Installing 2nd Gen Nest

After much pulling of hair, I’ve installed my Heat Link and (almost) my 2nd Gen Nest stat… only I’ve run into a problem.

The issue is, the 12v wiring between Heat Link and the wall behind the Nest stat, doesn’t go directly there. It passes through the controls for the hot water and - somehow - the 12v output (I’ve tested with a multimeter, and it is 12v) is converted to 230v by the time it reaches the wall behind the Nest stat (obviously I can’t connect up the stat, or I’ll cook it).

There is another wire that does run direct from the boiler, which I thought I could use to send 12v… it does send 12v, but apparently if I don’t use the original one (that heads up to the water system) the whole heating system doesn’t work (I end up with a functional Nest stat, but no heating!

Note also, I have to wire the 230v to the Nest stat base, in the wall (no Nest attached of course), for the heating to continue working, too.

Here’s a diagram showing where I’m at.

Does anyone have an idea of how I work around this, keeping the heating working (so dealing with the need to keep the 230v wires in place), and sending a 12v feed to the stat please?

Thanks for your help/suggestions


I should have said… I’d rather not use the USB charger, too.

I have a couple of set up in the way your interested.

I use what are called Switching Relays with internal 24vac transformer.
Make sure you use one that can handle the load. If it’s switching elements, motors or a heavy load you might want to look into a Contactor Relay.

The Nest needs at least 20VAC in order to charge its battery correctly I have been told.

Thanks JP. I had an engineer tell me that my issue is that I need to move the Heat Link from next to the boiler into the hot water (airing) cupboard. He said I can intercept the mains power there, and then the 12V output from the Heat Link will then actually go to the stat without any further issues. Kind of makes sense. I was thrown by the idea that I had to swap my old controller (by the boiler) with the Heat Link. Not so (and not very clear installation instructions from Nest, to be fair).

Thanks again for your suggestion.