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 and made my way to Argos where I picked one up for £199.

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 reposition 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 Argos for an eye-watering £29.

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!

21 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. 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!

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