Your help required on a Nest Wiring Diagram ... Thank you

13 Nov 2012
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United Kingdom
I hope everyone is well, and thank you for your help.

I installed a Gen 3 Nest Thermostat and Heat Link to our System Boiler yesterday.
The heating is working perfectly well.
While the Hot Water controls and schedule are showing on the display and on the App there is no hot water being produced.

I think the Cylinder Stat wires are wrongly connected.

On the original programmer Hot Water OFF was to Grey on Y Plan and Cyl Stat 2
On the original programmer Hot Water ON was to Cyl Stat Common

On the Nest Hot Water OFF goes to Grey and Cyl Stat 1
On the Nest Hot Water ON goes to Cyl Stat Common
On the Nest Live Orange goes to Cyl Stat 2

It is a Honeywell L641A Cyl Stat

Can you tell me if the Cyl Stat wires are wrongly connected

Thank you
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On the original programmer Cyl Stat 1 was connected to the Live Orange from the Y Plan valve
Have you checked the valve is still working? Cylinder thermostat connections look ok to me.
You have the cylinder stat wired backwards, swop 1 and 2 .
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Reactions: CBW
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Thank you for the quick replies,
The valve is relatively new and was OK prior to fitting the Nest.


Presumably I need to change the Cylinder Stat 1 & 2 over leaving the Common as it is ?

Is the problem caused by the layout of my particular Cylinder Stat ... I seemed to follow the instructions on YouTube and the paperwork that came with the Nest?

Thank you again for your help,


Yes common stays as it is, 1 is call for heat and 2 is satisfied so just swop 1 and 2 around.
A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a
lampholder having no exposed-conductive-parts and suspended from such a point.

The bi-colour combination green-and-yellow shall be used exclusively for identification of a protective conductor and this combination shall not be used for any, other Purpose.
Single-core cables that are Coloured green-and-yellow throughout their length shall only be used as a protective conductor and shall not be over-marked at their terminations, except as permitted by Regulation 514.4.3.

Some people feel with a multi-core cable you can over sleeve, however since you need to run a circuit protective conductor this could only really be done where multi multi core cables feed the same device, so the cable to tank thermostat needs changing for a 4 core cable.

I know BS 7671 is not law, but when you get an EICR done this will likely be flagged up, better is corrected at your leisure rather than having to do it in a hurry.
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Reactions: DP
Thank you for the input on the earth cable Eric,
I just need to get some 4 core - there's only a metre of cable between the Honeywell connection box and the cylinder stat so it won't take much changing.

Here's the correct wiring diagram for my system boiler ...

Nest Wiring.png

Thank you again for your help

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