Nest Thermostat 3rd Gen Install

This is my experience of swapping out my old thermostat for a shiny new Google Nest Thermostat 3rd Gen. I’m mainly writing this as a reminder to myself on what I did should I ever need to go back and work on it again. I’m also doing this as I found any on-line help from forums etc to be very poor. When I was researching the Nest I joined a few DIY groups and posted some questions about installation, most of the replies were less than helpful & many people, possibly professional installers, simply said ‘Get a professional installer to do it’.

Before starting this project I knew nothing about how a home heating system works, but do have an inquisitive mind and a working knowledge for reading wiring diagrams and how to use a multi meter. These sorts of projects are great for learning and saving a bit of cash is also attractive.

Reasons for Purchase

With a bit of bonus money from work in the form of PC World vouchers I decided to invest in the new 3rd Generation Nest thermostat unit and hopefully save some money with the energy savings it promises. Up till this point these units have only been able to control the central heating (CH) and not the hot water (HW), therefore you have to keep your old timer just for the hot water, which seemed messy and dumb. Be careful when buying the unit and make sure you get the correct one, when I went to PC World they were only displaying the older models, I had to ask for the 3rd gen edition which they brought in from the stock room.

Now my old heating system was controlled by a traditional two a day on/off programmer unit which worked absolutely fine and never missed a beat except on the very odd occasion when we had a power cut and it lost all it’s memory. It did suffer from three main issues to my mind.

  1. The CH & HW would come on regardless if anyone was home. There was no finesse to it, 06:30-08:30 CH on, no matter if you were already gone for work or away on holiday and had forgotten to turn it off.
  2. If the hot water tank was cold you could ‘advance’ the timer for the HW to toggle it on and then twenty minutes later it would be hot enough to have your bath, however I would always forget to turn the hot water back off, so it could remain on for hours unnecessarily. Luckily the tank is very well insulated so I’m not sure how inefficient it was to leave the HW on all this time, but it will certainly not be saving me any money and it’s causing more wear and tear on the boiler and pump etc.
  3. There were times when I was working an odd shift such as nights and I really should have adjusted the timer for both CH & HW to fit in better with this, however altering the programmer was less then easy and could only be done whilst hunched over the kitchen top trying not to bang your head on the boiler above you whilst navigating through a non-intuitive set of menus. As it was a simple ‘one day is the same as every day’ I could not plan ahead on days I was free to do so. Setting the timer after a night shift was never something I would attempt.

So these were my three main reasons for upgrading to the Nest Thermostat. The Nest is not the only option however.

The Competition

We had a new Worcester Bosh boiler installed in December of last year as the old one was failing, we did at the time have the option for a free British Gas Hive unit, which does the same thing as the Nest.  I do find it strange to buy a gas saving device from a gas company that’s in the business of selling you as much gas as possible. That’s a bit like buying a petrol saving engine from an oil company or an ink saving printer from an ink supplier. I would be a bit worried about conflicts of interest and that they were not as devoted to making the best product possible. So I decided to go with Googles/Alphabets solution, especialy as I’m already pretty invested in the Android/Google systems. Now you could argue, ‘What’s so complex, it just turns heating and hot water on and off’, which is true, but there is a bit more to it. There are security concerns, mainly in that its a unit that can tell when you are home and more to the point, when  you are away. If in the future this information should become available via some hack then it would be a burglars dream. I have more trust in Google implementing security in this area than a gas company.

I also believe that Google will innovate faster and with more features, it is already in it’s third iteration. Then again Google are notorious for dropping products that fail to take off, such as Reader, Wave, Glass etc. They may drop Nest if it fails to make money, they did a similar thing with Revolve, a smart home hub, recently. These early days of Internet of Things (IoT) is a bit like the early days of computers, which one to invest in, which one will come out on top? Caveat emptor. I have no real dealings with the Hive, a couple of people have them at work and think they are great, it certainly appears to be the more popular option in the UK. So certainly don’t disregard that option, it also sounds easier to install.

My Old System

As mentioned before, I knew nothing about household heating systems. The mess of wiring was a total mystery. If ever anything should fail and if the reset button on the boiler did not put things right then I called a plumber. So first off I needed to reverse engineer the current wiring on my existing system to figure out how it worked.

Y-Plan Water Plan

Y-Plan Water Plan

I found out from a friend at work that most home systems came in standard wiring setups, mainly S-Plan and Y-Plan. The S-Plan uses two separate valves to divert the flow of water to either CH or HW. My system has a single 3 port valve which diverts the flow of water from either feeding the CH or HW, lastly it has a central position to feed both simultaneously. This Y-Plan is the system I’m going to focus on in this blog, however I believe it’s a very similar install for Nest on S-Plan. Both these systems are all 240 volts AC, at no point is it low voltage which initially surprised me, I thought that the room thermostat, which for my install was in the living room, would have to work on low voltage as I imagined it to be a simple bimetallic strip employed to make and break the connection depending on the room temperature. This unit was mains powered and a bit more complex than a simple bimetallic strip, however it still just made a make/break connection at a desired temperature. This AC thermostat would however prove to be the main problem for me when planning for the Nest Thermostat. More on this later.


TimerWhatever system you have there will be a timer/programmer unit somewhere to turn the CH and HW on & off. Mine was in the kitchen just under the boiler. When I removed it there was a simple diagram on the back showing the connections and what they do. There was a permanent neutral and live to power the unit and a HW & CH on/off connection. The live would then continue on to be switched by the timer in the following way:

  1. CH ON: If a call for central heating (CH) was initiated, the timer would toggle the live so that it was switched to terminal #4.
    CH OFF: When the CH is off the live is routed to terminal #2.
    Note: My setup did not have anything connected to the CH off, which turns out to be normal.
  2. HW ON: If a call for hot water (HW) was initiated, the timer would toggle the live so that it was switched to terminal #3.
    HW OFF: When the HW is off the live is routed to terminal #1

Where the live then goes in the loom is a bit round the houses, you can trace it out using the wiring diagrams above. Suffice to say that the respective order is received and executed.

The Nest has a very similar diagram to the old timer with the addition of a couple of Open Therm (OT) terminals and a 12 volt DC +  -.  The Open Therm is probably not going to go unused unless you have a very new setup, it’s a standard communication system between heating systems, but not part of a normal setup. So these two connections can be ignored. Therefore the only difference from the old timer to the new Nest is the requirement for a 12 volt supply for the thermostat.

Nest Heat Link

Nest Heat Link

Nest Thermostat

Nest Thermostat

Now may be a good time to note that the Nest Thermostat system comes in two parts, the actual thermostat itself, a rather nice looking glass and chrome circular unit, which is your interface, and a square plastic back end control unit that has the relays for turning the CH & HW on/off. This is called the ‘Heat Link’.

By all concerns the Nest looked simple to install. At the end of the day it requires some mains, cables run to the call for heating/Hot Water and a 12v feed for the Thermostat. See the rather simplified diagram below.

Nest Heal Link Wiring Diagram

Nest Heal Link Y-Plan Wiring Diagram

When I looked at this I figured that the easiest way to install the Heat Link would be in the same place as the old timer. For the Nest Thermostat I planned to mount it in place of the existing thermostat. Makes perfect sense. Next I mapped out my entire wiring loom which extends to three main locations, the main wiring box upstairs by the HW tank, Downstairs in the kitchen where the timer was & finally the living room where the room stat lived. Although it looks totally different from the Y-Plan Diagram above, it is actually the same.

Old Wiring System. Pre-Nest

Old Wiring System. Pre-Nest

The Problem

Unfortunately, the Nest Thermostat runs on 12 volts DC and the old thermostat ran on 240 volts AC. Initially I did not think this would be a problem at all as I assumed I could simply re-purpose the now redundant AC wires to carry DC instead. Great idea, however the way my system had been wired means that the mains loops through the thermostat position and on to the main loom upstairs by the HW tank. No matter how I looked at it there was no way to get around this. The only solution I could see was to place a small transformer/bridge rectifier behind the Nest Thermostat to step down the mains to 12v DC. This seemed like possible fire risk to me and decided against it.

As the Nest Thermostat can communicate with the Heat link via wireless I could place it anywhere and run power from a wall socket USB or adapter to it via the micro USB connector built into the device. As there is a mains socket below the thermostat this would be easy, but it would look messy with a cable running up the wall. I could bury the cable in the wall but then I would have to come out near the socket to plug it in. Again messy.

A simple solution would be to buy a purpose built stand for the thermostat and sit it on a shelf and power it via a mains/USB adapter and plaster over the old thermostat position. So I started to look at alternative locations. It has to be at a certain height to work best, not by a radiator, not in direct sunlight, not on an external wall or in a draught and it should also have a good view of the room that’s not obstructed by doors etc. This last requirement is of no importance to the old dumb stat, but is important for the Nest Thermostat. The reason being that it has a motion sensor in it that can see if the room is occupied. After taking all these requirements into consideration I came to the conclusion that the perfect spot for it was in the original position!

The Solution

Rather strangely the solution hit me at about three o’clock one morning. By placing the Heat Link in the cupboard with the HW tank, instead of downstairs in the kitchen, this freed up exactly two wires from that position down to the living room thermostat. Perfect. The new wiring diagram I is as follows:

Current Wiring System

Current Wiring System

It took me about three hours to install everything. I could have probably done it in less than an hour but I wanted to buzz out each cable to ensure it was going to the position I expected it too, with the mixture of mains voltage and low voltage in the same loom I was not taking any chances. I also took everything very slow as I did not want to screw anything up. I had initially planned to not touch it at all till the hot weather came, if it ain’t broke, don’t fix it and all that.  Initially when I first turned the power on and ran the tests nothing happened, rather scary. This was because I was assuming that, like the old timer, the permanent live would feed the common for both CH & HW. It did not. This was because the Heat Link had been designed to be used with low power systems if required. I had suspected this may be the case and simply connected the permanent live in the Heat Link to ports two and five and now we are cooking with gas, well heating with gas.

Living with Nest Thermostat

So I have only had the Nest Thermostat running for a couple of days and it does seem to work very well. No longer being restricted to a simple two on, two off, one temperature setting like the old system I have created a new more specific schedule. The Nest has a self learning schedule option, however I’m very dubious that it would be able to figure out our schedules, especially with my random work times including nights. So I have this disabled. I have set it up so that if it sees either my wife’s or my phone at home, or it sees movement, via the thermostat motion detector, then it sets the system to ‘Home’ mode. Otherwise it sets itself to ‘Away’. I guess you could then select a desired temperature all day and leave it at that. This way whenever you are home and if it’s below the target temp the heating will come on. When you leave it will turn off.

One interesting point is that the system soon learns how long it will take to raise the temperature of your house to the target level, therefore if you set the temp to turn on at 17:00 to 20c it will turn on before that to make sure it is 20c at that time. Neat.

scheduleOne thing I had not considered was the ability to have different temperatures during the day or night. Now I have it set to just take the chill off in the morning as we are getting ready for work, say 16c then later in the day when we get home it will go to 20c. Near bedtime it will go down to 18c then switch off at night. Again, if at any point neither of us are home it will turn the heating off. Actually you can’t turn the heating off fully, you can set a minimum temperature of 9c but no lower, I believe my old thermostat only went down to about 10c so perhaps this is some standard frost protection thing.

As the programming for the schedule is very simple thanks to either a phone app or web interface it’s easy to change it from day to day, which I may well do if working nights. Of course you are free to change it wherever you are, not just when you are in the kitchen. It would be good to have a few memory settings to recall for Normal week, Nights etc to quickly change the timings, possibly this will be a future update. As there is at lease 7 days shown you can plan ahead for the next few days if required. As it is so easy to change and can be done anywhere this may give me something to do whilst waiting for a train or bus.

The Hot Water schedule I have set is very simple, I have it coming on 45 mins before we wake up for an hour or two then on again for half an hour before bed. If you want a bath and suspect the water to be too cool, then it’s easy to select a ‘Boost’ option which turns the hot water on for just 30 mins. No more turning it on and forgetting to turn it off. Now I can turn the HW on when im on my way home and have the bath water nice and hot when I walk in the door.

As well as the web and phone app control it also works with your phone by voice. You can say things such as ‘OK Google, set he heating to 21 degrees c’ and off it goes. Even works with my watch.


I’m extremely happy with the Nest Thermostat and glad I could figure out how to install it. The visual design and feel of the unit is top notch. I love the small features like it has a very simple display when it detects you are far away from the unit, then as you get closer the display changes to give you more information. You can also modify the display from target temperature to a nice analogue or digital clock.

historyThe system gives you a daily run down on how long the heating was on and at what temperature. It will also let you know how well you are doing in saving energy using it’s ‘Nest Leaf‘ icon, the gamifiction of energy savings is here. I believe you also get a monthly report and breakdown.

Unfortunately as I got a new boiler in December then the Nest Thermostat three months later I don’t have a good baseline to calculate savings. The new boiler should be much more efficient than the old one so if I see any savings, which I certainly should, I don’t know how much of this was thanks to the boiler or Nest. According to Google it should save you 10-12% a year, obviously that’s Google’s own survey, I would like to see an independent review. However if it is true then the unit should pay for itself in a year or two. I certainly can’t see that it would ever cost me more money in energy, unless someone hacks it or some Demon Seed or Skynet Judgement Day scenario unfolds. I’m rather hoping a future AI will have better things to do than mess about with my heating.

I hope this blog will help anyone else make a decision if this sort of smart device is right for you and possibly help answer some questions on installation. If you do have any doubts and the diagrams make absolutely no sense, I would consult a qualified fitter, no sense in killing yourself and/or setting fire to your home.


  • Hot Water ‘Boost’.
  • Being able to set as many on/off settings and changes of temperature.
  • Auto away mode should save energy and money.
  • Ability to control the CH & HW anywhere via phone.
  • Super simplicity of use.
  • Works with IFTTT recepies for added functionality.
  • If you also have the Nest Protect unit, a smoke/carbon monoxide detector, and if gas is detected it will talk to the Thermostat and shut the heating down. Future Nest products could add more functionality.
  • The unit also displays humidity levels which I guess could be useful.
  • There is a PIN lockout option to stop other people from changing the settings/temperature. Much nicer than placing tape across the thermostat with a message saying ‘Don’t even think about it’


  • It’s expensive at £200. However it could very well pay for itself in a couple of years.
  • Installation is not as easy as it could be. If the Heal Link unit had the standard mount that most dumb timers use and if the thermostat worked on mains voltage the install would be greatly simplified.
  • You can’t seem to get the display to show the current temperature, just the target temp. If you wake up the display it will show the current temp in small characters, however I would prefer the current temp to be the dominant display. This could be a future feature.
  • As it’s software and open to the internet it could be vulnerable to bugs, as happened last year, and security issues. The latter can be mitigated by running all IOT devices on a separate router isolated from your computers and NAS drives etc. A future blog post I feel.
  • Long term support not guaranteed.


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