NEST Thermostat with standalone Transformer

DieterReuther

I have a 3 zone setup with TACO zone valves. 2 are controlled by NEST thermostats, 1 by a traditional thermostat. Only 2 wires are running to each NEST and it is too complicated to add a C wire.

Everything works fine, but I am getting a buzzing sound in the NESTs when they steal power to charge their battery. This happens about every 15 seconds. The HVAC tech is also concerned that the 2 NESTs might overload the heating system transformer when both NESTs and all 3 zone valves kick in.

Does somebody have experience adding a standalone 24V AC transformer to a NEST to power its battery? I would connect the transformer to C and RC. The heating system wires are connected to RH and W1. NEST Support is not supportive at all ("... I can't recommend any resolution to this matter").

Thanks,
Dieter

Bob Bona_4

Run the W from the stat to W on TT on the Taco.
Run the R from the stat to R on the Taco, and also pigtail it with the R on a transormer. 40 VA is more than enough. I dimly recall the Nests pull 15? Could be wrong.

Then run the C from the transformer to the C on the Nest.

DieterReuther

The transformer unfortunately won't be close to the Taco. It will be a standalone transformer in the proximity of the NEST.

Zman

You need to be careful when you put more than one transformer in a system. If they end up on different phases, you could end up with 48 volts rather than 24 volts.
The safe way to do what you are describing is to install a relay to isolate the new transformer from the old.

Bob Bona_4

Gotcha. Only 2 conductors to Nest?

DieterReuther

Yes Bob. Only 2 conductors running to the NEST. It works, just the buzzing in the NEST is concerning.