

@Larkey106

p.s.

I think your new stat is a T4

Is that right?

 

[Larkey106]

Hello @JohnD

thanks for your quick reply.

Sorry your right, light isn’t the best.

I have the following connections in the existing T40:

1. Yellow and Green to earth
2. Grey to number 2.
3. Black to number 3
4. Brown to number 1.

Spot on with the T4.

So yellow and green not needed.

Which other should I lose?

wiring diagram inside the old T40 is attached.

 

[denso13]

Which other should I lose?

Terminate 2 safely and use 1 and 3 on the T40.

 

[Larkey106]

Thanks @denso13.

so earth and 2 capped.

sorry to be a pain but do you know which to which on the new T4?

photo attached again.

I assume 1 to A and the other to B?

sorry for being simple!

 

[denso13]

I assume 1 to A and the other to B?

Doesn't really matter as it is only for switching, but yes, that is ok.

 

[Larkey106]

Perfect. Thank you.

I’ll give it a go