Nest on Vaillant ecotec plus 630 H sys A

I’m looking to add a Nest stat and have been looking at how the current system is wired.
I have a Honeywell stat that sends to the boiler.
The boiler is in the garage and the programmer and tank are in the bathroom above the garage.
The junction box by the boiler has mains power, going to the boiler and the programmer and coming down from the programmer is the stat cable, which also goes to the boiler.
When I access the wire point there is no 345 and 678, like people have on posts I have read.
This is what I have
The manual shows me

So I don’t understand how I control the heating and water or even how it works now.
Any ideas?
Thanks
Iggy

This boiler is a combi and provides instantaneous hot water internally which is controlled by hot tap being turned on.

The existing thermostat would be connected to the brown wire in the terminal marked RT 230V. When your thermostat says that heating is required this wire becomes live and the boiler fires up to heat the radiators.

Normally if as above, this is one of the simplest systems to add a Nest to, but, and there is a but.....

You mention it being connected to a "tank" Do you mean you have a hot water cylinder connected to it also? If so, there will be other components such as a cylinder thermostat and motorised valve(s) that will need to be taken into account.

EDIT
If the above is the case and you do have stored hot water, you will need to provide details of the entire system and all of its associated components. Just saying you have a programmer without stating a make or model doesn't help much. There are so many variants out there, and almost as many different ways of wiring them up.

Thanks for the reply.
It’s not a Combi, i.e. I need hot water in the cylinder tank for showers etc.
So the programmer and all the other bits are in the airing cubboard. I guess this is were the heating and hot water are controlled from. Some more investigation needed I think.

IggyT said:

It’s not a Combi

Click to expand...

OK, thanks for the clarification, there are two variants of the ecoTEC plus (VU and VUW) and it's difficult to tell them apart just from the circuit diagram. However under closer scrutiny I can't see that yours has an 'impeller sensor' anywhere which agrees with what you say.

So to install a Nest thermostat here are the steps I would follow:

1. Identify what each of the wires going to your existing programmer is for, by noting what terminal each wire is currently connected to, and what it actually does. If you don't have the instructions for it, or can't find them online, there should be a diagram showing this information fixed on the programmer somewhere. For example: 'Live' 'Neutral' 'Heating On' 'Hot Water On' 'Hot Water Off'. etc., identify the function of each wire and label them.

2. Once you have worked out what each wire does, then if you look at the Nest receiver instructions, you will find that it has terminals that have the same functions. Connect the wires to the terminals that carry out the same functions as those that they came from.

3. Any existing room thermostat will need to be decommissioned to prevent it will overriding the Nest. It can't just be disconnected, otherwise the heating wiring will be 'open circuit' and not operate. This can be done by tracing the wires back from the thermostat and finding where they originate from. Then disconnect them at their origin, and the two terminals where the switching wires came from should be joined together. Any neutral, if present can just be disconnected.

4. Mount the Nest thermostat and either wire it to T1 & T2 of the receiver to provide it with power, or use a separate plug in power supply.

Thanks for the advise. I will take the programmer cover off and see what it is controlling.