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Wiring up New Nest Thermostat a bit of a Conundrum

Postby JamieMartin » Tue Dec 06, 2016 1:37 pm

As a disclaimer, I have some, but limited home DIY electrical wiring experience, but am very keen to learn. My experience so far includes replacing wall switches, wall sockets, ceiling lighting, and wiring up my Combi-boiler's new Heat Link. My question is regarding trying to wire a new Nest thermostat into my home's existing wiring. It's bit of a read, but please bear with me - here's a bit of background:

I have installed the Nest in our hallway - it sits where the previous thermostat was located. Unfortunately the Nest thermostat requires mains power (not batteries) to work, but our previous thermostat was just a battery-powered unit. When I removed the previous thermostat I discovered it had been concealing an existing hole in the wall, with a mess of wiring inside. I haven't touched them (yet) but assume they're disabled (but can't be certain - hence not touching them). I've fitted the Nest over the hole and for now have it plugged into the mains wall socket below via the supplied power cable - a temporary solution (I hope). The Nest can also be wired into the house's power supply, if a previous thermostat that was mains powered existed. This is obviously the preferred option. The problem is that the Nest cannot handle the normal UK 240v mains power - it needs to be reduced to 12v before it can be fed into the Nest. This is what the supplied power cable and plug does (it's actually a USB power cable, providing 5v I believe). If we had had a mains-powered thermostat, the process would have been: install the Heat Link box at the boiler (done), connect it to the wiring that leads through the house to the previous thermostat, and then connect up the Nest to the end of that wiring. The Heat Link would act as a transformer; reducing the output voltage to 12v to provide a safe amount of power to the Nest. As far as I'm aware, we don't have this option - I don't think the mess of wires located in the hole in the hall wall lead back towards the boiler (it's too far), instead I think they lead upstairs. And this is where my question comes in.

The airing cupboard at the top of the stairs was where the house's original boiler was located. At some point it was removed, and a Combi Boiler was installed in the utility room at the other side of the house. I have a suspicion (needs investigating though) that the cables hidden in the ground-floor hall wall (behind where our previous thermostat was located) were connected up to an old dial thermostat that lead directly up to the boiler situated in the airing cupboard above. If this is the case, do you think there would be a way for me to utilise this wiring to provide power to the Nest? Otherwise I'm left in a situation where I'm likely going to have to dig a channel in the wall to bury the nests USB power cable, and then have it plugged into the wall socket below - not the best solution in my eyes.

So firstly there is the question of where the cabling in the hall wall leads to, and how to test it? Would someone be willing to guide me through this process?
Secondly, if the existing wiring proves to be usable, what can I do to make it provide 12v to the Nest? A small transformer?

I've attached a photo of the switches that are located in the upstairs airing cupboard as a starting point towards hopefully solving this mystery. As mentioned, my theory (or hope) is that one of these switches directs power down to the wiring in the hall wall.

Thanks for your time.

Airing Cupboard Wiring

Nest thermostat mount plate
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Simply Build It

Postby ericmark » Thu Dec 08, 2016 9:02 am

First point many combi boilers are condensating and that means it likely has anti cycle soft ware and is designed to work with thermostatic radiator valves (TRV) not a room thermostat. Unlike the old boilers which simply switched on/off the new boilers alter flame size to keep the return water at the correct temperature.

In that case NEST and HIVE are only there to allow you to switch on and off remotely and the temperature setting does not really matter so they can be put anywhere.

The EvoHome does talk to the boiler and allows you to control each room independently using a wet heating system but once you see the price you will see why not used that much. But most the other systems are designed really for hot air systems where one thermostat can control whole of the house.

You could fit motorised valves and Hive or Nest to each valve but again cost makes that silly.

So you are installing a compromise, a cheap system to allow you to turn heat on and off remotely but the temperature is rather hit and miss. The lock shield valve and TRV control temperature but you can set one room to heat slower than rest and then the NEST or HIVE or any other thermostat can control that room independent to rest, but the lock shield valve needs setting so that room is slowest to heat, in the main we really want the reverse, we want main room to be first to heat up.

So first question is where the NEST is being sited the best place anyway? The idea of hallway or other cold draughty room may not be the best any longer.

I looked for 12 volt DC power supplies for a door bell, not easy to find any other than plug in type. Not fitted NEST I fitted a simple programmable thermostat it was not as good as I expected, with just TRV the rooms stayed around constant temperature, but with thermostat there is a sine wave of temperature.
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