Nest - No power to existing thermostat

10 May 2021
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United Kingdom
Hi Everyone,

I have replaced my existing Myson MEP2C with a Nest 3rd Gen. Everything went well, except there seems to be no power to the existing stat where I was planning on hard wiring the new Nest thermostat.

Here is the wiring setup before at the conrol where:
3 = HW on
4 = CH on


Here is my new wiring. The three N and L have gone into two different terminal blocks and come out as one wire each going into the Nest's N and L feeds
Links between L > 2 (Common) > 5 (Common)



Going into the existing stat, I have a brown wire (should go to most right terminal), black wire with brown sleeve and grey wire with blue sleeve (should go to most left terminal).

Below is the wiring behind the control which is a bit hard to see.
The white cable to the right goes to the boiler.
The only grey wire coming into this box is coming from the second grey cable which is grey wire sleeved blue. This wire going into a terminal block with other neutral wires and then connects to the N of the Nest
The live browns go into a terminal block and connects to the L of the Nest
The black wire into the terminal block goes to call for HW
The brown wire into the terminal block goes to call for CH


Anyone have any ideas why the stat has no power? To be honest, I can't work out which wire is which!

Thanks in advance for the help!
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'The operation was technically flawless but the patient died' is where you're at.
Most of your pic uploads have failed, try again
Haha - I like that! I guess that's true.

Reuploaded photos...
I can see the pics. Have you tested anything?

The thermostat wiring brown and black from your old room thermostat go into T1 and T2, doesn’t matter which way as there’s no polarity in Nest and terminate safely the grey sleeved blue, unless you’re using the usb, in which case join them together. If using T1 and T2 then you need an earth wire as well. These join T1 and T2 at the heatlink.
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If you've wired it correctly you won't get any mains at the thermostat position- t1 and t2 will be some sort of very low voltage signalling. Looking at your pics you haven't got anything into t1 and t2, if you shove mains up the Learning Thermostat it'll not like it
T1 and T2 are expecting 12v go to the Learning Thermostat. How can I get that there without using USB?
T1 and T2 are expecting 12v go to the Learning Thermostat. How can I get that there without using USB?
Existing wiring. You need to trace the wires from room stat to wiring centre, to nest heatlink. If and it’s a big if the colours are the same, it’s likely to be them, but colours generally mean nothing in heating systems.
My apologies, it appears I’ve made a faux pas. The existing thermostat will need to be decommissioned (removed) or the wires joining together. The Nest will either need to use either the usb or fresh cable from heatlink to thermostat.

Although, where is that wiring center in relation to the heatlink?
The heatlink is under the boiler to replace the existing control downstairs.

I was wondering if it was best to move it to next to the wiring centre upstairs in the airing cupboard. But then I'm not sure what to do with the existing wires under the boiler as I don't want to break the circuit.

or do i just leave all the L's and N's connected in the choc block and leave the HW and CH links taped up, then connect the heatlink to the wiring centre like below?

Then move the existing stat wires going into the wiring centre to the T1 and T2

You sound like you have the same issue I had, you can refer to my thread that I've just updated too.
Forget about T1&T2 for now we can deal with them once we have the Nest up and running, at the old thermostat, the brown wire and the black wire with Brown sleeve need to be joined together, the neutral needs to be safely isolated, then the heating will work , but that is not the problem if you dont have power at the nest, how far are you getting power and how are you testing this ?
sorry think I mis read your question, if you just mean that you dont have power to the nest thermostat, you can use the usb charger supply to power with it, or you can use T1 & T2 using the existing wires but you will need to move the Nest receiver to the wiring centre or identify the existig room thermostat wires and totally disconnect them and use the wires to connect to T1 & T2
  • Thanks
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I think I gathered I now need to move it to the wiring centre and plumb it in that way, thanks everyone.

One more question: What size cable should be used for this?

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