Replace Drayton LP522 & Honeywell DT90 with Nest

[aksp]

Hello there. I am looking for some help with my switch from my current setup of Drayton LP522 and Honeywell DT90 to Nest 3rd gen thermostat. I have followed prevoius posts and tried to replace lp522 with Nest HeatLink. Here is a look at my lp522

And here is a look at the hairy junction box.

The leftmost cable is to lp522 and I have tried to replace it with Nest heatlink and nothing has blown up. Here is a look at my honeywell-dt90

what I dont know are:
1. how to handle the live and switch wires coming into dt90? I cant fingure where these 2 endup in the hairy junctionbox
2. how to wire t1 and t2 from Nest HeatLink? I assumed (at the start) that I will be able to route it via the junctionbox. But, I cant figure out the cables
3. what connection change should be done in the junctionbox itself?

can someone help?

thanks a lot,
Sri

 

[stem]

Blue Wire in LP522 N [Neutral].....>Nest N
Brown Wire in LP522 L [Live].....>Nest L
White Wire in LP522 1 [Hot Water Off].....>Nest 4 [Hot Water Satisfied]
Black Wire in LP522 3 [Hot Water On].....>Nest 6 [Hot Water Call for Heat]
Red Wire in LP522 4 [Heating On].....>Nest 3 [Heating Call for Heat]

Nest also needs to have a link made between terminals L, 2 and 5

The Nest thermostat is connected to the Heat link terminals T1 & T2 (or it can be powered using a separate plug in power supply) If you are using T1 & T2 for the thermostat, the earth terminal just to their left, also needs connecting to earth.

Finally the old thermostat needs to be decommissioned. The simplest way is to leave it in place and put both wires in the same terminal so that they are electrically connected.

Or if you want to remove the old thermostat and cable, you can trace the red and yellow wires back to their point of origin, disconnect them from there and then join together the terminals where the two wires have just been removed from, so that they are electrically connected.

 

[Mister Banks]

View attachment 291709

Blue Wire in LP522 N [Neutral].....>Nest N
Brown Wire in LP522 L [Live].....>Nest L
White Wire in LP522 1 [Hot Water Off].....>Nest 4 [Hot Water Satisfied]
Black Wire in LP522 3 [Hot Water On].....>Nest 6 [Hot Water Call for Heat]
Red Wire in LP522 4 [Heating On].....>Nest 3 [Heating Call for Heat]

Nest also needs to have a link made between terminals L, 2 and 5

The Nest thermostat is connected to the Heat link terminals T1 & T2 (or it can be powered using a separate plug in power supply) If you are using T1 & T2 for the thermostat, the earth terminal just to their left, also needs connecting to earth.

Finally the old thermostat needs to be decommissioned. The simplest way is to leave it in place and put both wires in the same terminal so that they are electrically connected.

View attachment 291705

Or if you want to remove the old thermostat and cable, you can trace the red and yellow wires back to their point of origin, disconnect them from there and then join together the terminals where the two wires have just been removed from, so that they are electrically connected.

Wouldn’t the red and yellow from the thermostat go to the wiring centre rather than direct to the programmer? There is no yellow at the programmer. But there isn’t a yellow and red at the wiring centre either.

 

[stem]

With the majority of installations, the thermostat is connected to the wiring centre, but not always. The programmer is usually the second most common place, generally because it is downstairs and closer to the thermostat than the wiring centre which can be upstairs in an airing cupboard.

Much depends upon the layout of the various components in your home, if the system has been modified or moved in the past, the personal preferences of the installer, and the materials they had to hand. Lots of variables really.

Also, whilst there maybe a nomenclature in the wiring centre lid stating the suggested function of each terminal, often it is ignored.

Another possibility is that the room thermostat and /or programmer have been moved at some point and the wires extended from another junction box using wires with a different colour.

I assume too, that the existing thermostat is actually controlling the heating at the moment. I've seen some installations where there's a thermostat with a cable in, but it wasn't connected to anythng at the other end.

Unfortunately, whilst the wires have to terminate at their correct points for the heating to work, there is no fixed way of getting them there. I have seen 100's of installations over the years and I'm pretty sure no two of them have been installed exactly the same.

Sometimes it comes down to having to physically follow a cable to see where it goes. I remember seeing one where someone had routed the thermostat N & L to a light fitting on the wall in the room behind, and then on to the thermostat.

 

[aksp]

Thanks for the kind responses. I know that my current thermostat is working. But, just not sure how its connected to the wiring center. Whoever did the wiring, must have had a clear reason for not following the nomenclature on the lid.

I will try this and let you know., thanks again.

 

[aksp]

Hello @stem thanks.. I have got the wiring done and when I switch on the mains, the HeatLink is up and running.

But, when I connect T1 and T2 to the thermostat, the HeatLink shuts down (all lights down). The moment I disconnect the T1, T2 cables from the thermostat, HeatLink resumes to work.

What could be wrong ?

 

[aksp]

I forgot to add. I have setup the thermostat with its own power supply and it is connected to the heat-link and it is ok. But, I want to connect it via the t1-t2 cables, so, I dont have to run extra cables. thanks

 

[stem]

You use one method of getting the 12V power supply to the Nest thermostat. Either T1&T2 of the Heat link connect to T1&T2 the thermostat, or if you don't want to run wires between the heat link and thermostat, use the separate plug in Nest power supply which uses the USB port. You don't need to connect both.

 

[aksp]

Hello @stem. Thanks. I actually tried to wire it up using T1&T2, that kept shutting down the HeatLink. And so, I decided to use the separate power cable. I actually want to use the T1&T2 based connection only. But, not sure why it is shutting down the HeatLink.

 

[stem]

Is the cable you are using only connected between the T1 & T 2 terminals of the Heat link and the thermostat T1 & T2 terminals.

If you are using an existing thermostat cable, has it been completely disconnected from the original wiring? is it in good condition and not damaged anywhere?

Are the wires connected cleanly? i.e. no wire whiskers poking out that are shorting the terminals or wires together. Are the conductors (not the insulation) securely under the connecting screws?

You also need the earth connection made if using T1 & T2

Have you inadvertently connected the thermostat to mains voltage at any time?

What happens if you try another cable and connect the thermostat to the Heat link?. To do this test, the Thermostat can be right at the side of the Heat link and connected using a short piece of flex.

If the thermostat is installed as per the diagram above, the wiring correct and the Heat link is still shutting down I suspect that the thermostat may be faulty.

 

[aksp]

Have you inadvertently connected the thermostat to mains voltage at any time?

I think so.. Could that be the problem?

The current cable has worked well with the current setup so long, I assumed that it should work. I will try a different cable this weekend and see if the thermostat behaves correctly.

 

[stem]

I think so.. Could that be the problem?

Very likely. Putting 230V mains voltage on a device that is designed for 12v is never a good idea. Unless it has specific built in protection against such an event, you have probably damaged it.

 

[aksp]

ok. thank you