Running Nest Wires

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Hi Folks,

I can’t see that anyone has answered this anywhere else…

Plumbers are coming next week to fit new boiler and nest thermostats, Replacing a pair of oldish wireless ones.

I have asked for the the thermostats to be wired in rather than wireless, which they are happy to do as long as there is a cable run in

My question is, what kind of cable do I need to run from the location of the heat links to the location of the thermostats? I’m assuming it’s 4 core flex 1.0mm?

Any help would be greatly appreciated.
 
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Any 2 (or more) core flex will do. It's low voltage 5-12V. if you want you can wire it to the nest thermostat using a cable that is just supplying 5-12v. Or use a usb charger at the thermostat end. The comms between the heatlink and thermostat is wireless.
 
Technically 3 core, Nest say if hard wired between heat link and thermostat there should be an earth in the cable, so twin and earth is normal, this is why I fitted Nest Gen 3, the three core and earth changes colours red, yellow, blue one end and brown, black, grey the other, and one core is open circuit, so wanted to have extra low voltage, low current, and control heating and domestic hot water with two wires. And it has done just that.

However most of the features have been turned off. It seems it is rather a poor system. It said before I bought it that it worked with Energenie MiHome TRV heads, to a small extent it did, if I used the Energenie app to alter Nest the TRV and Nest would auto change together, however when Nest program said change target temperature the TRV failed to follow, and a phone call it seems when Nest became Google support for using TRV heads was withdrawn. Plus it was working wrong way around, every other system the TRV tells the main wall thermostat what to do, with Nest the wall thermostat tells the TRV's what to do.

It seems in USA Nest has released a set of three temperature sensors which connect to the USA version of Nest so it can monitor 4 rooms, but to date not released in UK.

It seems in the UK only EPH wall thermostats can you have more that one wall thermostat and use OpenTherm, the thermostats can be set master and up to 9 slaves, but these also will not work with TRV heads, which makes sense in a way, either you use motorised valves, or you use TRV's as with electronic TRV's they are in essence motorised valves.

It seems only some makes of boiler have given the option of using OpenTherm, some if you want the thermostat to connect to the boiler ebus, then you must use their make of wall thermostat.

The first 'Smart' thermostat was likely the Honeywell EvoHome, which unlike most of the others allows you to adjust room temperatures from the wall thermostat, you don't need a phone or PC. Drayton Wiser claims to have TRV's with built in algorithms which work out how long it takes to heat the room, so reheat the room faster. Tado not a clue what it does. Hive seems odd one out in that it does not have OpenTherm option, and the wall thermostat needs to be in a room which is kept under 22°C as once at that temperature it stops acting on demand for heat from the TRV's.
 
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Technically 3 core, Nest say if hard wired between heat link and thermostat there should be an earth in the cable,
Having looked into this recently, the cable connecting the heatlink to the thermostat doesn't need to have an earth (There is no earth terminal on the thermostat base!)

However, the earth terminal in the heatlink, should be connected to an earth (in the heatlink itself, the earth connection is directly connected to T1).

I believe the idea is, that should a fault develop that introduces mains to the ELV circuit, the RCD should trip, rather than having the wires floating at mains voltage.

If you intend to have a standard metal back box behind the thermostat base, a 3 core cable may be sensible. Otherwise a 2 core should be fine.

Personally, I have used 2 core 0.75mm2 mains flex for a 15M run between thermostat and heatlink and that works well.

I have seen reports that some nest setups are prone to interference, and a good quality screened cable could be the best option. Something like this:

https://www.cef.co.uk/catalogue/pro...rol-flexible-cable-cut-length-sold-by-the-mtr

I hope that helps :)
 
If you have existing cables, then the run is already there for the thermostat(s). If you have a wiring center then heatlink would be best placed here for the thermostat. It would help if you said what existing heating and hot water setup you have and if you’re planning on changing.
 
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If you have existing cables, then the run is already there for the thermostat(s). If you have a wiring center then heatlink would be best placed here for the thermostat. It would help if you said what existing heating and hot water setup you have and if you’re planning on changing.


As far as the current setup goes;

There is a valiant 30kw system boiler in a utility room downstairs.
Above are two 250L unvented cylinders
In another room there are two Honeywell boxes and a programmer
Then two wireless Honeywell thermostats one in the kitchen and one on the landing.

We will be replacing with;

Worcester Bosch Greenstar 8000 Style 35kw
Removing one of the 250L tanks
Installing the heat links to replace the Honeywell programmers
And my hope is to run cables and hardwire the nests to avoid any issues with RF signals.
 
Having looked into this recently, the cable connecting the heatlink to the thermostat doesn't need to have an earth (There is no earth terminal on the thermostat base!)
Correct however BS 7671:2008 says "A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point." but I have never run an earth with a separated extra low voltage supply, i.e. door bell, or fitted a RCD and there is nothing in the BS7671:2008 that says the RCD requirement is only for low voltage.

So common sense says no need for an earth, which is why I said technically it needs an earth as Nest says one should be fitted when hard wiring the thermostat, but see no reason for it.

More of an issue is the inability of Nest to work with electronic TRV heads. Basic idea is a wall thermostat in a room kept cool, so when summer arrives room is warm enough to not switch on heating, on the lower floor and heat raises, with no alternative heating and no outside doors. Don't know about you but with my house no such room.

My first house was hot air gas central heating, ducts to each room, and vents in doors for air to return, and a single wall thermostat worked fine as air circulated. Second house open plan, so TRV's upstairs to stop bedrooms over heating, but Myson fan assisted radiator ensured again air circulated so again one wall thermostat was enough.

It was only when I moved in with mother to look after her, that I realised the problem with one thermostat down stairs. She had bay windows facing east and west, and if the sun came out then front and rear of house were heated alternately as the sun moved around during the day, so it was important that the TRV heads controlled room temperature not a central thermostat, and the radiator was just warm enough, no more, so when sun came out it could cool quickly.

I fitted two thermostats in parallel one in kitchen and one in hall, and also a TRV in the hall to speed recovery when front door opened. And electronic TRV heads in most rooms, which were take to this house when that house was sold.

This house has two wall thermostats and two motorised valves, and the flat under the house has one thermostat and the main house the other which is Nest. But main house the problem is not heating but cooling. Every down stairs room has outside doors, we have four doors to go outside, living room has an open fire, and kitchen clearly we cook in, so hall seemed the best location, but hall is centre of house and last place to cool, so any over shoot and rest of house gets cool before boiler switches on again.

But the point is every home is different, and there is no one system fits all. And it seems Worcester Bosch has not signed up to OpenTherm so either you use their own thermostat which can modulate boiler output, or a simple on/off thermostat. If the home is split into two sections there are thermostats designed for this, Drayton Wiser does one, but likely any simple on/off thermostat will do.

I fitted Nest Gen 3, and not impressed. In hind sight even a cheap Hive would have been better. But it was fitted so the heating did not rely on batteries to work, or RF, being a radio ham relying on an RF link is not good. And it is hard wired and it works of sorts, good enough not to be worth replacing.
 
I would say use triple and earth even if it only needs two core with no earth, so you can at a latter date fit a low voltage (mains powered) thermostat if you want to.

The number of threads where people want to change their thermostat but are limited by existing wiring I have lost count.
 

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