How to connect two new Nest 3rd gen heatlinks? (photos)

11 Oct 2010
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United Kingdom
Two Nest 3rd generation units (PDF) need to be installed for dual zone central heating (but not hot water) with one in the living room (zone 1) and one in the hallway for rest of property (zone 2).


The combi boiler is an Ideal Logic 35 which is wired as follows:

I have situated both Nest base units on the walls and connected them to the previous thermostat wiring as follows:

The previous owner appeared to have two Honeywell thermostats, possibly later replaced by a single Salus unit, with the junction box wiring currently as follows:


Two of those three lower grey 3-core cables were connected to the previous two thermostats and are now connected to the Nest bases, I have no idea what the third cable was for.

I have 2-core and 3-core cable ready to connect to the Nest heatlinks. I would like to situate both heatlinks in the same cupboard next to this junction box for convenience.


How should I wire up these two heatlinks to the junction box and the combi boiler for dual zone central heating operation please?
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The wires that go to the two existing room thermostat need to be rerouted to the two Heat links.

Zone 1 thermostat goes to Heat link 1

Zone 2 thermostat goes to Heat link 2

I can't advise any more as you haven't provided details of the two existing room thermostats, make / model and wires connected to them.

No need to alter the wiring at the wiring centre or boiler.

The Nest thermostats are powered by 12v. The thermostat T1 and T2 terminals should only be connected to the Heat link terminals T1 & T2, or they can be powered via a separate plug in power supply.

Are you aware that with Nest units the thermostats are in the base units?
As far as I am aware the thermostat does not connect to boiler but it connects to the motorised valve which in turn connects to the boiler. I would agree it does not seem to make sense, but that is the normal method.

So the box
1) 2 x brown = zone valve activate seems both zones are connected together.
2) Unused
3) Permanent Line feed to valves (Grey) and boiler (Brown).
4) Neutral Boiler and two valves.
5) Earth
6) Unused
7) Unused
8) Unused
9) Unused
10) Unused
11) Unused
12) 2 x Orange supply to boiler run (Black).
So to fit heat link N to 4, L to 3, a link in heat link L to 2, and the 1's go to 3's however to use two thermostat the 1's need separating and one of the 1's goes to one heat link and other goes to other heat link.

The wires to thermostat should go direct to heat link (T1 + T2) better not to mix 230 volt and 12 volt.

Personally if my boiler was OpenTherm enabled which I think yours is, I would only use one Nest unit and use programmable TRV heats to make the zones, and do away with zone valves. But since you seem to have problems working out how to wire, maybe easier to use zone valves now fitted.
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Are you aware that with Nest units the thermostats are in the base units?

No they aren't

This is the Heat link where the heating control wires go...


And this is the thermostat...


The thermostat sends the heating control signals to operate the heating to the Heat link wirelessly, all the the Heat link is, is an on/off switch that will turn the heating on and off when the thermostat wirelessly tells it to. However the thermostat doesn't have batteries, so needs to get the power operate from somewhere. The only wires that connect to it are for that purpose. T1 & T2 at the Heat link are only there to provide it with a 12v power supply.

I think you are getting confused because the wires that used to to to the original thermostats which included the on/off switch as part of it internally, now go to the Heat link instead because that is where the switch on/off switch is now located.
No they aren't
And this is the thermostat...

It looks like we have been using the terms "thermostat" and "base unit" to describe the same thing.

The Nest documentation calls these thermostats the "base units". So we are on the same page.
The item which you describe as the "thermostat" is in fact the Nest "base unit"

It is described as the base unit throughout the Nest installation manual.

The base unit is just the plate that you screw to the wall.


The thermostat fixes onto the base unit


In reality, it doesn't really matter what you call them, it won't changed how they are installed. :)
I was going by this picture on page 5:


I must admit, their installation manual tries to be so simplistic that it ends of creating confusion.
@ericmark - Thank you for your detailed reply.
Here is a diagram of my understanding of your advice so far:

What should I do to complete the connections when you say:
- "a link in heat link L to 2";
- "and the 1's go to 3's"; and
- "to use two thermostat the 1's need separating and one of the 1's goes to one heat link and other goes to other heat link"

Please help me understand these final three steps.
You still haven't provided details of the original room stats or how the wiring was connected to them. So assuming that they follow convention and have a Live, Switched Live & Neutral then they are removed and the wiring that went to them redirected to the Heat link thus: [The colours of the wires maybe different in your case, there's no fixed colour scheme]

Nest Heatlink Model (1).jpg

The second thermostat & Heat Link will be a repeat of the above. The earth terminal needs a connection to earth if T1 & T2 are used to power the Thermostat. (or base ;))

PLEASE NOTE: If the original thermostats were battery powered they may not have a neutral present and if so, the wiring will be different, if that's the case please post back.

Also, this assumes that the original room thermostats were of the 'programmable' type and included time control meaning that there isn't a separate time switch or programmer elsewhere.
Last edited:
@stem - Thank you for your reply and apologies for my omission.

The previous living room thermostat was as follows, with the hallway thermostat being a simplified equivalent:


Does this accord with your excellent diagram?
PS: I can confirm that the original thermostats were not batter powered and there was no separate time switch or programmer elsewhere. Thanks
Indeed it does correspond :). [Except for the wire colours used] You even have a brown loop you can reuse to link Heat link terminals L and 2

If you want to use the existing thermostat cable, for T1 & T2 because it runs out to the thermostat, you can install a new cable between the wiring centre and the Heat link, and then use the disconnected cable for T1 & T2. Here's a version showing that now with the correct wire colours:

Thank you for the clarification. Here is a updated wiring centre diagram:


Heatlink and thermostat base are now connected to each other at wiring centre blocks 8 and 9.

Both of the Nest Heatlinks' live and neutral cables are connected to wiring centre blocks 3 and 4 respectively, joining to the permanent live feed to the valves and boiler.

Should the cable for Heatlink terminal 3 'Heating Call for Heat' be connected to 'Zone Valve Activator' at block 1?

Should the Zone Valve Activators also now be connected to both blocks 1 and 2 with both Heatlinks as above?

Is everything in this wiring diagram correct or have I missed anything?

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