Nest installation query

[dalespitfire]

Hi Guys,

I'm looking at installing 3rd generation Nest as we renovate our house.

I'm hoping to get some advice on the installation if possible as reading multiple threads has me confused....I'm guessing that not all the posts relate to the latest version.

I have a Vaillant Ecotec Pro 28 Combi Boiler in the bathroom which has a Digital Battery powered Honeywell T40 thermostat on the landing and a generic UT2 programmer downstairs in the kitchen.

I've enclosed a couple of pics of the wiring in both. The first being of the thermostat which has me confused. I was expecting 3 wires here and only have 2....

The 2nd is the Programmer. There doesn't appear to be any junction box at the boiler.

This doesn't appear to match up with what I'd expected to find (including a similar thread regarding a T40 thermostat which reference 3 wires at the thermostat.

I don't mind blanking off the existing thermostat (mating the wires together in a block) and plugging in the Nest thermostat into a socket using a base.

Question is if I do that can I simply remove and replace the existing programmer with the Nest Heatlink in this case or is it not going to be as straightforward as I'd hoped.

I appreciate these things come up a lot so I'm grateful for any responses on this specific thread.

Thanks

 

[stem]

First of all the thermostat is presently wired correctly. Only the two switching wires in terminals A & B are required. The thermostat is battery operated and so doesn't need a neutral, and terminal C is not used.

As you have worked out, you need to join the two thermostat wires together in a suitable junction box, or alternatively find the other end of the two wires and disconnect them there. Then where you have removed them, from join the wires that they were connected to together.

The Nest Heatlink would then be installed where the existing programmer is. The British Gas UT2 (aka Drayton LP711) wiring can be identified from your photo as follows:

Blue wire = Neutral
Brown wire = Live
Red wire in terminal 1 = Heating Common
Black wire in terminal 3= Heating Call for Heat

They need to go to the correspondingly named terminals at the Nest Heatlink.

That's all you need to do, nothing needs changing.

 

[dalespitfire]

Many thanks stem. That's much appreciated.

Not sure it's worth tracing back the wires. It's probably not the ideal place to have the thermostat anyway so may well relocate the nest to the living room and house on a stand.

I'd be wanting to completely cover over the old thermostat. Can a slim block connector be used so it can be boarded in and plastered over flush?

 

[Dan Robinson]

No

 

[stem]

Another alternative, is to leave the existing thermostat in place and put both of the red and yellow wires in the same terminal, it doesn't matter which terminal. If you wanted to, you can leave the batteries in so that it still gives a visual ambient temperature display.

If the room where the new thermostat ends up has a TRV on the radiator, don't forget to remove it, or set it to maximum so that it doesn't interfere with the correct operation of the Nest.

I suspect that there will be a blue wire somewhere behind the existing thermostat, if there is and you do remove the thermostat, make sure that the blue wire is insulated and don't connect it to anything else.

 

[dalespitfire]

That's actually not a bad shout Stem as it would be interesting to see the temp differential anyway. Thanks again. Much appreciated.