Nest Thermostat wont stop calling for heat (Y plan)

26 Oct 2022
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United Kingdom
Hi all,

I have just completed a rewire in my house and as part of that I moved the wiring for the boiler and nest heat link into/next to the wiring centre in the airing cupboard upstairs.

Originally a I did a swap from drayton lp522 to nest, as part of that I redid the wiring centre with the heat link downstairs and everything was working fine and as expected. So everything has been working before!

After the rewire, everything now runs from the wiring centre upstairs, the heat link is wired in directly and I ran new 5 core cable to the boiler which is also now wired into the wiring centre. I believe these were the only changes made, but I must be missing something

The issue now is that when the thermostat calls for heat it turns on but never turns off. Hot water is fine. If i restart the system (turn off at spur, powering down everything), and no call for heat is made (e.g. thermostat is set to away), the boiler remains off. If the heating is still firing, even when thermostat is not calling for it and I turn the power off on the boiler (front switch) and back on again, boiler fires up again.

Some wiring info:
Wiring centre is powered from a spur, which powers everything except the tank heating element.
Boiler 5 core is wired into the back of the wiring centre, Red and white being pump and switched live (8)
Forgive the tacked on earth, that was an addition today, as I realised that when powering thermo from the heatlink earth is required. Also I appreciate that heatlink 4 needs to be sheathed and will be when Iv figured out whats going wrong!

When the thermostat is set to a low heat after CH has been running,
Voltage reads 240v between neutral and terminal 8 of the wiring centre
I still get 240v on terminal 3 of the heat link, I hear clicks form the relays in the heat link both on call for heat and when I turn down the thermostat
(if i then pull out the cable out of heat link terminal three heating stays on).

Im sure this is a wiring issue, maybe its something simple that someone can see that I cannot!

2022-12-14 14.49.19.jpg
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Not clear in the photo where in the Heatlink the first brown Live link is connected. Is it from L to terminal 2 as it should be?
Can you post a photo taken from the left side of the wiring centre. It's hard to see exactly what's connected to the LHS of the terminals.
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Apologies, I'm still struggling to follow the red wire from Heatlink terminal 3. It should be connected to the 3 port valve white wire but I can't see where it goes.
Hi Jackthom,

Red from Heatlink goes to (4) in the wiring centre, which is bridged to (5) replacing the old thermostat, where the white wire of the 3 port valve is.

I would say either hot water off isn’t done properly or another wiring fault. Are you saying you still get 240vac if you remove wire from terminal 3 on heat link? Also the green/yellow shouldn’t be used let alone sleeved for showing it’s live.

Yes thats right, pull the cable out of HL 3 and it both has 240v and the boiler is still going.

It looks a bit like everything is in the right place so i think I will just go through and do a rewire make sure everything is making contact properly

I will swap out the cable as suggested (ill use it as earth for the Heatlink)

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@CBW I rewired the whole thing today, every wire out (swapped the yellow and earth cables int he heatlink) and rewired checking for solid connections but have the same issue.

I noticed today that on the HW heatlink terminals 4 + 6, I was getting 240v on both when the HW was supposed to be off.

I think I will quickly wire the old LP522 back in tomorrow and see if I get full control of the CH and see if I can eliminate anything that way
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The 3-Port valve has an orange wire which when the valve is in the CH position is made live by an internal switch. This wire should go to the boiler and is what keeps it running. I suspect this microswitch is somehow also keeping the valve in the CH position somehow.

If my theory is correct, when you remove the power, the valve would be pulled by its internal spring back to the HW position and the orange wire is no longer live. When power is reapplied, nothing happens until Nest triggers the heating, the valve opens moves to the CH position operates the switch and once again it stays there.

So to start with, I would check that:

1. The orange wire from the motorised valve only goes to the boilers switched live connection along with the the 'call for heat' wire from the hot water cylinder thermostat (this is responsible for running the boiler when the HW is operating)

2. The white wire from the motorised valve goes to Nest CH 'Heating call for Heat' terminal 3 only. There should be no other wires connected to it from anywhere.
Hi @stem

Thanks for the response I think I have found part of the issue. Originally I had the orange wire wired into (8) of wiring centre along with the switched live of the boiler, the pump (also housed in the airing cupboard), the boiler pump live and the cylinder thermostat 1.

I removed the boiler pump live and now the heatlink is able to stop the central heating and so far all is normal again. I do now have a dangling boiler pump wire and am not sure where it goes.

The boiler pump was wired into a 5 core from the boiler but I was never able to trace it to wither wiring centre or where the old LP522 was situated. I clearly missed something.

The attached photo is of the current boiler wiring, the electrician who swapped in the new five core (that now runs to the wiring centre) says he like for like swapped in the new cable.

Should the boiler pump live to go the pump live for the pump in the airing cupboard?

Many Thanks


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With some boilers, the pump is controlled by the boiler to give what is known as a pump override, In this instance, the boiler keeps the pump running for a few minutes after the boiler goes off to dissipate residual heat. In this case there will be special wiring terminals inside the boiler for the pump to connect to. The example below is from a Baxi boiler.


Some boilers don't need a pump overrun so in this case the boiler and pump are wired together such that when the boiler is on, the pump is on. And when the boiler is off, the pump is off. I forgot to mention that in my first post so if this case for you, then the boiler and the pump should be connected to the orange wire, along with the hot water cylinder thermostats 'call for heat' wire.

Should the boiler pump live to go the pump live for the pump in the airing cupboard?
Sorry I'm not sure I understand that, but I hope that my comments above will help.
The brown pump wire that is in with the orange valve wire at the moment should be put in a separate terminal with the red wire that is going to the boiler.
Last edited:
The white wire that's going to the boiler is correctly connected to the orange valve wire.

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