Replacing Drayton LP111 with Nest Learning Heatlink

[JordanW]

Good afternoon,

Im looking for some advice re the wiring for a Nest Learning Thermostat which will replace a Drayton LP111 programmer currently connected to a Glowworm compact 24C.

The current programmer is wired as in the attached image and any guidance would be greatly appreciated!

 

[RandomGrinch]

Hi,
I'm only a DIY'er, I'm sure a pro will be along soon!
That wiring is a little busy isn't it?
There is a third neutral wire connected in there, that I wouldn't expect.
Do you know what it is attached to?
Even if the third neutral is legit, the terminals on the nest are notoriously small - it is often a struggle to get two wires in, let alone three!

 

[stem]

Here's a link to someone else's thread that asked the same question:

LP111 to Nest

Have a look and if you have any questions, post back. In the thread, it's an older version of Nest, yours will have some additional terminals 4, 5 & 6 for hot water control. But as the LP111 is a single channel programmer [just for central heating] it doesn't have hot water control, so you won't need to use them.

Don't worry too much about the busy wiring. Where the connections are made varies from installation to installation depending on the preferences of the original installer, what you have is not unusual. You might have to use additional terminals to get the number of wires down to get them into the Nest as @RandomGrinch has pointed out the Nest terminals are small and close together, and can be tricky for those of us used to wiring them.

 

[RandomGrinch]

Where the connections are made varies from installation to installation depending on the preferences of the original installer, what you have is not unusual.

Yes, I'm sorry - I didn't want anyone to worry about it.
It may be spending too much time on the electrical forum, has made me suspicious!
I just wanted to check that the third neutral was still connected to part of the heating system, rather than something silly, like being stolen for a nearby smart switch!

 

[JordanW]

Thank you both for your replies. It's very much appreciated.

Just to confirm my wiring, because I don't want to get anything wrong!

Thermostat has a LIVE (BROWN), NEUTRAL (GREY, SLEEVED BLUE) and CALL FOR HEAT (BLACK SLEEVED BROWN). The neural goes to the neutral of the programmer. The live goes to call for heat of programmer and the call for heat is connected to the boiler’s call for heat via a cable connector.

The boiler has a LIVE (BROWN), NEUTRAL (BLUE), EARTH (GREEN/YELLOW) and a SWITCHED LIVE/call for heat (BLACK). Live goes to live of programmer, neutral to neutral, earth to earth and the switched live goes to the cable connector to link to thermostat’s call for heat.

Then I have the power input from a fused switch. LIVE (BROWN), NEUTRAL (BLUE) and EARTH. Live to common of programmer, neutral to neutral and earth to earth.

Does this sound correct?

I've included a few more photos (Programmer, Stat and Boiler), just in case it helps!

 

[RandomGrinch]

I'm confident you've got it - though it's worth waiting for the expert @stem to confirm!

Were you happy with using the existing thermostat wiring to connect the Nest thermostat to the heatlink, or were you planning on moving the thermostat elseware?

 

[JordanW]

So when migrating to the heat link, should the power input from the fused switch still go into (2) i.e. common... or into (L)? (Although they are linked with the little red wire)

In order to decommission the old room stat, I'd be looking to run a cable from (3) to the call for heat (RT) at the boiler? That leaves two wires to the old room stat, which I can plug into (T1) and (T2) for 12v power? That'd leave two neutrals into (N) from the fused switch and boiler.

 

[ianmcd]

So when migrating to the heat link, should the power input from the fused switch still go into (2) i.e. common... or into (L)? (Although they are linked with the little red wire)

In order to decommission the old room stat, I'd be looking to run a cable from (3) to the call for heat (RT) at the boiler? That leaves two wires to the old room stat, which I can plug into (T1) and (T2) for 12v power? That'd leave two neutrals into (N) from the fused switch and boiler.

If you are using T1 &T2 you will also need an earth

 

[JordanW]

If you are using T1 &T2 you will also need an earth

No problem... specifically into the earth of the heat link... or just how it is currently will suffice?

 

[Deleted member 267285]

.

 

[RandomGrinch]

No problem... specifically into the earth of the heat link... or just how it is currently will suffice?

The cable you have running from the controller to the existing thermostat has 4 cores.

Choose any 2 cores (apart from earth!) to connect to T1 and T2.
Connect the other end of those 2 cores, into the appropriate connections on the Nest thermostat base.

The spare core can be wrapped up in insulation tape and folded out of the way.

For information, the earth connection on the heatlink is not technically needed - it is directly connected to T1 on the PCB. It is only used when connecting existing thermostat wiring, or with a shielded cable.

Edit: In your case, as you are using existing wiring, the earth should be connected.

If there is a metal backbox to your controller, it would be better to have the other earth's terminate in the earth lug on that. And they will need connecting!

Does that make sense?

 

[stem]

So when migrating to the heat link, should the power input from the fused switch still go into (2) i.e. common... or into (L)? (Although they are linked with the little red wire)

It doesn't matter as you point out, they are electrically connected so L and 2 are effectively the same terminal.

In order to decommission the old room stat, I'd be looking to run a cable from (3) to the call for heat (RT) at the boiler? That leaves two wires to the old room stat, which I can plug into (T1) and (T2) for 12v power? That'd leave two neutrals into (N) from the fused switch and boiler.

Once the old cable is completely disconnected from the existing wiring you can use it to connect T1 and T2 of the Heat link to T1 and T2 of the thermostat. Also, as per @ianmcd I always connect the earth when using T1 and T2 for the thermostat as per the Nest instructions below, it offers protection should the 12v wiring accidentially come into contact with 230V. Any metal back box should also be connected to earth.

But please tidy these wire up as a matter of urgency. There's a big risk of a short circuit here. It just takes one strand of the wire in one terminal to touch the other. There should only be the tiniest sign of the copper conductors showing outside of the metal terminal

 

[RandomGrinch]

Also, as per @ianmcd I always connect the earth when using T1 and T2 for the thermostat as per the Nest instructions below, it offers protection should the 12v wiring accidentially come into contact with 230V

Ok, I can see that - the expert I asked about this didn't see the nuance

Should the ELV of the heatlink come into contact with 230V, having T1 linked to earth would trip an RCD; rather than having the ELV wires floating at mains voltage.
As the option is there, the heatlink should be connected to earth when using T1 and T2.
Although the risk is probably low; the gen 2 heatlink doesn't have this option.

...and it may be beneficial not to connect it to earth, if T1 and T2 aren't being used!

 

[RandomGrinch]

...so for the sake of my understanding, I believe this is how it should be connected.
And I'm happy to be corrected

Two blue neutrals (from fused switch and to boiler) -> Nest N

Live from fused switch -> Nest L
Link from Nest L -> Nest 2 (common)
Live to boiler -> Nest 2
(The same layout as existing, is probably easier to wire with only 2 wires per terminal)

Black (boiler RT) -> Nest 3 (call-for-heat)

4 core cable to Nest thermostat appropriately connected to T1 and T2

Earth's appropriately connected to back boxes and to Nest earth, ensuring continuation of earth from fused switch to boiler.

 

[ianmcd]

No problem... specifically into the earth of the heat link... or just how it is currently will suffice?

into the earth on the heaatlink next to T1 & T2