DIY Doctor

Nest Thermostat 3rd Generation and 240v Wiring

Postby bmrpl » Sun Nov 11, 2018 11:27 am

Hello.
I'm planning to install nest thermostat 3rd generation to replace my old wired thermostat. Whilst trying to remove the Drayton original analogue thermostat i realised that this unit is powered by 240v mains and nest requires 12v. What's the best solution to swap these please, ideally I do want to have my nest thermostat to be in the same place and don't want to have cables hanging on the wall. Any ideas what transformer to use? It would have to be small enough to hide it in between plaster walls...
Thanks for your help in advance!
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Postby ericmark » Sat Nov 17, 2018 11:30 am

Nest normally comes in two parts, the main bit replaces the programmer and the temperature sensor control is wireless for the control and is either fed with 12 volt from main unit or USB 5 volt.

Depending how originally wired but often possible to locate the old thermostat wires and reuse them for 12 volt.

When fitting Nest first thing is check what the boiler can do, there are two ways to control a modern boiler, one with return water temperature, the other is to the eBUS, with the former there is an inherent problem, it has to circulate water to find if heat is required, so we fit basic thermostats so on a warm day the boiler does not fire up, these thermostats are not designed to control room temperature, they are often fitted in the hall, their sole job is to turn off central heating on a warm day.

The TRV is designed to control room temperature, however I will admit it took me some time to get mine set so four main rooms are at temperature set.

However Nest has a follow command, so you can in theory put Nest and Energenie e-TRV head in the same room and they will change together, never tried it, I have not got Nest yet, I only have the electronic heads on the radiator valves.

However with an old non modulating boiler then often the wall thermostat actually controls the room temperature directly, so thermostat is fitted in main room and one hopes rest of house will be near enough. You can still use TRV's on radiators and net result is the same, but in real terms it was easy with old boiler, although you still had TRV and lock shield valves all you did was turn boiler off/on.

New modulating boilers (condensing) however have a variable output and aim is to cycle (turn off/on) the boiler as little as possible.

We are told fit the wall thermostat in the down stairs room kept the coolest, with no outside doors and no alternative heating, don't know about your house, but in my house there is no such room. Fitting a thermostat in the hall is not ideal, in my case it has a large radiator to be able to reheat hall after front door has been opened. However the installers did not fit a TRV to hall radiator as general instructions so to get other rooms warm the lock shield valve was throttled right back so when front door opened the hall took hours to recover.

I fitted a TRV to hall radiator against all advice, it took some setting up, but yes it did work, hall reheats fast then slows down, I then swapped the head for an electronic one, and now works spot on, intention was to fit Nest in the hall, with a follow command so I can vary the two devices together, but not go around to it, and not sure it will work.

TRV set to 19°C wall thermostat set to 18°C if follow command used could not do that.
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Postby bmrpl » Sun Nov 18, 2018 12:37 pm

I've done some more homework and understand much more about the installation now.

The combi boiler I have is Potterton
Promax Combi HE plus

I have programmer - Drayton LP711 with lots and lots of cables :
3x neutral (2x blue and 1x grey)
2x live (dark brown and light brown)
1 - common (light grey)
3 - call for heat (brown)
There also 2 earth cables at the back of the box and 2 black cables connected together in a block

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I tried to show the back of the programmer box as well, hopefully it does show how the cables look like there.


Thermostat in the hallway Drayton RTS1 with the below:
Neutral (grey) which is not needed and will be insulated in terminal block
Live - black
Brown in number 3 which is switched live (call for heat)
Earth which I will put in terminal block

F8AD16AF-9016-419F-9124-7962F5618A3F.jpeg


So of course the programmer will be replaced with heat link and the thermostat will be swapped for the nest itself.

From what I understand I need to connect both live cables from the programmer box in a connector block and then feed one cable to live in the heat link
Similarly, neutral - all 3 in connector block and then with a single cable to my heat link.
Common cable - light grey - from 1 on programmer goes to number 2 on heat link. I also need to use a jumper cable between live and 2 on heat link.
Call for heat goes from number 3 on programmer to number 3 in heat link.

I will be using existing cables from thermostat to connect to my nest controller. I am not too sure about this connection though as I will be using the black and brown only but at the moment these are 230V I guess and I only need 12V for the nest.

Which cables are going to T1 and T2 in heat link please? Do I need to connect earth to the heat link at all? Everything is plastic and the nest won't even have earth connected...

Thank you very much for any help in advance!
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Postby ericmark » Tue Nov 20, 2018 1:04 pm

The main Nest unit could replace the Drayton LP711 and you will need to bridge the cables in old thermostat or turn it full on.

However there is likely a wiring centre, and if Nest replaces the wiring centre you may be able to use the old wires to thermostat to supply power to Nest thermostat.

Some how the control needs to cause boiler to either heat radiators or domestic hot water, or even simply heat domestic hot water at all times and switch heating on/off, in the main to do that we use a motorised valve.

There are very basic systems which were really designed more for back ground heat rather than full central heating, years ago that is how this house worked.

So 7 am the heating switched on and 10 am it switched off, if the wall thermostat in hall got warm enough it would turn pump off, leaving just thermo syphon for domestic hot water, however in real terms upstairs continued to heat up. But since switched off at 10 am not really a problem, around 5 pm it would fire up again for a couple of hours so off at 7 pm.

The coke followed latter by gas fires heated the main living room during day and evening, the central heating just removed the chill.

The domestic hot water got really hot, even with boiler running just 5 hours a day, and all the time programmer was on the boiler would cycle on/off even if thermostat showed room warm enough.

By adding a cylinder thermostat then one could leave the central heating on without it continually cycling, however domestic hot water was as hot as radiators, often 80 deg C rather than 55 deg C which is more normal today.

Next stage was add a motorised valve, then we could at last stop the tank getting stinking hot, and control central heating and domestic hot water at least to some extent independently. I say to some extent as central heating pump off once upstairs radiators were hot, they continued to be hot due to thermal syphon.

All the three stages were all called C Plan. Once you added a second motorised valve it was called S Plan.

There is also a system with a single motorised valve with three ports, and the W plan heats domestic hot water first (either two valves or a single three port valve) then it does central heating.

Or the Y Plan used a mid position three port valve, this does both domestic hot water and central heating at the same time.

I have not read how to program Nest for the different systems, but the old programmer had two options either 10 programs (no motorised valve) or 16 programs (with motorised valve) and often a mechanical and electrical stop and switch to change it from 10 to 16 programs. The basic fact was with early C plan there was no way you could have central heating without domestic hot water, you simply could not turn off domestic hot water without also turning off central heating.

I would guess Nest was never designed to work with the very old C plan, and if you don't have a motorised valve, adding a motorised valve must be your first job.

When done on installation the motorised valve is normally near the hot water cylinder, it makes the cabling to the cylinder thermostat easy, however when doing my dads central heating I wanted to keep running wires down to a minimum. So motorised valve fitted near boiler, and I used a wireless tank thermostat.

As it transpired the cold water tank started to leak soon after, so whole system was replaced with a combi boiler, so mod was short lived.[attachment=1]C-Plan_old2.jpg[/attachment]This is basic C Plan [attachment=0]C-Plan.jpg[/attachment] and this is with both tank thermostat and motorised valve added, hope pictures work never had much luck with pictures.

From what happened with my dads house, first thing to look at is if there are any other faults, I was rather peeved to have spent so much time and effort updating only to see it all ripped out when new boiler went in.

As to new boiler v old boiler not easy, whole idea of new boiler is to adjust output to match requirements where old boiler was simple on/off. I am not sure the best way to control new boiler, but if fitting a new one, I would say if it does not support OpenTherm reject it, there are so many options not open to you if it does not support OpenTherm I would say discount any boiler not supporting it.
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C-Plan_old2.jpg
ericmark
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