Replace British Gas UP2 Programmer with Nest 3rd Gen

5 Feb 2016
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United Kingdom
Hi guys, like the subject suggests I try to replace my by up2 with a nest 3rd gen thermostat. My up2 controls both the hot water and heating and the nest can do both. The issue I have is the up2 has two calls, one for HW and the other CH and they seem to share a common. See picture 1. The Nest though seems to need commons, one for the HW and CH, in not sure what to do about this now. Can I split the common for the Nest, only connect one common and the Nest internally will share the common or would i need to add an addtional cable from the boiler? The boiler is a Potterton Precision if that helps.
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OP, everything you need to know is in the very simplistic wiring diagrams that come with it and can be found on line.
That's not helpful, I've read the Nest manual and the boiler mammal and the programmer mammal more times than I care to admit. I'm not sure where to get the extra common from or if I can split the single common that I do have. I've mail Nest and they won't advise besides that I need two commons, but I don't know where to get the 2nd from.
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As said all the info you need is right there in the manual.

nest print screen.png

When you've finished with the mammals you can thank me later (y)

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Earth core being used as a switched live boils my pizz... @ least they used a SL to close HW port....something I suppose.
It's one of the better installed heating wiring from what has been shown recently! Though we can only see one small part of the install... I wouldn't use g/y for anything other than earth but it is permitted if it is properly identified at all terminations.

OP you have a Y plan system, the wiring diagram clearly shows you how to connect each wire. Just to help you further:
Brown wire is Live supply
Blue wire is Neutral
Green and yellow (which needs a brown sleeve) is HW off
Grey (which needs a brown sleeve) is HW on
Black (which needs a brown sleeve) is CH on

You will need to alter your existing room thermostat wiring if one is fitted, which is also explained in the manual.
Any suggestions? The heating is done by a combi and the hw is from a hot water tank. I've just bought the house and still trunking piece this together.
Jesus this is painful. Ok keep this under your hat @silversurfer79 I just want a quiet night.

Edited till the post above is clarified. If we can't get a boiler type right I'm not helping with the wiring!


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