Help Disconnecting Old T40 Thermostat to replace with nest (v3)

[chrisjroyce]

Hi All,

Firstly I have successfully swapped out my programmer with the nest heatlink and all is good in terms of using this to turn on and off my heating and water using the nest wirelessly.

I want to disconnect my old thermostat and install the nest in it's place. Removal of the old thermostat (Honeywell t40) is easy enough, but working out which wires belong to it in the junction box is proving tricky!

Thermostat:


Upstairs set up and junction box.


This is all upstairs in the airing cupboard with my boiler in the loft. There appears to only be one collection of wires coming "up" into the junction box but the colours do not match those leaving the thermostat.

Here is a close up of the wire coming up.


So my questions are..

A) How does the thermostat work, ie if I disconnect it entirely, will the heating permanently fire (assuming the programmer/nest says so) or will it be permanently off? What I mean is when the thermostat is at temperature does it send a signal to stop CH, or does it instead stop sending a signal to keep the CH on.

B) Is there a practical way I can confirm that 3 wires (red,yellow and blue) turn into the wires in the above close up (brown, black and grey) by tracing them to a component?

Thanks in advance, this is the final piece of the puzzle!

Chris

 

[flameport]

A - if disconnected, the heating will not work. The thermostat is a switch, when heat is required it connects the red and yellow wires together.

B - disconnect the wires and use a multimeter. Any cheap one will do.

 

[chrisjroyce]

Hi,

Thanks for the reply. Done some further digging, it looks like the "ch on" from Programmer goes down to the thermostat and back up again to the valve. I should be able to determine the neutral easy enough and I can just disconnect that then wire the ch on directly to the valve skipping the thermostat.

This is just from reading, I'll see if that matches my junction box tomorrow!