Google Nest Thermostat v3 - power from Lighting transformer?

[Ben B]

I've installed a Google Nest Thermostat that I picked up used from CEX. The Heat Link powers up, and the screen powers up by USB cable. All is working, synced, controlling the heating etc... however -

If I connect the heatlink T1 and T2 to the screen - using the wiring already in the wall (in the position of our old thermosat), then the screen and the heatlink are both dead. But they work fine without t1 and t2 connected and using USB power instead.

Not sure if I've done something dumb here! The mains cables that were connected to mains voltage, at the cylinder, leading to the wall-mounted thermostat, now carrying 12v along t1 and t2 to the Nest Thermostat screen unit instead of the 240v they used to carry.

Seems simple enough, so if I come to the thinking that the unit has something faulty (perhaps the previous owner put 240v through it? - you have to ask why it would be sold!) can I use another 12v transformer to create the power needed for the screen unit? I was thinking a 12v low voltage lighting transformer, plugged in at the cylinder again, and sending power down those same wires would do the same just as the Heat Link, for sending the 12v the screen unit needs.

Is a 12v lighting transformer suitable? Or anything else small and simple enough?

 

[ianmcd]

no you have to use the supply from the heat link, it doubles as a communication cable from the stat to the heat link, so you dont get wifi signal problems

 

[stem]

If you connected the thermostat as below, it should work.

I've done several using the old thermostat cable. If the thermostat has been damaged then even if you used a separate power supply, then it wouldn't work with another power supply either. You can check T1 & T2 at the Heat link with a multimeter to see if it is providing the 12v supply.

no you have to use the supply from the heat link, it doubles as a communication cable from the stat to the heat link, so you dont get wifi signal problems

Was that established? I remember questions being raised before, but not the answer. The Nest manual shows wireless communication between the two units even when wired together.

If the thermostat is powered with the separate mains supply providing USB power as per the OP's first post there won't be an electrical connection between the two sets of T1 & T2 terminals.

EDIT
Found this old thread where the OP replies claiming to have contacted Nest with the same question. The bit about the power supply is towards the end.

https://www.diynot.com/diy/threads/upgrade-nest-to-nest-v3.498853/#post-4106012

 

[ianmcd]

Was that established? I remember questions being raised before, but not the answer. The Nest manual shows wireless communication between the two units even when wired together.

I was at a call as a nest agent, stat kept losing signal, hard wired it from heatlink, and never been back, that was over 2 years ago

 

[stem]

Ah OK, so it will work without being physically connected, but better to be, especially if wireless communication between the thermostat and heat link is poor.

 

[Ben B]

I've seen it in writing I'm sure, in the installation manual, 'make sure it's kept away from metal objects, as they communicate wirelessly, even if connected by wires'.

I may have to just check with a volt meter, see if t1 and t2 are carrying power

Thanks for the help!

 

[ianmcd]

Ah OK, so it will work without being physically connected, but better to be, especially if wireless communication between the thermostat and heat link is poor.

Yes , I have fitted over a hundred now, and most are just working away wirelessly, but on the odd occasion where there is a large distance between the stat and the heat link, more so with sandstone buildings, the wireless connection can drop out, connecting the two units using the old stat wires stops it from being a problem