Nest 3rd generation replacing a British Gas UP2

[Frances T]

Hello all,
I would a very quick bit of advice. I am planning to purchase A Nest thermostat this weekend but I want to understand how I can wire it first with my setup. Please in advance forgive me if I say anything slightly stupid but I have a good understanding of electric circuits so I should be fine

I have included a bit of extra info but this my setup:
My boiler is a Worcester Greenstar 18Ri
I have a hot water tank upstairs in the airing cupboard
My programmer is the British gas UP2
My current thermostat is a standard drayton (the warranty sticker on it expired January 1996!)

I have attached the wiring of the programmer and thermostat but there reason I having a bit of confusion is my current programmer has pins 1,2,3 and 4 with no pin 2 whilst the nest heat link has 6 pins and i believe requires pin 2? Any advice would be much appreciated, I am sure it is probably a common setup.

As a quick note too the cable on the programmer going straight down just connects to a junction box with a fuse which also provides power to the boiler. The cable going up doesn't go into the boiler at all and just goes straight up into the ceiling but another cable comes down next to it into the boiler.

 

[stem]

I believe that the British Gas UP2 is a re-badged Drayton ACL Lifestyle, if so the wires at your existing programmer will be:

N-Neutral
L-Live
1-Hot water off (live)
3-Hot water on (live)
4-Central heating on (live)

That leaves terminal 2 as 'Central Heating off' but your system doesn't need it.

If you look carefully, you should see a diagram on the UP2 confirming this.

The existing thermostat is L, N and a live which is energised when the heating is required. The nest thermostat requires a 12v operating supply, but doesn't need the neutral

All this will become clear when you have the Nest, because it comes with its own very comprehensive instructions which if you will be able to follow, now that you know what the existing wires are / do.

Don't think that you can just swap the wire that goes from the existing terminal 1 to the terminal 1 on the Nest, it doesn't work like that, no more than you would expect every aircraft leaving Gate 1 of all international airports in the world to end up at London.