Replacing room thermostat

Help Support

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Established Member
5 Oct 2009
Reaction score
Hi all

Recently moved in to a fixer-upper, so apologies for the first in a long line of noddy questions...

The old-style simple dial thermostat isn't working, so I am replacing it with a programmable type. The old one is 4-wire, and the new one is 3-wire.

First question: do I just cut off the exposed copper on the redundant wire, fold it over and give it a good taping-up?

Second question: I am currently using the rudimentary timer control on the (Worcester combi) boiler itself - the wheel of little tabs representing 15 mins that you flick up or down for on or off. In order to use the timer on the new thermostat, presumably all the tabs will need to be in the on??

The 2nd q isn't so important, as I can just have a play when it's all hooked up, but would be handy if someone happens to know the answer!

Much obliged.
To answer your first question, you need to understand what signals on what lines the boiler is expecting.

Have a look in the boiler manual and it should tell you how to wire it to the controller. If it only requires three inputs then make a note of which three lines you connect to the boiler, connect them appropriately at the other end to the controller and just ignore the fourth wire. You are right to tape it up to make sure the loose conductor does not short on another connection.

Your second question. If the dial really is purely mechanical then i would expect that putting all the switches in the ON position and using the controller to overirde them would be the likely solution. However its also possible (though much less usual) that the dial is set up to override the controller. Again you can tell for sure from the boiler manual installation instructions.
dont cut off the unused cable(you might need it again in the future) just use a single bit of "choc block " to terminate it safely in the back box,if present, or inside the new stat
It's worth mentioning that some boilers now have proprietary controllers wand to instal a 3rd party / replacement controller you may need to throw a couple of dip switches in the boiler, or move/remove a jumper cable or two. Again this should be in the boiler manual.

Sorry if I wasn't clear - I have ascertained which wires I need, I just wasn't sure of the best way to safely isolate the unneeded 4th. As suggested above, I don't want to chop it off in case I want to change to another 4-wire.

Thanks both.
No problem. You said noddy question - i wasn't sure how noddy! :)

Chop block is the correct solution but if you are short of space then a few wraps of insulation tape are ok. These cables are carrying low voltage signals not mains voltage power.