12V supply heating up Nest Thermostat!

[sanderr2]

Thanks all - to answer a few questions. The electrician has used stranded cable - I'm not sure of the exact gauge but looks like what I'd expect in a 13amp domestic cable. There is actually a second Nest thermostat piggy-backed from the first but only a foot or so away from the first on the other side of an internal wall. If I remove the Nest thermostat from its mounting plate I can feel warmth from the wires to the mounting plate and this heat seems to be confirmed by developments (see later). I looked at the alternative power supply that comes with the Nest thermostat - a wall plug that provides a micro USB connection which says it is "7W 5V DC". So I have used the 8V output from the BG Bell transformer instead of the 12V and as a result the ambient temperature behind the stat has gone from 30C to 25C - so a definite step in the right direction. The unit is using Wifi for signalling back to the Heatlink so I'm guessing the answer is probably to find something that will convert mains voltage to 5V DC. Presumably this must fit in the nest plug (which is no bigger than a standard UK plug ie no wall wart or additional PSU) so hopefully it's doable. I will get googling but if anyone knows of such a thing I'm all ears. Thanks again for all the advice and suggestions, Rob

 

[SUNRAY]

The manufacturers are idiots.

I seem to remember you being accused of being rude before.
There is NO NEED to be downright rude.
If you'd been bothered to read the supplied information and seen this bit: '8, 12 or 24V DC,' then youwouldn't have made a fool of yourself again.

 

[winston1]

If the manufacturer had described it correctly as a power supply it would not have happened. As I said the manufacturer is an idiot.

 

[Alec_t]

I've checked the spec of the BG Bell Transformer and it is outputting 12V DC...

I wonder if it really is outputting DC. The picture in your link shows the output terminals apparently joined by transformer windings. That's ok for AC, but not DC!

 

[EFLImpudence]

Do they supply power?

 

[ericmark]

At work we had ASii modules on the PLC which were 24 volt powered and used same cables for signals, we had to use the special power supply, as some could cause interference and some could damp out the signal. I am not saying Nest is the same, it may not be,

If the manufacturer had described it correctly as a power supply it would not have happened. As I said the manufacturer is an idiot.

that is unlikely to be true, but it is possible some one on the help desk can get it wrong, after fitting Nest myself I realised the boiler was firing up when no DHW or CH had been scheduled to run, and it was not showing a record of the firing on the software, I rang the help desk and basic all I got was we can send out an engineer to sort it, then I went onto a forum where some one replied that will be the anti legionnaires software it can be disabled, which is the answer I would expect from Nest help desk.

So because some one has been told

The Nest thermostat can be powered by any 12v power source over the existing thermostat wires, it will need to be at least 0.2A (200mA) as that's what the thermostat draws.

that does not actually mean it is designed to run from any 12 VDC source, it may run from non Nest supplied power supplies, but that does not mean it will run from a non Nest supply.

The supply would need to be smoothed, not just a simple rectifier, but as to how smooth only Nest knows, it could be also that like a satellite dish the voltage tells the thermostat what to do, i.e. use wifi or cable connection between the heat link and thermostat. Also although it says 200 mA that may be only on switch on when charging internal battery.

Again I got caught out, plumber asked for central heating to be switched off night before he was due to visit, so the boiler would be cold, so I switched off the FCU, hind sight I should have simply turned off at the thermostat, because when finished had to wait ½ hour for battery to recharge.

I wonder if it really is outputting DC. The picture in your link shows the output terminals apparently joined by transformer windings. That's ok for AC, but not DC!

I also wonder if smooth DC or just rectified. I would ask Nest if you can get a back to mount on wall and power from USB. Like the desk mount but for the wall.

 

[bernardgreen]

it could be also that like a satellite dish the voltage tells the thermostat what to do,

or the thermostat sends signals to the host ( power source ) by varying the current it takes from the host while the host maintains the supply at a constant voltage.

Always remember the meaning of (eff) which is short for effective.

12 Volts DC (eff) may be 36 volt pulses that average out to be 12 Volt

A Power Supply Unit ( PSU )that produces pulsed voltages that average out to be 12 volts will have the same heating effect on a filament lamp as steady state 12 volts DC.

The effect of pulsed voltages ( eff ) on electronic equipment ( LED lamps, thermostats etc etc ) can be disastrous, from "it doesn't work" through to "it blew up )