nest wiring

[andyG2]

Hi all,

Have searched many threads but no one seems to have the same setup as mine.

I'm a fairly competent DIYer and like learning how to do things so thought I'd have a go at installing a nest myself. Doesn't look too hard but before I fry everything just thought I'd do a sanity check and ask you very knowledgable chaps (chapesses too).

So my setup:

Glow worm 30si heat only boiler
Heatrae Sadie cl170 tank
Potterton 3002 programmer
Honeywell t40 thermostat
Wiring box
I believe I have an S plan (2 Honeywell valves next to the tank).

I'd like to change location of the thermostat but that might be an additional headache so might leave that for now.

So, the heatlink will be replacing the programmer. Please see pic for wiring, the N and L I'm guessing will transfer straight to the heatlink. I'll add a link to 2 then on to 5. Then I think move 3 to 6 and 4 to 3. That would work right?

The problem is with the thermostat. The wiring box isn't labelled, it's a bit of a mess for an OCD demon like me. So in there are 2 white 4 core (blue yellow red and earth), one of which is to the therm, the other I guess back to the boiler. (Right of the picture) How do I know which is which?

The wiring box is quite far from where I'd out the heat link (in place of the programmer) so powering the thermostat I'd have to add a cable up to the heat Link. Based on that it's probably easier to move the thermostat and put it in a better location. This is where I'm getting a bit stuck at deciphering a wiring diagram. I'd need to neutralise the thermostat within the wiring box. Please help me with that!

Thanks in advance

 

[stem]

the N and L I'm guessing will transfer straight to the heatlink. I'll add a link to 2 then on to 5.

Yes. Link the L to 2 and 5

Then I think move 3 to 6 and 4 to 3. That would work right?

Yes

How do I know which is which?

It's a Honeywell 6360 thermostat so there should be a wiring diagram inside the lid. Anyway in your case it is:

1 Red (Live)
2 Blue (Neutral)
3 Yellow (Switched Live)

Because the Nest carries out the functions of the room thermostat, the old wired thermostat needs to be 'decommissioned' to prevent it overriding the Nest. This can be done in three ways, in order of preference, they are.

1. Find the opposite end of the thermostat cable to the thermostat, and disconnect it. Then link the terminals where the two switching wires (ie Red and Yellow) came from together.

2. Remove the existing thermostat and replace it with a junction box, connect the Red and Yellow wires together and isolate / insulate the neutral.

3. Leave the existing room thermostat in place and set it to the highest setting. (Or better still put the red and yellow wires in the same terminal)

If you want to power your thermostat directly from the Heatlink and reuse the existing thermostat cable for T1 and T2 then option 1 is the only solution. Otherwise you can run a separate wire from the Heatlink to the thermostat, or power the thermostat directly from a 'plug-in' power supply.

Edit:
If you are going to go for option 1 above, and cannot physically follow the thermostat cable to identify it at the junction box, then it may be identified, because the yellow wire from the thermostat should be connected to the brown wire of the motorised valve controlling the central heating.

Once you have identified the suspect cable, I would recommend that if you have a multimeter and are able to use it safely, that you should check it first to be certain before proceeding.

 

[andyG2]

That's brilliant and exactly what I was after thanks stem. Will give it a go this week.