How to install the Nest Learning Thermostat (3rd Gen) in a Y-Plan System

Back in February I looked at my house and decided that it was time to drag it kicking and screaming into the 21st century. This meant embarking on a rather ambitious plan that involved 3 months of DIY and learning some new skills. The first job on my list was to invest in some smart heating technology that would not only be a bit of a man gadget (I’m not gonna lie, it totally is) but would also save some money on my heating bills due to improved energy efficiency.

The choice here only really involved the 2 leading players in the industry – Hive or Nest. Having read some reviews and spoken to a few friends who had each system I settled for the new Nest learning thermostat.

What’s in the Nest thermostat box?

3rd-gen Nest Learning Thermostat

3rd-gen Nest Learning Thermostat

3rd-gen Nest Learning Thermostat

3rd-gen Nest Learning Thermostat

That’s it! The key components here are:

  • Nest Display (learning thermostat)
  • Nest Heat Link
  • Nest Base
  • Nest Trim Plate and screws
  • USB cable
  • USB plug
  • Installation guide

Identify your existing home heating setup

Before you go any further with the installation your first challenge is to identify what the existing heating setup is in your property as there are numerous ways to install the Nest system. The Nest installation guide that’s in the box clearly outlines the different options that are compatible:

  • S-Plan system
  • Y-Plan system
  • 230 V combi boiler
  • Low voltage/dry contact combi boiler
  • OpenTherm boiler
  • District heating with electrical control valve

For the purpose of this post I am focusing on my home’s heating setup which is a Y-Plan system.

Installing your Nest thermostat into a Y-Plan system

A Y-Plan system is one that involves a 3-port valve – in a nutshell this means you can have only the heating on, only the hot water on or both the heating and hot water on at the same time. To identify if you have a Y-Plan system, look for your 3-port valve which should be located in your airing cupboard attached to the pipes coming from your water cylinder – this is what I have in my property where you can clearly see the 3 different value positions for Hot Water (W), Mid position (M) and Central Heating (H):

3 Port Valve

Installing the Nest Heat Link

Upgrading your heating system to a Nest system requires you to replace your old heating controller/timer with the Nest Heat Link. The Heat Link then becomes the unit which is wired directly into your boiler and heating system and due to its wi-fi capability, allows you to then control your heating and hot water from your smartphone.

So how did my setup look? Well, let’s get started.

Step 1: Turn off the power

Firstly, I killed the power! Now although there’s a fuse running off of the programmer which could be pulled out, when it comes to the electrics I don’t take any chances and tend to always kill the mains power – that way your have ensured your entire system is free from any unexpected wiring loops and minimise risk of electrocution.

Kill the mains power

Step 2: Disconnect your existing programming unit

With the power off I located the programming unit in my kitchen – in this instance a really old one from Danfoss and disconnected the face plate as shown (charge remains in the capacitors thus the screen is still live).

Danfoss Programmer

With the face plate removed this will allow you access to the wiring that sits behind it – this is the wiring that will run to the boiler and the other parts of your heating system.

Danfoss programmer wiring

Step 3: Identify the function of your existing terminals

As per the above photo, chances are you’ll be faced with a spaghetti junction scenario like I was! I can’t stress enough at this point that BEFORE you disconnect any of these wires, make a note of which wires are going to each terminal (I wrapped a bit of masking tape around them and wrote the number on) as you’ll need to reconnect these shortly after.

I’m not sure if this will be the case on all programmers however luckily for me on the back of the unit there was a terminal key so I knew exactly what each of the existing wires and terminals were doing – if this isn’t available on your programmer you may need to Google it to identify the purpose of each terminal.

Danfoss terminals

Danfoss terminals

This terminal key made things really simple:

  1. Hot Water (HW) = Off
  2. Central Heating (CH) = Off
  3. Hot Water (HW) = On
  4. Central Heating (CH) = On

Step 4: Map your existing terminal wiring to your Nest Heat Link terminals

The next step is to get hold of your Nest Heat Link, unscrew the bottom and remove the cover to reveal the Heat Link terminals:

Nest Heat Link

Nest Heat Link

Nest Heat Link Terminals

Although there are a lot of terminals, things here are actually very straightforward. If you take a look at the Nest Installation guide this clearly outlines what each of the numbered Nest Heat Link terminals should be used for:

Nest Heat Link wiring terminals

Nest Thermostat Heat Link Wiring

As we are focusing here on the Y-Plan setup, terminals such as the OT1 and OT2 don’t apply as they are used for OpenTherm boilers. The Y-Plan setup is outlined as follows:

Nest Thermostat Y-Plan Setup

So at this point, this is where you need to know what the wires on your existing setup’s terminals do so that you can map them to the terminals on your new Nest Heat Link. This should be as follows:

Function Existing Terminal Nest Heat Link Terminal
HW OFF 1 4
CH OFF 2 1 (Not used)
HW ON 3 6
CH ON 4 3

As per the diagrams in the Nest installation guide above, there are 2 switches present – Heating (terminals 1, 2 & 3) and Hot Water (terminals 4, 5 & 6). Terminals 2 and 5 are both “Common” and will require you to run a live power feed to them.

As you can see from my wiring below I have done this so there is a live power feed heading to L, 2 and 5 – as I needed to add these 2 additional live wires I’ve simply taken my live wire in the wall, added a block connector to it and split it out into 3 live wires (you can see the block connector below).

Nest Thermostat Heat Link Wiring

With these wires in place, you have completed the installation of your Nest Heat Link! The next stage is to install your Nest Display i.e. Learning Thermostat.

Installing your Nest Learning Thermostat

So the first thing is to know what I’m referring to – the thermostat. As per my original programming unit, my existing thermostat was also made by Danfoss:

Danfoss Thermostat

This is the switch in your heating system that asks for more heat when the house temperature falls below a set level. In my house, the thermostat was located in an awkward wall position behind where my sofa usually sits which has made hanging pictures an absolute pain.

Existing Thermostat

So when it comes to your existing thermostat and your new Nest Learning Thermostat, there are 2 installation options available to you:

  1. Replace your existing wired thermostat in situ with your new Nest thermostat
  2. Remove your existing wired thermostat entirely and use your new Nest thermostat elsewhere either hard wired or via a plug socket

Whatever option you go with, you will need to remove the existing thermostat so we will start from there.

Step 1: Removing your existing thermostat

The first thing that we need to do is get to the wiring on your existing thermostat. Now if your thermostat is anything like mine was, these things are designed to be put on the wall and not to be taken off the wall. I’m not going to lie, getting mine off of the wall was pretty difficult. Eventually I managed to get the cover off however to see what was going on inside:

Old thermostat wiring

Danfoss thermostat wiring

With my thermostat removed this is what I was left with – nothing more than a simple power circuit:

Old thermostat wiring

Step 2a: Installing the Nest thermostat in situ

So one of your options is to simply remove this existing thermostat and replace it like for like with your new Nest thermostat.

This should be a really straightforward installation that requires you to take the Nest Base and do nothing more than supply it with power to the T1 and T2 terminals. These then need to be wired to the Heat Link’s respective T1 and T2 power terminals.

Nest thermostat mounting bracket

Once you know how to do the installation you can screw this Base to the wall and use the optional Trim Plate to cover any holes/rough edges – the Nest Display then simply pushes and clicks into this Base.

Step 2b: Removing the existing thermostat to use the Nest thermostat elsewhere

Due to the aforementioned wall position of my existing thermostat I went down the route of removing the thermostat from my wall completely to tidy things up. By doing this however you are going to be left with live wires in your wall and not forming a circuit so there is additional work to be done. In addition, you are going to have a hole in your wall so you will need to consider that it will likely need to be re-plastered and re-painted too as per my image above.

As the thermostat is nothing more than a switch, the new Nest thermostat would simply become a wireless switch for my house and so my existing hard-wired thermostat needed to be removed and have these switch related wires joined back together to re-form the circuit – this is where things got a little complicated for me!

Step 3: Identify your existing thermostat wires in your junction box

So, with a Y-Plan system these typically use a junction box that combines the switched wiring for the various components of the system:

  • Boiler
  • Thermostat
  • 3 port valve
  • Cylinder thermostat
  • Pump

This junction box is likely to be located in your airing cupboard and be encased in an electrical box:

Y-Plan system junction box

Opening up the box presents you with a lot of wiring which I hadn’t anticipated before starting my installation and so from here things got a bit tricky.

Y-Plan system junction box

I knew that the wires coming out of the existing thermostat went straight up the wall and this would certainly explain the thermostat’s awkward wall position in the lounge as I figured they must head straight to this junction box. The issue I had was knowing which wires related to the thermostat as none were labelled, they all looked the same and there were 3 pieces of cable coming out of the wall here where any 1 of these could be the one powering the thermostat.

Without electrical testing equipment this made the task near impossible so I tried to identify every cable one by one which meant I figured out the entire wiring setup within my house and by process of elimination identified the thermostat wires.

This is how my wiring looked and should be useful for anyone with the same setup as me to see what’s going on:

Y-Plan central heating system wiring

So what I have done here to keep things easy is keep the original programmable unit (control unit) in my diagram and map those terminals to the new Next Heat Link below it so you can see where they should be going.

With regard to the thermostat, if you follow the “Lounge Thermostat” top left I’ve labelled this as cable 1 – I managed to identify that this cable became what I’ve labelled as cable 6 in my airing cupboard so I know all 3 wires here relate to the thermostat switch.

Therefore to remove the thermostat and re-connect the circuit I’ve done 2 things:

  1. Remove all of the wires located in cable 6 from the junction block (terminals 1, 2, 3 & 6). I’ve marked the original wire positions with dotted lines.
  2. Move the live wire that was coming out of cable 7 (red dotted) to power the thermostat (via junction block terminal 3) from terminal 3 and moved it into terminal 6 as indicated by the pink circle.

This meant that all wiring to the thermostat was now redundant and the circuit reconnected and made safe. You can therefore safely remove the old thermostat wire from the wall and with a bit of force pull it through:

Remove old thermostat wiring

And that’s everything done! Well, other than patching up the new hole in your wall and repainting anyway!

Setting up the Nest Stand and Display

So the final stage is to get the wireless Nest Display (which is also your thermostat) up and running. For this there are a few options:

  1. Mount the Nest Display on a wall
  2. Mount the Nest Display on a stand

I wasn’t keen to dig any new holes in the wall to re-position it or have wires running up my wall from sockets so I opted to go ahead with the Nest Stand which I picked up from Amazon.

What’s in the Nest stand box?

Nest thermostat stand

Nest stand

That’s it, a simple plastic stand and 2 screws! £29!

Step 1: Gather the parts needed for the Nest Stand

To get the Nest Stand set up you need to gather together the key components.

Nest stand components

This requires:

  • Nest Stand
  • Nest Base
  • Next Display (Learning thermostat)
  • USB cable
  • USB plug
  • 2x screws

Step 2: Join together the Nest Stand components

Connect the USB cable to the Nest Base.

Connect Nest Base to USB cable

Screw the Nest Base to the Stand.

Screw Nest base to the stand

Push the Nest Display / thermostat onto the base until it clicks.

Clip on the Nest display to the Base

Attach the plug.

Connect the plug

Step 3: Place your Nest thermostat in a location within your home

For the purpose of me testing the setup I located my Nest thermostat in my kitchen by the boiler – this allowed me to easily test the unit and whether or not the wiring was correctly installed with the boiler firing up as required.

Nest thermostat setup

Also note that if you install your Nest Heat Link in place of your old programmer, you are going to encounter an issue such as below where the hole in the wall is a different size – again you should plan for this to avoid an unsightly install. With my installation as I am going to be re-fitting the kitchen later this year, this issue wasn’t something I was concerned about.

Step 4: Switch the power back on

The final step of the installation is to switch your power back on. If everything has been done correctly there should be no blown fuses, with the house icon appearing on the thermostat display and a light illuminating on the Heat Link as per the above photo.

Step 5: Follow the on-screen setup instructions

With the power back on, your Nest thermostat’s display will take you through a series of questions to connect it to your wi-fi, link it to your phone app (you will need to download this) and choose your minimum temperature.

Nest thermostat setup

The Nest system will then learn over time what temperatures you want at different times of day in your house and gradually plan for these and automatically adjust your heating, saving you money.

Step 6: Test your installation

With the app installed on your smartphone you can now test your system.

Step 6a: Hot water test

From the Nest app if you tap the “Hot water” icon this should change the status to “ON”:

Nest thermostat hot water test

You should then find that your 3 port valve moves the “W” position allowing the flow of hot water to fill your system. If the temperature of the water in your cylinder isn’t high enough then your boiler should also fire up.

Nest thermostat hot water test

Step 6b: Central heating test

The second test is to check the heating works. For this, tap the hot water icon to disable the hot water and tap the temperature setting, adjusting it to a number above the current room temperature:

Nest thermostat central heating test

This should make the temperature number highlight in orange to indicate it is on and the on the valve, the switch should move to the “H” position indicating Heating and allowing hot water to flow through your heating system and radiators. Again, if the temperature in your cylinder is too low your boiler should fire up too at this point:

Nest thermostat central heating test

Step 6c: Hot water and Central heating test

The final test is to ensure you can have both hot water and heating at the same time. To do this, on the app enable both the icons as previously shown:

Nest thermostat central heating and hot water test

This should then move the switch on the valve to the “M” (mid) position which should allow for both hot water and heating via your radiators.

Nest thermostat central heating and hot water test

With everything working as intended I can now move my thermostat to any position in the house where there’s a plug socket – it’s now sitting in my lounge on a table.

Nest thermostat

Nest installation summary

Now when it comes to electrics I’m massively inexperienced so I was a little nervous doing the installation myself. That said, looking at some of the wiring diagrams and a few videos on YouTube, the installation and setup should be pretty straight forward. Heck, if I can do it anyone can!

The only thing that I struggled with during my installation was tracing wires that ran inside the walls to know which ones I could safely remove as without electrical testing equipment this is extremely difficult.

I must say however that this is a brave install to do yourself if you don’t know what you are doing – just remember that if you balls up the wiring you are going to end up with no hot water and no central heating! Therefore if you really don’t have any idea what you are doing I fully advise you seek professional help from one of Nest’s approved installers.

Personally this was a massive DIY challenge for me as a few years ago I wouldn’t have even considered undertaking such a task. I’m glad I took it on however as the system is working flawlessly and I’ve got a really great sense of achievement pushing myself to try something advanced like this and get it right!

Buy today on Amazon

       

If you’ve read this post during your research to buy a Nest unit, stop hesitating and start buying! It’s an amazing piece of kit that you won’t regret installing in your house. Pick up the 2 components on Amazon here:

67 Comments

    1. Thanks Roger, after days struggling to find something useful myself I thought I’d document my journey to help someone else!

      Are you looking to do your own installation?

      1. Yes, I got a reduced price nest from that Amazon deals day the other day – it arrives on the 24th July so am trying to do my research (as a complete novice before then). I was ok with replacing the programmer as that is just a case of unplug – after taking photos – and then plug in to the new nest heatlink but I couldn’t get my head around what happens to the room thermostat. This tends to get glossed over. Anyway, I read yours and now understand how it works but I must admit I am very tempted to join the wires on the thermostat together so that the circuit is completed and I don’t have to do anything complicated at the junction box.
        I fear that if I try and make it live at the junction box I will plug it into the wrong bit!

        You really cut your teeth on this one – welll done buddy!
        You also have a really readable writing style (if readable is even a word!).

          1. Hey Nick, The nest went in last weekend – piece of cake really (replacing the programmer with the heatlink). I have been a bit lame with the room stat though and just set the old room stat to 30 so that it is permanently live – When I get a bit of time I’ll maybe revisit it at the junction box end but it will do as a workaround in the meantime 🙂

          2. Hi Roger/Nick, thank you for some great documentation. I am nearing the same point, as in I have the heat link in (replacing old Danfoss 715 programmer) and I have hot water working great. The CH is not working yet but I am confidant that I can achieve this if I put the room thermostat back together & set it to the max (like you have Roger). Ideally though I would like to use the existing thermostats’s (Drayton RTS1 240v) wiring with the Nest thermostat but this is above me at the moment. I am doing a lot of reading right now!, especially as the Nest thermostat is low voltage. It may be that I don’t gain the knowledge and just use the thermostat in its stand & may have to look at the Junction box like you have Nick. Wish me luck 🙂

          3. Hi Tao, thanks for commenting – it’s interesting how many people are attempting to DIY this installation but getting stuck around the same point with the wiring. Did you manage to get your installation done yourself in the end?

      2. This is a great article. The only part I am failing to grasp is the last part of removing the wall stay “Move the live wire that was coming out of cable 7 (red dotted) to power the thermostat (via junction block terminal 3) from terminal 3 and moved it into terminal 6 as indicated by the pink circle.”
        No matter how many times I read this I don’t understand.

        1. Hi Brian, so I guess the simplest way to explain this is that if you choose to completely remove the in line thermostat you need to reconnect the circuit. Moving the power cable from terminal 3 to 6 achieves that. Nick

        2. Brian, draw out your wiring diagram, only needs to be basic but it will help. I had three attempts to get it right and on the third attempt drew my wiring diagram. This helped me work out where the open circuit should be closed. On my previous attempt, I tried every combination I thought that was required but the correct one! Hope this helps.

          1. Thank you. Unfortunately my wiring is dreadful. No wiring centre just a whole bunch of wires. I think I’m going to purchase a heating wiring centre first and gradually put everything where it needs be.
            But I have identified the two wall stat wires it’s just the very last part that I don’t know yet.

    2. Hats off to the man who took the trouble to write this excellent guide. This is what I call ‘serving the community’. I don’t even have a Nest but I still enjoyed reading it.

      1. Thanks Ed! I’m not gonna lie, it took forever to write this post but glad so many people are finding it useful! I struggled to find anything of use online when I was looking to install it so figured others likely had the same issue and would appreciate some help too!

  1. Hi roger, you didn’t say what you did with the CH wire at the heat link that isn’t used? Did you just disconnect both ends?

    1. Hi Dave,

      The CH OFF wire that would originally have been in position 2 of my programmer and position 1 of the Heat Link has simply been disconnected and made safe – in my original Danfoss installation there wasn’t a wire connected at all for CH OFF, it was simply in a block connector and made safe.

      Nick

  2. Hi Nick, great writeup, I’ve got some suggestions.

    What you could’ve done is install the heatlink near your junction box and feed 12v through T1/T2 over your current roomstat wires (close the circuit off at junction box). so you can re-use the existing wiring and put the nest stat in its place.

    You could’ve left your programmer as is.

    Also, you might want to re- consider punching out the back plastic on your heatlink and feed the wires from the back so you won’t have a gaping hole in the kitchen wall.

  3. Hi Nick/Jan,

    This is exactly what I ended up doing. It took a while for me to locate the existing thermostat’s wires in the junction box but once I did I managed to wire the T1/T2 to the existing wiring so that the voltage was correct. I had to feed the wire down through a stud partition but I eventually did it & am very happy to now have the Nest thermostat in the old thermostat’s position. I have also added in two hard wired smoke alarms so we are looking pretty good here.

    Thanks for everyone’s advice.

    The nest pro contacted me approx 2 weeks after I submitted a help request online!

  4. Fantastic guide helped me get my nest up and running with my Y plan system no troubles. Shame Nest cant make there instructions as good. many thanks.

    1. Hi Ronnie, it depends what you’re trying to achieve with your setup. With Nest you receive a new digital thermostat so your existing one would either be replaced in situ or as I did with my installation, be completely disconnected to use the new thermostat wirelessly. I think you can run the system with multiple thermostats e.g. upstairs and downstairs but unfortunately I’m not that clever to know how to wire that in!

  5. Picked mine up on a Black Friday deal, so will be installing over the next few days. This article clearly explains what I am about to do, although will also have to decide whether to replace the old thermostat in place or go completely mobile. I am looking to place my heat link in the airing cupboard replacing the older controls unit, I can’t see this should be a problem.

    1. Best of luck Will! It can be quite daunting when you first buy the kit as it’s a major part of the home to attempt as a novice (as I was) but hopefully my wiring diagram makes things super easy for you! Only pain with going mobile was that I had to gauze and replaster the hole in the wall. It wasn’t too bad for me as I was completely renovating but certainly won’t be for everyone. Let me know how it goes and if you have any tips for others!

      1. Attempted today getting totally dismantling the controller and thermostat. My thermostat was the same as yours, which I thought operates as 230v. Popped out and bought a multi-meter and this confirmed it, so decided to leave for now.
        My controller also had two wires going from the 3 port valve into the controller point 3, HW on, which didn’t make sense to me. I think I figured out the feed from the junction box to the thermostat, but there’s no play on them to disconnect and be sure. There were also some wires just tapped off in the junction box which was like spaghetti junctions evil twin. These may be from the original boiler but I am not sure. Going to do some more research.

        1. Ignore the part about those wires going to the three port valve I have doubled checked and that’s not right, they go into the junction box.

      2. Having gone over this thread and another again, I am going to retry next weekend. I might do it in two phases, 1 with the old thermostat in place, switched to max and then swapping for the nest.

      3. 2nd attempt failed. Wired it all up following the instructions, replacing the old thermostat with the nest. First issue was the nest thermostat keep switching off as I tried to set it up. A couple of firm pushes later and this was working. Test of hot water (haven’t tested turning the cylinder stat down to see if it cuts off as it should do) no issues. Testing central heating did not work. Reviewing everything tried again and still did not work. then it did work after testing hot water. Testing on its own, now it will come on for about 5 – 15 secs, then switch off the pump and boiler. Rewired the old room thermostat switching to max and again, the heating will only come on for 10 secs.
        Nick, you moved your wire from wire 7 block 3 to 6. did this originally go into the controller and hence now nest heat link?
        Any advice from anyone appreciated.

        1. Hi will, sounds like you’re having similar issues to what I had. I had a few failed attempts where my boiler would fire up temporarily and then shut off. I then had issues with having central heating but no hot water from the taps so I identified that the switches weren’t quite right somewhere in the circuit. The first thing I would check is 1) have you added the extra power to points 2 and 5 on the Nest heatlink? 2) With regard to my old thermostat, if you look at my wiring diagram the old thermostat (wire 6) had 4 wires coming out of it – green, blue and yellow dotted were completely disconnected. The red wire that ran from wire 7 to wire 6 (via point 3 on the electrical block – shown as dotted red in my diagram) was then simply moved into point 6 of the electrical block. If you have the same Y-Plan set up as I had it may be that your wires are in different positions in the block so you would need to trace which wire is where.

          1. Third time lucky! Thanks Nick for your guide and help.
            I cured my problem of the heating switching off after about ten seconds by following your guide and the explanation of the wiring diagram. The wire from my room stat went into connection blocks 3 and 4 in my junction box. 3 had a wire coming from the controller (now heat link) and four had a wire coming out the other side to another block (9), into which was wired my three port valve. I moved the wire from 9 to 4, to now be 9 to 3. This closed the circuit and allowed the heating to stay on.
            I have also tested that the cylinder stat turns the hot water off once it reaches the preset temperature.

            Nick I owe you a beer! failing that a massive thank you.

          2. Fantastic, great perseverance! When mine wasn’t working it was a combination of panic of no heating and being a bit deflated but just had to keep trying!

          3. Cheers Nick and once again thank you for the great blog and you quick responses, its much appreciated, i’m sure by all who stumble across this thread.

  6. Hey Nick,

    Just wanted to say a big thank you for your time in putting this documentation together. You were spot on and precise leading to my installation taking less than an hour.

    I was lucky and my heating system uses a honeywell wiring center (effectively a break out pcb board for all your connections for boiler, pump and valve, so no chasing wires in choc blocks)

    I did a hard install of my thermostat in the original position, for this all I had to do was remove the old thermostat wires, place a jumper on my honeywell wiring center board and then join the old thermostat cable to the heat link box giving 12volts to the controller . Very easy. That was the only mod.

    As for the live wires to pins 2 and 5 I just made jumpers from live across to keep it neat in the box.

    All up and running in less than an hour, could not be more happy with it. I think you need a nest pro code for all your help you have give us. My 20 year old boiler suddenly feels very new and modern lol.

    I have photos if you are interested of the installation and the honeywell board.

    Many thanks again Carl

    1. Karl, I too was looking at jumping wires to positions 2&5. Can you explain how you converted the feed to your thermostat down to 12 volts? Was it converted coming out of the junction box?

      1. My old thermostat wires went to a junction box I disconnected them there and used a connection block to extended them onto the heat link box.

        It depends on where your old thermostat wires connect too.

        It would be easier with photos to show you but can’t post them here

        1. Hi Carl, I may expand this post with additional detail to help others so if you want to drop me across any photos and directions to nick@lifeofman.co.uk I can see if I can include them at a later date. Nick

  7. Hi, I have managed to install 3rd Gen and have placed the Nest thermostat in a new place but only Hot water works from the Nest thermostat, it turns Red when i increase the temp but the boiler doesn’t fire up, it still works from the old thermostat ,it’s a Honeywell T40 thermostat.Could you please let me know which wires should i join to re form the circuit and remove the old thermostat.There is junction box in the airing cupboard and has got loads of wire in it.Any suggestion would be appreciated.
    Boiler Logic Heat 15
    Thermostat Honey Well T40

    1. Hi VJ, I’m certainly not an electrician so it’s difficult for me to advise on your circuit as t may be different to my own. It sounds like the switch maybe isn’t firing – is your wiring the same as per my wiring diagram? If so I’ve outlined on there which wires need to be re-routed if you’re removing your existing thermostat to reform the circuit. Nick

    2. Mine did that until I switched the wiring in the junction box to run from the T1 & T2 into the junction box and joined to the existing thermostat wires. Ie changing the voltage. Whilst I had the old thermostat on the circuit I could only get the heating on by turning the old thermostat to the max temp.

  8. Thank you very much for these instructions. It gave me the confidence to do it myself and only took a couple of hours. Luckily the thermostat wires were labeled so not to much cable hunting. This filled the gap as I could not find any 3rd gen video instructions for hot water tanks.

  9. As other have said: THANK YOU EVER SO MUCH! Very nice step-by-step set of instructions with pictures. My main concern was to close the thermostat circuit at the “spaghetti junction” but with your instructions I could figure it out. In my case was a bit different as yours but essentially the same: the Live cable coming from the thermostat to the terminal block had no other cable in the same hole but on the other side of it, not 2 in number 3 in your diagram but one in 3 and another one in 11 . What I did was to remove the ends of Live (1 in the picture of your thermostat) and Call (2 in the picture) cables from where they were in the terminal boxes and run a cable between the positions in the terminal boxes they were. I am not sure I explained myself, a picture could probably be a bit more clear.
    In any case, thanks again!

    1. No problem Paulino, glad it helped! Based on the comments here it seems like a lot of people are attempting this themselves at the moment so thanks for sharing your own experience which may help someone else. Nick

  10. Just a quick question. When removing the old wall mounted thermostat you mentioned switching a wire in to junction box 6 pink oval annotation. What is the reason behind this?

    1. Hi Tom, it’s because the thermostat is an in-line switch so if you remove the hard wired thermostat you’ll have a gap in your circuit. Moving those wires means you re-connect/close the circuit

    2. Tom, look at my thread above to see what issues you may have if you can’t work this out. As Nick says, you need to close the circuit for it to work.

  11. Fantastic step by step instructions. I was running the exact same system so this guide made installation easy and straightforward.
    Many Thanks for the hard work that has gone into this. 😉

  12. Hi Nick,
    Can you please advise how to go about installing for my configuration? I have a honeywell ST9100c controller and two honeywell thermostats CM707. There is a potterton pro max boiler in the kitchen and a heatrae sadia water storage in the airing cupboard. This is a 3 months old house so everything is new. I can see a junction box as well in the airing cupboard with cables from three electrical cover boxes leading into it. What is the easiest way to find out what type of wiring system I have, should i open the junction box or check the controller wiring? I have two nest 3rd gen thermostats with heatlinks. Will I have to install both the heatlinks or just one?

    Any help would be greatly appreciated. I can provide more details if required.

    Kind regards,
    Nandan

    1. Hi Nadan,

      Thanks for commenting however as previously mentioned in this thread I’m no heating engineer or electrician so I can’t provide any advice with this I’m afraid. My post only outlines how my own installation went.

      Maybe someone else in the thread will be able to offer you some guidance.

      Nick

    2. Nadan, there is info on the Nest website about having to use both Heatlinks if set up is multi zone.

  13. Hi Nick

    Great write up – quick question, if you’re hard wiring the nest in situ of the original, is it easier to install the heat link in the airing cupboard as it’s a bit difficult to get the T1\T2 wires from the nest stat back to the heat if I install the heat link in place of the programmer?

    1. Hi Pav, from the comments made by others here then yes that’s absolutely possible but as I didn’t do that with my install I can’t really advise on how to do it I’m afraid! Maybe someone else on this thread will be able to help you out… Nick

  14. Hi Nick,

    As everyone has already stated great write up.

    It appears my install is very similar to yours. I have a Danfoss programmer which controls the hot water from the boiler to a megaflo and central heating. What I’m confused about is the wiring to my existing programmer does not have a ‘HW off’ wire fitted (nor does it have one to CH), which made me think I need the S-Plan.

    I wired in the S-Plan and the heating perfects perfectly on and off, however once the HW is turned on it does not turn back off. On reflection I think this must mean I need the Y-Plan wiring like yours so the satisfied command can turn the HW back off again. What I do not understand is how my current Danfoss programmer can turn the HW off without this wire!?

    Would appreciate any help if you have any ideas.

    Thanks,
    Sunny

  15. As it is wireless how close to the router (wireless) does the unit need to be? As the signal may have to go through walls etc.

    Chris.m

  16. Really good write up.
    The nest own guides were nowhere near as good as this
    Couldn’t have done it without your info
    Thank you ??

    1. Hi Jerry, I completely agree, the Nest instructions are pretty unfriendly for DIY’ers! Glad my post helped!

  17. Hi, I have a Daiken Antherma Airsource Heat Pump and seperate Hot Water Cylinder system so its an all electric system.

    I have an existing wired thermostat/programmer in my livingroom (i can set heating schedules and turn the heating on and off, adjust room set point temps. this does not control the Hot Water though)

    I can only set hot water schedules from the boiler upstairs as it has a built in programmer for water only.

    Does this suggest my nest, once installed will only have the fuction of controlling heating/room temp and not the hot water?

    Nest compatibility checker does say its compatible with Air Source Heat Pumps, but im confused that i’ll only get the heating fuction but not the hot water control too?

    Going by your very well put together guide, i think my install will also be done using the Y plan as i have a valve that allows just heating, just water or both at the same time. any advice appreciated, thanks.

  18. 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 as there doesn’t appear to be provision for uploading attachments
    ?

  19. Best write up I have seen, quick question someone maybe able to answer can you not terminate the thermostat at its source rather than changing the wiring in the junction box, leaving a terminal block behind the wall in position and what connections would need to be made to complete this circuit.

    1. Hi Jason, thanks for reading! I went for a simple connection with my installation by re-routing the wires but I expect there are other ways to achieve the same thing. I’m not an electrician however so I daren’t advise on any way other than what I did which works. Perhaps someone else in the thread can offer some insight.

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