Nest Gen 3 installation help please

23 Mar 2019
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United Kingdom
Good afternoon,

I’ve trawled through a few of the threads on here but unfortunately not found an answer to my specific question, although I’ve definitely learned a lot!

I’m looking to replace my existing Siemens RWB29 controller and Honeywell thermostat with a Nest Gen 3. All seems straightforward enough - heatlink replaces controller and thermostat replaces thermostat. My issue is that the current Honeywell thermostat is 240v but I’m keen to mount the Nest one in the same place rather than have it on a stand. Is there a way of getting around this? In my head I’m thinking there must be a link between the controller and the thermostat therefore is there a way of using this cabling to provide the 12v supply the Nest thermostat craves?

The rest of my set up is boiler and separate hot water tank, all currently told when to come off and on by the Siemens controller.

Any help would be gratefully received. Thanks in advance. I’ve included pictures of the current set up and Nest heatlink terminals.


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Yes the sensor of Nest 3 sends info using wifi so that leaves wires which can be used to power it, the problem normally is identifying which wire is which, in the main where we use a single boiler for domestic hot water and heating we have motorised valves which often connect to a wiring centre.

Often we need to work out the wiring in that centre to enable us to use the wires for other things. Since there is no standard wiring this can be quite a problem, so our first task is to try and work out what we have. We tend to name the plans used C, Y, S, W etc and so your first thing is to try to work out what you have.

In some cases the motorised valves are inside the boiler however it seems in your case the domestic hot water is controlled external?

Since the colour of the motorised valve wires is reasonably standard we often use them to help us work out what we have, often physically following the wire through the wiring centre, we may be able to do it from photos, but much easier when one can put a wire to be sure one if following the correct wire.
Yes I have a separate hot water tank.

So basically I need to figure out which wires are currently going into the programmer from the thermostat, and connect those to T1 and T2 on the heatlink to use them as 12v power supply to the Nest thermostat?
It is unlikely that the thermostat wires will go directly into the programmer - as you can see, they are different cables.

Do you have a wiring centre near the boiler or valve?
If so, the controller should be placed near this where all the necessary cables will be and can be reconfigured.

The wires in the programmer look rather old so it might be advisable to do away with them - so this would be a good oportunity.

It looks like you have a Y-plan system.
The wiring diagram on page 23 of the Nest instructions.
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That makes sense - I was thinking there weren’t as many wires as I would expect and they’re not the same as the ones going to the thermostat.
I don’t like the look of this!


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Yes I have a separate h hot water tank.

So basically I need to figure out which wires are currently going into the programmer from the thermostat, and connect those to T1 and T2 on the heatlink to use them as 12v power supply to the Nest thermostat?
As you say T1 and T2 are 12 volt supply to the remote thermostat, however it seems anywhere between 5 and 12 volt will do, and you can run it from any 5 volt DC supply for example a USB outlet.

Honeywell make wiring centres designed for the job with cable clamps etc. However many builders want to save pennies and have for years used socket back boxes with a length of terminal block and blanking plate as a lid. And also even save on the 10 ways shown by Honeywell and get away with less terminals so you can't simply say terminal 1 in Line and terminal 2 neutral and so on as they have used their own diagram.

However the motorised valves do seem to use standard colours, so a two port valve brown and blue powers the motor, orange is common, white N/C and Grey N/O.

There are different plans but counting the number of valves and noting how many ports one can normally work out what plan has been followed, so with two x 2 port valves than likely S plan output from thermostat will likely connect to brown wire to motorised valve, there will be two thermostats but often the tank thermostat is close to the wiring centre so you can work out which is which.

So you slowly draw out the connections, so you have a 10 terminal connector block, you have two brown wires from motorised valves so you can identify the thermostats, the that wire goes to a cable so you find other cable from thermostat, likely orange connects to boiler so that's another wire, so soon you identify all 10 terminals.

As to what cables can be reassigned depends on exactly what you have, if your boiler is OpenTherm enabled then you will want OT1 and OT2 connected direct to boiler, to be frank my boiler is not OpenTherm so don't know how it is used when also doing domestic hot water, and you may have to alter the wiring to zone valves?

But Nest 3 does not have to use OpenTherm it is just an option, which seems great with a gas boiler able to run between 6 and 28 kW however with an oil boiler able to run 15 to 24 kW not sure it's worth it?

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