Nest, time for a professional?

4 Jan 2017
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United Kingdom
Hey all,

I've read numerous threads on here about installing Nest 3rd generation into the home (such as this good one - kudos to @stem). I'm really tempted to do this myself but my wife will kill me if I screw up the heating at this time of year! If anyone can give me some pointers, it would be much appreciated! I'd like to keep the existing thermostat where it is and hoping to use the existing wires to feed the 12V to it from the Heatlink. But how do you find out where the existing thermostat wires are coming from?! Either from the airing cupboard junction box (which looks a complete mess btw) which is directly above the thermostat or from the existing programmer by the boiler.

Current setup is:
  • Honeywell T6360B Thermostat (in Hallway - existing to replace with Nest Thermostat)
  • Honeywell T45 Programmer (next to boiler - expecting to replace with Nest HeatLink)
  • Airing cupboard (cable nightmare - out of my comfort zone here!)
Pictures here
If anyone can give me a few tips on how to trace the wires that would be great :)


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out of my comfort zone here

The airing cupboard may be the place where the wiring centre is then.
EDIT, just seen the pictures :eek:
But quite honestly, there are 100 different ways that this could be wired and this isnt going to be sorted by asking questions on the Internet.

You need an electrician/heating engineer who knows about Nest. In my opinion.
And looking at the pictures, one that likes a challenge!
Well, I had typed the text before I noticed that there were some pictures. Normally they are displayed as part of the post.

I should have said "spaghetti centre":ROFLMAO:
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Ok, some if it's a bit of a rats nest, but depending upon how you proceed, you might not need to touch that part. Most of the connections look good, in that there isn't a lot of visible copper on show. A grommet would be nice, and the metal box should definitely be connected to an earth.

If you are competent to do the work, take your time and carefully identify and mark each wire before removing it from the existing programmer.

You need the dual channel Nest that includes hot water control.

At the Programmer, moving the N and L wires is straight forward as both the Honeywell and the Nest use the same identification.

For the other wires you can work out from the diagram on the existing programmer what they all do, and then it's just a matter of looking at the Nest wiring diagram and finding the corresponding terminals. NC (normally closed) corresponds to 'Satisfied' and NO (normally open) corresponds to 'call' So in your case that means.

Honeywell 8 = Hot Water Common = Nest 5
Honeywell 7 = Hot Water NC (or satisfied) = Nest 4
Honeywell 6 = Hot Water NO (or call) = Nest 6
Honeywell 5 = Heating Common = Nest 2
Honeywell 4 = Heating NC (or satisfied) Not required for your system
Honeywell 3 = Heating NO (or call) = Nest 3

Because the Nest carries out the functions of the existing room thermostat, the old wired thermostat needs to be removed to prevent it overriding the Nest. This can be done in three ways, in order of preference, they are.

1. Find the other end of the thermostat cable and disconnect it. Then link the terminals where the red and yellow wires came from together
2. Remove the existing thermostat and replace it with a junction box, connect the two live wires together (red and yellow) and isolate the neutral (blue)
3. Leave the existing room thermostat in place and set it to the highest setting. (Or put the red and yellow wires in the same terminal)

Option 1 is the only one that will allow the existing thermostat cable to be used for the Nest's 12v supply, but only if it runs from the existing programmer, to the thermostat. If it goes to the ahem...wiring centre instead (both methods are OK, but depends on personal preference of the installer) it won't be suitable.

The only way to trace the thermostat cable is by physically following its route, or by disconnecting each cable that has red, yellow and blue wires and electrically testing it with a multimeter. It may be possible for some one who understands 'Y Plan' wiring thoroughly, to physically inspect the installation and work out what goes where, but this would be neigh on impossible to do via a forum.

If you still want to DIY, and can't follow the thermostat cable, then swap the existing programmer for the Nest Heatlink, choose option 2 or 3 above for the existing thermostat and use the plug in power supply for the Nest thermostat.
Last edited:
@stem - thanks so much for your detailed instructions and taking the time to reply. I'm going to try and trace the wiring this weekend, may have to lift some floorboards above the thermostat location - seems obvious now, but I hadn't actually thought of that :)
I fitted the nest 3rd gen in my house a couple months ago, the previous wiring looked much like yours in the sense that it was a complete nightmare! I ended up completely re-wiring the whole system from scratch as the previous wiring made no sense whatsoever! Quite a lot of work and research to do it so I would only recommend if you are pretty confident at electrics and troubleshooting, definitely one of the most involved DIY jobs I have ever done!

I did the following:

Removed the old controller by boiler, thermostat from the lounge wall and disconnected everything in the airing cupboard.

Bought a wire tracer like this:

Used the tracer to label up each existing cable from the boiler to airing cupboard and from thermostat in the lounge to the airing cupboard.

Then used a new honeywell wiring centre:

Put this in the airing cupboard and followed the Y-plan schematics on the honeywell website to wire up to the wiring centre. located the Nest home link above this with power and control wires connected into the honeywell wiring centre. then used the old thermostat wiring to send 12V supply to the NEST unit on the wall in lounge.

Where the controller was by the boiler I used terminal blocks to connect the boiler to the airing cupboard cabling for power supply, pump overrun, call for heat. then placed a blanking plate over this to keep tidy.

All works perfectly now and I am really pleased with it, was a hell of a lot of work though so I probably wouldnt recommend taking it on in winter! I can supply photos of my install if it helps!
@Iknowcraig - thanks for sharing, that gizmo looks REALLY useful - I've got a whole bunch of tasks that will benefit from that. Ordering one now :)

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