Nest 3rd Gen thermostat voltage requirements

[Deleted member 137318]

I have installed a 3rd generation Nest thermostat with requires a 12 volt supply normally taken from the Nest Heat Link. In my installation that option is not possible so my question is: can I used this LED driver to transform my 240 volts into the 12 volt required by the thermostat? (see pic)

 

[ericmark]

No that driver is fixed current it will not work. However it seems the remote thermostat can be also powered from a USB or a battery, although it seems latter does not last that long. So it would seem there is a large latitude in the supply volts from 5 to 12 volt. I have looked many times at how to use USB to power Nest and it seems there is no real option but have a local USB outlet.

 

[ianmcd]

Have a look at the nest stand and use the usb cable supplied it can be hidden well with the stand

 

[Deleted member 137318]

Yeah. There is a usb supply port on the thermostat. Trouble is, it’s wall mounted so it won’t really be able to accommodate the type A USB connector.

Looks like I’m back to having some really long wires cleated up the wall.

 

[ianmcd]

Yeah. There is a usb supply port on the thermostat. Trouble is, it’s wall mounted so it won’t really be able to accommodate the type A USB connector.

Looks like I’m back to having some really long wires cleated up the wall.

that is why i suggested the stand you can site the nest anywhere then and hide the usb cable

 

[Sam Roberts]

that is why i suggested the stand you can site the nest anywhere then and hide the usb cable

This sounds like the best option, you may need to short out the thermostat terminals in the boiler/old stat back box depending on how it’s wired in. I’ve put mine on the wall for now, but to get hot water I had to link the stat cables to the boiler stat terminals from the nest receiver and wire a permanent supply to the boiler. If the 12v wires are “always on” though, this still may not work (I’ve literally just done this!)

 

[Sam Roberts]

Regarding the 12v stat terminals in the receiver, I’ve linked them to the stat terminals and permanently supplied the combi (as said in previous post). Have switched to eco mode but Boiler still seems to be heating, so I guess the terminals are purely to charge the stat, and the control is purely meant to be the stat wirelessly telling the receiver to turn the boiler power on/off. Don’t really see how they can say this is compatible with combi boilers which need a permanent supply to provide hot water? My boiler is an old glow worm (mains in and no volt stat terminals, everything else controlled on pcb) so maybe more modern boilers work with nest? I’m going to have to install a workaround until I get a new boiler (just bought the house)

 

[muggles]

What model of Glow-worm boiler do you have? It sounds to me like you haven't wired it correctly

 

[Sam Roberts]

What model of Glow-worm boiler do you have? It sounds to me like you haven't wired it correctly

I’ll check when I get in but know it ends in 24 - there’s only 230v supply and the stat terminals to connect to, so to make it work with nest (as per nest manual), you wire the CH call for heat to the 230v supplyon the boiler. As the only other connection is the 2 stat terminals (it has a built in analogue timer which has to be set at always on), you can either short the stat (boiler will heat when nest powers it up) or leave them open (boiler won’t heat). The outgoing 12v stat terminals on the nest receiver seem to just be there to power the nest thermostat

EDIT: only work around I can think of at the moment is to connect the supply to the boiler back into the call for heat on the receiver, the boiler would then power down when not heating. I’ll then install a switch in the cupboard which I’ll connect across the boiler stat terminals, meaning when that’s switched off the boiler won’t heat when it’s on - the boiler will then be switched on “manual” at the receiver turning power on to the boiler so I can have hot water without heating. Not ideal, so I’m open to suggestions!

 

[ianmcd]

Regarding the 12v stat terminals in the receiver, I’ve linked them to the stat terminals and permanently supplied the combi (as said in previous post). Have switched to eco mode but Boiler still seems to be heating, so I guess the terminals are purely to charge the stat, and the control is purely meant to be the stat wirelessly telling the receiver to turn the boiler power on/off. Don’t really see how they can say this is compatible with combi boilers which need a permanent supply to provide hot water? My boiler is an old glow worm (mains in and no volt stat terminals, everything else controlled on pcb) so maybe more modern boilers work with nest? I’m going to have to install a workaround until I get a new boiler (just bought the house)

Any combi boiler can be controlled by the nest, the nest takes no part in the HW demand, a sensor inside the boiler notices when a hot tap is opened and tells the boiler to activate in HW mode

 

[ianmcd]

you are making this too complicated for yourself, you say that you have an internal timer on the boiler, access that and it will have 4 wires a L&N and the other two are your switched wires, L&N from timer to L&N on the nest heat link and the other 2 wires to 2&3 on the heat link,( either way round it doesnt matter,) then usb into the nest and plug it in and its done

 

[Sam Roberts]

Any combi boiler can be controlled by the nest, the nest takes no part in the HW demand, a sensor inside the boiler notices when a hot tap is opened and tells the boiler to activate in HW mode

But the boiler needs a permanent supply to supply hot water, nest instructions show that supply going through the receiver, so it completely powers down boiler when not heating (as it’s currently wired).

you are making this too complicated for yourself, you say that you have an internal timer on the boiler, access that and it will have 4 wires a L&N and the other two are your switched wires, L&N from timer to L&N on the nest heat link and the other 2 wires to 2&3 on the heat link,( either way round it doesnt matter,) then usb into the nest and plug it in and its done

I was hoping someone could make sense of it to me (I’m good at missing the obvious!), I’ll have a go at this when I get in

 

[ianmcd]

The nest gen 3 can be used to control a HW cylinder and the CH circuit, but you just need the CH side, just forget the HW stuff

 

[Sam Roberts]

you are making this too complicated for yourself, you say that you have an internal timer on the boiler, access that and it will have 4 wires a L&N and the other two are your switched wires, L&N from timer to L&N on the nest heat link and the other 2 wires to 2&3 on the heat link,( either way round it doesnt matter,) then usb into the nest and plug it in and its done

Worked a treat, didn’t need the L+N from the clock as the receiver is next to the spur, just connected the outgoing pair to CH 2 + 3 and took a permanent supply to the boiler mains, easy when you know how - as the limited info with the boiler stated all controls are volt free, I was blinded to the 230v timer wiring, thanks again.

 

[ianmcd]

o problem glad to have been of assistance