Upgrading LP522 to Nest Wiring Diagram

[Alan Shaw]

As a precursor to upgrading my LP522 Controller to Nest I set about documenting the wiring in the junction box. I have now disconnected the LP522 and on checking my diagram realise I have made a mistake. You can see from the attached the call for hot water goes to 5 on the junction box but there unfortunately it ends, there are no other connections to complete the task. I have not disturbed any of the wiring apart from the connections between the LP522 and the junction box. Just wish I had taken a photograph but too late now. Can anyone assist please?

 

[misterdubya]

Stem will be along soon. Work it out yourself eh.

 

[Alan Shaw]

I have now updated diagram to give more information. Assuming I have summarised the connections correctly at foot of diagram it doesn't appear to make sense as terminal 7 appears to be handling both H/W & C/H. Cannot understand how this can of worms could have occured when only the LP522 controller was disconnected.

 

[stem]

Stem will be along soon.

There is no need to disturb the wiring centre. They tend to be wired differently from installation to installation, depending upon the personal preferences of the installer and the layout of the various components in your home, so it's difficult to give advice about that over a forum and not being physically present too inspect it. So, I would suggest that you put everything back as it was and once you have confirmed that it's working, then proceed as below:

The LP522 wiring terminals are thus:

The existing wires are simply moved from the terminals of the LP522 to the terminals of the Nest Heatlink that have the same function. Don't worry if there are some terminals that do not have wires connected to them, if your system doesn't need them now, you won't need them with the Nest.

So:
LP522 N = Heatlink N
LP522 L = Heatlink L
LP522 1 HW off = Heatlink 4 Hot Water Satisfied
LP522 2 CH off = Heatlink 1 Heating Satisfied
LP522 3 HW on = Heatlink 6 Hot Water Call for Heat
LP522 4 CH on = Heatlink 3 Heating Call for Heat

The Heatlink common terminals (2 & 5) need to be linked to Heatlink L

The Nest Thermostat can be powered by wiring it to Heatlink T1 & T2 terminals, or alternatively by using a separate plug in 12V power supply for it.

If you don't have an existing wired room thermostat, that's it job done. If you do have an existing wired room thermostat it can't simply be disconnected as this would leave the heating wiring 'open circuit' and it would not operate. So you can:

1. Leave the existing room thermostat in place and set it to its maximum setting and leave it there, this will prevent it overriding the Nest. Or,

2. Find the origin of the thermostat cable. Note where the wires are connected and disconnect the cable. Then link the terminals where the two ‘live’ switching wires came from together. (This will probably be in the wiring centre) The Neutral, if present is simply disconnected. Or,

3. Remove the existing thermostat and replace it with a junction box, connect the two ‘live’ switching wires together and isolate the neutral (if there is one). Or,

4. Leave the existing room thermostat in place and join the ‘live’ switching wires together in a separate connector that you can tuck away in the corner of it.

 

[muggles]

Why have your wiring colours changed between the LP522 connections and Nest connections?

 

[Alan Shaw]

Hi Muggles,
The existing LP522 cables have been disconnected as I wanted to re-site the Heat-Link and install using fresh 5 core for switching and ground for 12volt Nest terminal.

 

[Alan Shaw]

Hi Stem,
Thank you for your comprehensive reply. I had already followed your advice but my concern is the H/W switched live from either the LP522 or Heat -Link isn't going anywhere in the junction box. I am reluctant to power up and risk damaging the Heat - Link. As I said earlier I must have made a mistake documenting the connection to junction box. In my defence it was stuck in the corner of airing cupboard with very poor access.

 

[stem]

The hot water 'switched live' from the programmer (or Heatlink) should be connected to the hot water cylinder thermostat 'common' terminal. Where the other cylinder thermostat terminals will connect depends upon whether you have a 3 port motorised valve, or two 2 port valves.

Edit:
From your annotated table I see that you have a 3-port valve, so the connections should be:

Cylinder Thermostat 'Common' = Heatlink Hot Water Call for Heat (6)
Cylinder Thermostat 'Satisfied' = Grey wire at motorised valve
Cylinder Thermostat 'Call' = Boiler Switched Live (also connected to the Motorised valves orange wire)

 

[Alan Shaw]

Hi Stem,
Apologies for the delay in getting back to you. I am pleased to report all is now working thanks to your advice. The tank thermostat proved problematic and had to be changed. Considering my original enquiry related to an orphan wire on the junction box I'm amazed how much had to be changed. It beggars belief that the system worked prior to the Nest upgrade. I have attached the final outcome and again wish to thank you for your help.