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Nest Learning Thermostat 3rd Gen Hot Water Installation

Published on Friday 04 December 2015
Categories: Electronics | General |

Nest Thermostat install

Today the new 3rd Gen Nest Learning Thermostat with hot water control arrived which we had ordered last month when the new UK model with hot water control was announced.

We needed the Nest Thermostat with the hot water control to replace our old Myson two channel timer which we had been using for many years to control the central heating and hot water.

We wanted to be able to have more control over the heating and hot water in the house and also wanted to remotely control the hot water when out walking with the dogs. They sometimes roll in fox or badger mess and get very smelly and so being able to get back home to have hot water ready to shower them off would be a big advantage.

There are other remote access systems available in the United Kingdom but they nearly all needed to be installed by a “professional” and would only be connected to a modern boiler. As our gas boiler is over 50 years old we didn’t think they would set everything up as the boiler doesn’t have all the “modern” features, but has lasted a lot longer than any modern boiler our friends have had installed in their homes which seem to need to be fixed very often or replaced every few years which didn’t seem to be very efficient when you add the cost of a new boiler into the annual running costs!

Upon opening the box, we found the following items:

  • Instructions and basic user guide
  • Nest thermostat
  • Nest Heat Link
  • Thermostat wall bracket
  • Thermostat flat plasic surround
  • USB cable and mains 5V adapter
  • 4 Screws

We decided to install the thermostat on our upstairs landing and the old central heating controller was installed in our Kitchen downstairs and so we had to run a two core cable from the old controller to the location of the new Nest Thermostat upstairs to power the thermostat controller without having to use the mains plug in supply.

With the PCB built the last thing to do was make the case.  We had some old sheets of delrin and acrylic sheet left over from previous projects so a case was designed in Adobe Illustrator and cut on the mill.  The base is machined out so the PCB bolts down in several places and the Raspberry Pi ports are recessed into the right side.  The sides of the box are made up of six layers of acrylic cut to allow all of the connectors to be accessible and shield the 240V part of the circuit.  The top and bottom of the case were left open to allow air to flow up through the Raspberry Pi keeping it cool.  A clear lid was cut for the top and secured in place with two bolts.

The thermostat mount has a small sprit level built into the base and so this made it very easy to align the mount on the wall and keep it upright. The 12V supply cables were installed into the T1 and T2 screw terminals and the thermostat was clipped onto the mount on the wall.

In the Kitchen we unscrewed the cover of the old controller and found that a section of the plaster was missing where the cables fed into the back of the controllers mount. We attached small labels to each of the cables so we knew where they needed to be connected to and then removed the old controllers wall bracket.

The Nest Heat Link box has cable access clamps and space at the bottom of the unit but as all our cables come from behind the controller in the wall we felt it would be a lot tidier and safer to have the cables come into the back of the Heat Link box. We carefully cut a pair of slots in the back of the cable area and we were then able to feed the cables from behind and the Heat Link box was fitted to the wall with two screws and wall plugs.

The supplied screws are not very long and would be ok if you are fitting the boxes to wooden walls but as we needed to drill and use wall plugs we needed to find longer screws for the Heat Link box. The walls in the kitchen are made of stone and are held together with lime mortar which is over 100 years old. One hole went into a gap between the stones and we had to glue the plastic wall plug into the hole with PVA glue as the lime plaster doesn’t hold very well with wall plugs and the other mounting hole went into one of the stones which took around 10 minutes to drill to approx. 60mm deep and took several steps with different drill sizes!

Once we had connected the cables back into the Heat Link connectors we found that our previous central heating controller was configured to supply power to the hot water circuit when the central heating was enabled and the relay control on the central heating side was only used to control a two-way water valve which redirects the hot water from the boiler to either the radiators and hot water tank or direct to the tank if the central heating is on its normally closed contact.

The Nest Thermostat doesn’t have the facility to turn on the hot water relay when the central heating is activated and so we needed to find a workaround to supply power to the hot water cable when the central heating output is enabled but also being isolated from the hot water output when the central heating is not needed and we only need to heat the water tank. 

We resolved this problem by adding an additional mains powered relay to supply power to the hot water wire when the central heating relay in the Heat Link is activated. We purchased a mains relay from Maplin Electronics and a small plastic box which we fitted to the wall below the Heat Link box. An alternative workaround for this is shown in the comments below which uses the internal relays to do the same function.

We fed the wires from the relay box into the lower cable entry on the Heat Link (see photos) and when the cover is on the Heat Link the cables are not visible. We will paint the relay box to match the walls to make it less visible.

After setting up the Nest Thermostat and connecting it to our wifi network we added the Nest apps to our iPads and iPhones and the remote access and interface seems very easy to use.

Overall the installation went without any problems apart from needing the additional relay to work with our older boiler. I felt that the Heat Link would have been better if the back of the cable entry area had the slots already installed in the case so you have the option to install the cables from below or behind.

Also the wire clamps are small and it isn’t possible to install more than one cable at a time and so linking the mains live cable to the centre relay contacts meant that we needed to join the cables in a separate connection block behind the unit in the wall. Having slightly larger wire clamps or maybe even adding cable jumpers into the back of the box to link the mains live to the relay centre contacts would have made the install simpler.

Nest are talking about releasing internal API access to the thermostat controller which means that we can then use our Raspberry Pi data logger to communicate with the thermostat and so we can then set the hot water heating times to be set based on the current hot water tank temperature.

Update 22/02/2018

Due to the changes to the comments system, I have created an archive of all previous comments including pictures as a PDF file.




08 June 2018 at 10:05 pm

Hello just found your post while searching for problem. Installed Nest 3rd Gen. All works well when heating on but concerned about hot water. I have oil boiler y plan system hot water tank is open ( filled with header tank) if I press boost on phone app for water only boiler fires up then stops immediately. Can I work around this and if heating gets to required temp does that mean my hot water won't heat??? Help would be appreciated.


09 June 2018 at 9:59 am

Hello Waddy, Do you have a make and model for the boiler which you can post? It could be a thermostat in the hot water tank shutting the boiler off.

andrew freeman

31 July 2018 at 5:53 pm

hi,i have been asked to fit nest room stat to a 3 zone heating system.do I need 3 room stats to do each zone of the house(upstairs,downstairs and extentsion underfloor).it also has hot water cylinder.


23 September 2018 at 1:09 pm

Great article, thanks for taking the time to write and publish it.
Would you mind telling me where you bought the relay, as I am looking to do a similar installation. Most of the relays I have found are huge and unsightly!


23 September 2018 at 1:26 pm

I got the relay from https://uk.farnell.com/ and most suppliers will have a suitable relay.


25 January 2019 at 11:07 pm

Before anyone goes down the route of adding a relay - check out the previous comment in the above PDF by Mark C. The nest does all this for you, all you need to do is wire it correctly by just adding a couple of jumper wires. .... It worked for me , no relay neccesary.


25 January 2019 at 11:07 pm

Before anyone goes down the route of adding a relay - check out the previous comment in the above PDF by Mark C. The nest does all this for you, all you need to do is wire it correctly by just adding a couple of jumper wires. .... It worked for me , no relay neccesary.


23 February 2019 at 7:51 am

Thank you for a very good explanation. My question is a little different.
There is a double-circuit gas boiler company Solly. From the boiler wire goes to connect the thermostat. The fact is that the heating mode in all boilers works when these wires are closed, but I have the opposite situation. The heating works when the wires are opened. When closed, the heating is turned off, that is, the boiler turns off the burner. I read in all forums that these boilers work according to the normally open relay mode.
The question is, what contacts heat link to connect my wires. There are contacts 1, 2, 3 and I see that everyone is connected to pins 2 and 3, since for all the heating operation mode works when the contacts are closed. What do i do? If I connect to pins 1 and 3, will it be correct?

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