Nest 3rd gen thermostat power supply

[Daithevet]

Hi,

I've got the thermostat nowhere near the heatlink to supply 12v, so currently using the usb power supply. This means that it's cannon fodder for a toddler...

I was hoping to wall mount it and supply it from a light switch via a 12V transformer mounted in a back box behind it. Would supply the transformer from a FCU. Any problem with this?

The Nest needs 12V 0.15A at T1 and T2 as I understand, but I've ordered a 12V 300mA constant current transformer, will this be suitable or do I need to find a 12V constant voltage transformer that's small enough to hide away?

 

[Rockingit]

Confused. Are you talking about powering the thermostat (round shiny thing) or the heatlink? Heatlink normally sits near your boiler out of the way and the thermostat power comes from a USB lead from a plug top.

 

[Daithevet]

Yes, the shiny round bit. It's irresistible to the toddler and the wire trailing from it makes it an easy target.
Since it could be powered by 12v from the heat link, I don't see any reason it couldn't be hardwired with 12v from another source?

 

[davesparks]

Do not use a constant current supply, that will almost certainly damage the nest thermostat. A constant current supply varies the voltage to maintain a constant current, so your 300mA supply will keep increasing the voltage as it tries to push 300mA through the nest which you say requires 150mA.

I always advise against supplying anything other than lights from a lighting circuit, but your electrician will be able to advise further on what is possible for your individual circumstances.

 

[James]

You need a constant voltage power supply.
I can’t see a problem with what you intend to do,
The other way you could do it is with a usb outlet plate

 

[Strima]

Is there a neutral available at your light switch?

 

[Daithevet]

Yes, the lights are looped at the switch, so pretty handy.

USB faceplate just gets me a different place to hang a wire though doesn't it?

Aren't smoke alarms and extractors on lighting circuits already? The thermostat will be next to the light switch so won't take much figuring out for the next guy.
As I understand, a constant voltage supply will overload an led if there is no resistive load, so it will supply as much current as it's capable of. But if a constant current supply is connected to a load, won't it only supply the current needed?

 

[Rockingit]

Teach your toddler, use the Nest USB and leave your warranty intact. That advice comes free from a parent and Nest Pro installer.

(and remember to 'lock' the thermostat in the app!)

 

[davesparks]

Yes, the lights are looped at the switch, so pretty handy.

USB faceplate just gets me a different place to hang a wire though doesn't it?

Aren't smoke alarms and extractors on lighting circuits already? The thermostat will be next to the light switch so won't take much figuring out for the next guy.
As I understand, a constant voltage supply will overload an led if there is no resistive load, so it will supply as much current as it's capable of. But if a constant current supply is connected to a load, won't it only supply the current needed?

Yes smoke alarms and extractors which need to be linked to the lights are normally supplied from a lighting circuit. An extractor which does not need to be linked to the lights, such as in a utility room is normally connected to a power circuit.
This does not mean that you can/should connect anything you like in to a lighting circuit in my opinion.

A constant current supplies the current that it is set to regardless of what current is needed by the load, so your nest which has a maximum required current of 150mA would instead have 300mA effectively forced through it destroying it.

An LED is a very different kind of load and it does require the current through it to be controlled externally, this is why we have constant current supplies.

You need to select the correct type of supply for your load, which is, as you have been advised, a constant voltage supply.

 

[EricMark]

I was under the impression that the thermostat sensor has to be in the circulating air, the idea is thermostat is opposite to the radiator, and since both sight and manipulation is required should be 1400 mm from floor level, you must have very tall toddlers.

OK I will admit my Nest 3 is not in the best location either, I was lead astray, I got 4 Energenie MiHome TRV heads which said they were designed to work with Nest, however when I got Nest 3 and the TRV heads followed when using app, but not when using dial or schedule, I phoned for assistance and was told Nest has withdrawn support, so it no longer works with a link to the TRV head.

However I have found simply setting the TRV to same schedule as the wall thermostat works reasonably well, and I had to set a temperature differential between the two, there is no real reason why the TRV should not be set to geofence same as the Nest Gen 3, but have found it is not required, turning the Nest Gen 3 down as it detects the phone has left the house turns heating down whatever the TRV does, if boiler not running it really does not matter what the TRV does.

But there is a limit, and a thermostat in middle of the room is not going to work, it needs to be in the natural circulating air, so on the wall, where you can read the display and reach the dial with ease. So eye level.

Mine is supplied for the heat link, as the cables were already in place between the flat under house and the hall so although the thermostat may work better in living room to hall, in real terms the thermostat in my case (Oil) only turns boiler on/off, the TRV controls room temperatures. The 9 programmable TRV heads seem to do a very good job, however my children are not a problem, the grandchildren do seem to think it is funny to tell the Nest Mini to do odd things, so I have removed the TRV heads from Nest Mini control to stop grandchildren altering heating, they do however seem to think it is funny to set an alarm at 3 am. And I am told we can't smack grandchildren, mainly as they can run faster than me.

 

[Daithevet]

Yes smoke alarms and extractors which need to be linked to the lights are normally supplied from a lighting circuit. An extractor which does not need to be linked to the lights, such as in a utility room is normally connected to a power circuit.
This does not mean that you can/should connect anything you like in to a lighting circuit in my opinion.

A constant current supplies the current that it is set to regardless of what current is needed by the load, so your nest which has a maximum required current of 150mA would instead have 300mA effectively forced through it destroying it.

An LED is a very different kind of load and it does require the current through it to be controlled externally, this is why we have constant current supplies.

You need to select the correct type of supply for your load, which is, as you have been advised, a constant voltage supply.

Point taken, use the right one!

Is it a regulation that only lights to be on the lighting circuit? Am I wasting my time getting someone in to look at this?

I was under the impression that the thermostat sensor has to be in the circulating air, View attachment 60697 the idea is thermostat is opposite to the radiator, and since both sight and manipulation is required should be 1400 mm from floor level, you must have very tall toddlers.

OK I will admit my Nest 3 is not in the best location either, I was lead astray, I got 4 Energenie MiHome TRV heads which said they were designed to work with Nest, however when I got Nest 3 and the TRV heads followed when using app, but not when using dial or schedule, I phoned for assistance and was told Nest has withdrawn support, so it no longer works with a link to the TRV head.

However I have found simply setting the TRV to same schedule as the wall thermostat works reasonably well, and I had to set a temperature differential between the two, there is no real reason why the TRV should not be set to geofence same as the Nest Gen 3, but have found it is not required, turning the Nest Gen 3 down as it detects the phone has left the house turns heating down whatever the TRV does, if boiler not running it really does not matter what the TRV does.

But there is a limit, and a thermostat in middle of the room is not going to work, it needs to be in the natural circulating air, so on the wall, where you can read the display and reach the dial with ease. So eye level.

Mine is supplied for the heat link, as the cables were already in place between the flat under house and the hall so although the thermostat may work better in living room to hall, in real terms the thermostat in my case (Oil) only turns boiler on/off, the TRV controls room temperatures. The 9 programmable TRV heads seem to do a very good job, however my children are not a problem, the grandchildren do seem to think it is funny to tell the Nest Mini to do odd things, so I have removed the TRV heads from Nest Mini control to stop grandchildren altering heating, they do however seem to think it is funny to set an alarm at 3 am. And I am told we can't smack grandchildren, mainly as they can run faster than me.

This is the problem really, the toddler is just a side issue. Since I'm using the plug in USB power supply there isn't really a power socket anywhere sensible to plug it in. The kitchen has a cooker, the lounge a fire and there aren't many sockets in the hall

 

[EricMark]

I know my Nest says it is hard wired for data, not sure how it knows if to use the 12 volt supply to send data, but may be because it is 12 volt not 5 volt?

But easy way to get power is swap the socket for a socket with USB outlets, I have looked at getting USB power to a wall mounted item, and the problem is you have a cable coming out of a socket and returning into the wall, never found a neat method.

Closest I found was a LAP 4 module grid plate with a twin output USB and brushes but still not ideal, but the same applies to a 12 volt power supply where are you going to mount it?

 

[Daithevet]

I took a feed from a light switch to a fcu and used that to feed a small 12v transformer that fitted in the box behind it. This is the other side of the wall form the nest in a cupboard, so it's not an eyesore