Nest V3 Installation (UK)

For anyone thinking of doing a DIY installation of a Nest Thermostat in the UK, it’s not so difficult.

Obviously, it requires a level of understanding of electrical wiring and confidence in what you’re doing, but the reality is pretty simple.

For most UK installations you’re likely to have a junction box, though I beat the wiring doesn’t look as neat as this diagram.   This is a “Y plan” system where a 3 position valve controls whether the heating, hot water or both has a flow of hot water from the boiler.

 

y_plan_wiring_diagram_HWON

 

Before doing any of this make sure you’ve killed power to the heating system.   If you need help doing that, you should be paying someone to install the system for you.

The nest heat link is pretty much a drop in replacement for the existing programmer, but with the wiring in a different order.

You’ll need to jumper terminals 2 and 4 to the live feed to the programmer, and swap the terminals as below.

Existing Terminal
Purpose
Hive Terminal
Notes
1
HW OFF
4
2
CH OFF
1
Not used
3
HW ON
6
4
CH ON
3
2, 5
Jumper to Live IN

Once this is done, the next step is to identify the wires from the junction box going to the room thermostat.    Confusingly in my system these are different colours in the junction box to those in the thermostat housing, so theres obviously a link somewhere along the way.

To identify these, I basically had someone to twiddle the ‘stat up and down while I held a multimeter across the terminals I suspected, and when it jumped from open to zero resistance and back I new I had the right connections.

Once I knew what wires were what, what I did was to replace the room thermostat with a link (between points 4 and 5 in the diagram above) to minimise the amount of wiring I actually touched (and to make it simple to revert if needed).  Then I took the existing wiring to the thermostat and linked it to the 12V feeds from the Nest Heat Link.

For safety I did the 12V join in a separate, new junction box so it’s not in danger of touching any 12V circuitry.

As a final check I powered up the system without the Nest thermostat attached, and checked for 12V across the terminals in the thermostat wall mount.

If you plan on using the provided USB power to relocate the thermostat then you can just short out the existing one and you’re done.

It’s now in learning mode – it remains to be seen how good it is – watch this space.

Update: I’ve corrected the wiring terminals since I originally wrote this.  Please use a professional installer if you’re not 100% confident in what you’re doing.

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