Installing nest thermostat

[Nickclarke2183]

So after reviewing my not so handy work it's working!

I miss placed one wire into nest 1.... Muppet.

Stem your instructions work perfectly and I have heating and hot water. Thanks so much for your advice.

 

[stem]

Well done! glad to hear it's working now, thanks for the update.

 

[Nickclarke2183]

Hi, so a bit of a follow up request for help.

This has all been working perfectly but I have decided I want to stick the thermostat on the wall rather than use the table stand.

When doing this I have found that the cables that went to the old thermostat still have a live supply going to it. So when I fire up the central heating, the wires that went to the old thermostat go live. Why is this?

I wanted to reuse these cables and plug them into t1 and t2 but struggling to find where they originate from at the boiler. Anyone have any ideas?

 

[CBW]

When doing this I have found that the cables that went to the old thermostat still have a live supply going to it. So when I fire up the central heating, the wires that went to the old thermostat go live. Why is this?

I think, This is because it was left in situ, and still part of the circuit. Read @stem’s reply (4t reply on this thread). If I understand it correctly (I’m trying to learn myself), the heatlink replaced the programmer on the red tiled wall. To fit the thermostat in place of existing thermostat it’s a case of connecting the relevant wires from existing thermostat into nest? Photo of thermostat wiring on wall and one of nest may help.

 

[Nickclarke2183]

So find attached all pics.

Nest is wired as per Stems earlier thread and all working a treat. I have also included a photo of the junction box and wires directly behind the nest.

When I heating only is switched on then the wires from the old thermostat go live. Pic with grey/blue wall.

 

[CBW]

Do you have t1 and t2 on your base for the nest?

 

[Nickclarke2183]

Not sure what you mean. Photos were attached. @stem can you help again pls

 

[CBW]

Apologies I’ll but out and leave to a pro.

 

[stem]

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. It can't simply be disconnected as this would leave the heating wiring 'open circuit' and it will not operate. The thermostat can be decommissioned in several ways. At the moment, your existing thermostat has been removed, the two live switching wires (red and yellow) connected together, and the neutral isolated.

If you want to disconnect the cable to use it to connect T1 & T2, then trace the cable from the thermostat back to its origin. Note where the red and yellow wires are connected, and then disconnect the cable completely, including the neutral and earth. Finally insert a wire to join the two terminals where the red and yellow wires have just been removed from together so that they are electrically connected. The original thermostat cable will now be electrically disconnected at both ends.

This would be good opportunity to make the existing wiring safe. That junction box is messy and rather dangerous. I can't see that it has an earth connection to the metal box itself (the thermostat back box shows how it should be done) and the wires seem to lack protection at the bottom. The wires should be enclosed with a sheath where they enter the box (to protect the wires chafing on the edges) and any holes that they pass through should have the additional protection of rubber grommets. Those at the top are correct.

 

[Nickclarke2183]

Thanks.

Big problem I have is that I cant trace those cables back to the boiler end. Could they possibly enter the boiler first and then into the junction box for the nest?

4 cables enter that junction box at the boiler.

I'm assuming two are the ring main, one for boiler power (switched) other for the rcd?

I was hoping one of them would be the thermostat but from what I can see that's not the case.

 

[stem]

There are so many ways to wire up these systems. Much will depend upon the positioning of the components of the heating system in relation to each other in your home, the physical layout and construction of your home, the personal preferences of the original installer, and if the system has been modified or altered at some point. So, it's difficult to advise without being physically present. To trace the cable, there ares some clues that can help:

The room thermostat will open and close a motorised valve, (I'm assuming here, as indicated by the wiring, that your system is fitted with a 3-port motorised valve) so the room thermostat won't be connected directly to the boiler.

You know that it's a 3 core and earth cable, so that will narrow it down a bit.

The blue wire (if it is connected) will go to a neutral at its origin. (So, it won't for example be the blue wire in terminal 4 of the the Heat link)

Of the two live wires (Red and Yellow) one will be connected to terminal 3 of the Heat link (usually the red one) and the other, will go to the motorised valve. (white wire if it's a 3-port valve) They probably won't go directly, but may pass through a junction box and may even change colour when doing so.

Once you have identified the cable it should be tested with a multimeter to make sure you have located the correct one.

 

[Nickclarke2183]

That's helps alot, thanks.
3port motorised valve? Not sure what that is sorry.

 

[stem]

They look something like this and are used to divert water from the boiler, to the hot water cylinder and / or radiators as required. They are normally (but not always) located next to the hot water cylinder.

The indicator that you have one is that your system has a wire for 'hot water satisfied' (in terminal 4 of the Heat link) which other types of system don't need.

 

[Nickclarke2183]

Absolutely have one of these.
So is it likely the wire from the thermostat goes here before going to the boiler?

 

[stem]

The room thermostat is wired to the motorised valve. It is not connected to the boiler.

[The motorised valve has a small switch inside that controls the boiler.]