Nest 3rd Gen - wiring

12 Jun 2017
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United Kingdom
Hi all

Wonder if someone out there would be kind enough to double check my thinking for wiring-in a 3rd gen NEST to replace my Drayton Lifestyle LP522 programmer.

I'm pretty confident with home wiring but have never amended the central heating electrics. I've had a good look at the electrics in my new place and am pretty certain I understand what to do but wanted to double check.

I have a Y-plan installation with the programmer in the kitchen, Room thermostat in the hall and wiring junction box in the upstairs cupboard next to the tank.

At the programmer box location I have the following set up:

I'm proposing to fit the following wires into the Nest controller:

Fit N & L to corresponding Nest terminals.
Run a wire from the Nest L to HTG common (2) and to HW common (5)
Attach the HW satisfied wire (HW off, 1) to the Nest HW normally closed (4).
Attach the HW On wire (3) to Nest HW Open (6)
Attach the HTG On (CH on, 4) to Nest HTG Open (3)

At the junction box:
Disconnect wires 2 & 3 (red & yellow that go to the roomstat)
Remove roomstat blue earth from N block
Connect 9 (HTG on) to 4 (motorised valve) (via red wire)


I think that is it but I'd be really grateful if someone let me know if this is not the case.
Many thanks
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Hi Alex

First of all welcome to the forum, and congratulations on a well presented and explained post. If only they were all like this!

Anyway, you have gone to all that trouble for me to say, yes that all looks correct to me. Well done (y)
Thank you for the welcome and confirmation my approach was right (I'd seen you'd helped others with similar quieries so was hoping you'd have time to look at my post). Writing it all up helped me understand how to get it right and also felt a good idea to ensure I remembered what I had done in 10 years time!!

Thanks again.
Hi Stem and Alex,

I'm preparing to also install Nest 3rd gen thermostat, and your system seems to be very similar to what I have.

But I was wondering, after the heat link has been installed in place of the existing Programmer unit,
and the changes made in the junction box (which is in my airing cupboard)
do you place the actual Nest thermostat in place of the existing thermostat?
I ask this as you mentioned about removing the wires to the roomstat in the junction box?

- Chris
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Hi Chris, welcome to the forum.

The reference to removing the old thermostat wires was because the old wired thermostat needs to be taken out of use to prevent it interfering with the operation of the Nest, but it must be decommissioned properly. It can't just be disconnected, otherwise the heating wiring would be 'open circuit' and the heating would not come on.

The Nest thermostat communicates wirelessly with the Heatlink, so wires are not required and it can be powered using a plug in 12v power supply. Although there is an option to wire it to T1 and T2 at the Heatlink which can provide the required 12v instead of the plug in power supply.

The location of the Nest room thermostat is crucial to get the best from it and ensure it works properly. Here are some guidelines:

The room thermostat will switch off all of the radiators in the house when the set temperature is reached in the room in which it is located. Therefore, it should be in the last room to get warm. Understand this concept and you will be able to work out the best location for your home.

Because properties are different in use, orientation, construction size etc., it's not possible to say your thermostat should be located here or there. But consider the following.

Do not locate the thermostat in a room where it will be subject to other heat sources such as cooking appliances, used fireplaces, or sunlight. Any extra heat that warms up the room where the thermostat is located, will switch off the thermostat, and all of the radiators in the rest of the property.

It should be located in an open space to allow air to circulate freely around it so that it senses the true ambient temperature of the room in which it is located, not stuck in a corner, behind curtains or furniture, or subject to drafts.

Do not fit a TRV to the radiator in the room where the thermostat is located. if you do, it could turn off the radiator before the temperature set on the room thermostat is reached. In this event, the room thermostat will never turn off, and so there is no point in having it. [Some will fit a TRV, and either remove the sensor head or leave it set on maximum, but at some point no doubt someone would turn it down, so personally I would not fit one in the first place]

Fit TRV's to all other radiators to prevent the other rooms overheating

Ideally it should be in a place that is the last room to warm up, so that the radiators remain 'on' in all of the other rooms first to allow them to heat up before the main thermostat starts turning the whole system off. You may have to downsize the radiator in the room where it is located to achieve this.

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