Nest Wiring Check

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Hi,

I've just purchased a 3rd gen Nest as they are reduced for Black Friday.

I was hoping to get a sanity check on my wiring plans. I understand that I have a Y Plan system.

Existing Timer (Honeywell C21):

N = 230v Neutral
L = 230V Live
Earth

1 = HW OFF
2 = CH OFF (NOT USED)

3 = HW ON
4 = CH ON

Nest Heat Link:

N = 230v Neutral
L = 230v Live

1 = CH Satisfied (OPTIONAL)
2 = CH Common
3 = CH Demand

4 = HW Satisfied (OPTIONAL)
5 = HW Common
6 = HW Demand

OT1 = OpenTherm (NOT USED)
OT2 = OpenTherm (NOT USED)

EARTH
T1 = 12V DC (to Nest)
T2 = 12V DC (to Nest)


So therefore I'm planning to wire as follows:

(Honeywell C21 - Nest Heat Link)
N - N
L - L
Earth - Earth
1 (HW OFF) - 4 (HW Satisfied)
3 (HW ON) - 6 (HW Demand)
4 (CH ON) - 3 (CH Demand)
Link commons to 230V Live by:
Linking Heat Link Live to Terminal 2
Linking Heat Link Terminal 2 to Terminal 5
Send 12V to Nest by:
T1 = Old thermostat wire
T2 = Old thermostat wire

Does this look correct?

If so I just need to find and identify where the wires are to the old thermostat. I understand it would have been wired in series with the programmer, so hopefully the wires will be behind the backplate!

I had read that when replacing the old thermostat the wires would need to be connected for the heating to function, but if they are being removed from the series wiring and used instead to power the Nest, then I guess this should work fine?

Any help much appreciated.

I can always post a pic of the current wiring but hopefully I've explained it clearly.

Cheers,

Phil
 
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After doing some further research I'm now more confident the wiring I've proposed is correct, the only thing I'm not sure about is the old thermostat.

It seems likely that the thermostat is going to be wired to a junction box in the airing cupboard upstairs, rather than the cupboard with the boiler and programmer in. This means I won't be able to easily connect the nest to the T1 and T2 terminals of the Heat Link, and I also suspect when I open the junction box nothing will be labelled and it will be difficult trying to find the correct wires.

What have other people done regarding getting power to the Nest when replacing an old thermostat?
 
Soldato
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That's my Plan B, and is the easier option (to just use the stand and put it on a table somewhere with the USB cable and mains adapter). However, I'd really like to have it on the wall in place of the original thermostat.

my experience was that my nest was in the wrong location so I moved it last week and mounted it on the wall via usb

I tried wiring it in but I gave up in the end as it started to impact another light in the airing coupboard etc so I thought forget it!

Don't know your setup but perhaps it needs to be moved anyway?
 
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my experience was that my nest was in the wrong location so I moved it last week and mounted it on the wall via usb

I tried wiring it in but I gave up in the end as it started to impact another light in the airing coupboard etc so I thought forget it!

Don't know your setup but perhaps it needs to be moved anyway?

It would be fine where it is I think (downstairs hallway). I just wanted a neat install and obviously using the existing thermostat wires would have them hidden.

What did you do about routing the USB cable to a plug socket?
 
Soldato
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It would be fine where it is I think (downstairs hallway). I just wanted a neat install and obviously using the existing thermostat wires would have them hidden.

What did you do about routing the USB cable to a plug socket?


Well when I got the whole house wired up
I made the area under the stairs the hub of the home!

4 double sockets under there
It is like media city lol
 
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For getting power to your nest thermostat:

Do the current wires feeding the Honeywell go to a wiring box? When I added my Nest I put the heatlink next to my wiring box which allowed me to use T1 & T2 .
 
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For getting power to your nest thermostat:

Do the current wires feeding the Honeywell go to a wiring box? When I added my Nest I put the heatlink next to my wiring box which allowed me to use T1 & T2 .

I need to check that out.

I suspect they go to the wiring box in the upstairs airing cupboard, whilst the programmer is next to the boiler downstairs in another cupboard!

Best result is the thermostat is wired from the programmer and the wires are where I need them to be, but I think that's unlikely.
 
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So I've traced the thermostat wiring this evening and as suspected it goes to a junction box in the airing cupboard upstairs. The programmer is next to the boiler downstairs.

This means I can't connect T1 & T2 using the existing wiring.

Next easiest solution would be to put a 12V transformer/PSU in the airing cupboard with the junction box and use the thermostat cable to supply the Nest.

Anyone done this, or see any issues with what I'm thinking?
 
Caporegime
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I fitted mine today. Didn’t work just mimicking existing (put heat link in place of old controller) so took my time going through the wiring.

Ended up linking the live and call on the existing stat and run a new wire from heat to nest. Also drilled the back plate for neat appearance.

For the wiring I had to swap 3&4 around and move 1 to 6. This was on a Y plan with mid way valve.

Horstmann H27XL got replaced, which to be fair was a decent controller.

Working fine now it seems.
 
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I like having my nest powered by usb from a plug socket. I’ve got it in a book case so the wiring behind is hidden. It’s been there for months, but it’s good to have it mobile for trouble shooting the inevitable heat link connection drop outs, or if you want to change the thermostat position. Ideally you want your thermostat in your main living area rather than a hall anyway.
 
Caporegime
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Depends on the house. My tv does a good job of keeping the lounge warmer than the rest of the house so this room would be a bad place for a stat
 
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OK so the current plan was to use a constant voltage 12v DC LED Driver to power the Nest thermostat. This will need to go in the upstairs airing cupboard as that is where the current thermostat wiring goes.

There is no permanent live in the airing cupboard (they are all switched) so I was going to take a live feed from the smoke detector mains circuit in the loft and send it down in to the airing cupboard. Seemed like the easiest way.

Was hoping to get this done over the weekend but Screwfix didn't have the right LED Driver so that should turn up today.

The more I think about it the more tempted I am just to get an installer. That seems like a bit of a bodge and I guess the smoke detectors are on their own circuit for a reason.
 
Caporegime
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Assume you cant add a new cable to run from heatlink to stat?

Don't think its a bodge, it just need a bit of power to keep it going (the USB adaptor is only 7W). It has an internal battery aswell so its not drawing huge amounts of current.
 
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Assume you cant add a new cable to run from heatlink to stat?

Don't think its a bodge, it just need a bit of power to keep it going (the USB adaptor is only 7W). It has an internal battery aswell so its not drawing huge amounts of current.

Unfortunately not without ripping up floors and/or channeling out walls.

I think the 12V supply is fine, when I say bodge I was meaning grabbing a mains feed from the smoke detector circuit.

There is an immersion heater in the airing cupboard which has a fused switch. Maybe that would be a better point to take a feed from. Not sure if it would require a second fused switch adding to supply the Nest, the 12V supply fed from the permanent live behind the immersion heater switch, or whether it could be powered directly without another fused switch.
 
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I ended up fitting the Nest last night.

Decided to remove the programmer from the downstairs cupboard and fit a blanking plate, use the existing wires from that to the wiring centre in the airing cupboard upstairs to send 230v and install the Heat Link next to the wiring centre. This then allowed me to use the existing thermostat cables to send 12V to the Nest via T1/T2 from the Heat Link.

Only took me about 5 hours to trace all the wiring and make the modifications, and install the Nest :rolleyes: worked first time though so no more messing about at least. Also, now I know how to wire up a central heating system!
 
Soldato
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I have been looking into Nest and Hive recently. Its just the wiring of the Nest that is putting me off. I want it in the hallway/stairway, but there are no sockets or existing wiring to power it up.
 
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