topic Re: Nest Wiring with Taco ZVC-405-4, common doesn't work? in Nest Thermostats

Nest Wiring with Taco ZVC-405-4, common doesn't work?

Khatva — Fri, 15 Apr 2022 19:05:18 GMT

I have a Taco ZVC-405-4 with priority, and a set of 4 Nest learning thermostats on 4 radiant heating zones. I have been running with 2-wires fine for a few years, but there have been some battery issues and I want to use the C wire, which the thermostats all have run to them.

The ZVC-405 actually has C terminals for each of the thermostats, but if I connect the C wire on the appropriate terminal in the ZVC to the actual Nest's C terminal on the other end, the Nest reports an error that the C wire isn't providing 24v.

Is the C terminal on the ZVC somehow incompatible with the C terminal on the Nest? How can we fix this so that the C terminal works and we get constant power to the Nest?

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

Patrick_Caezza — Sat, 16 Apr 2022 03:19:30 GMT

I have never had any luck getting a Nest thermostat to work 100% with any Taco valve zone controller.

In theory, you should be able to just connect the three wires and it should work, but it never does.

Have you tried measuring the voltage from the R to W terminal and the R to C terminal? Are you getting 24VAC?

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

Khatva — Sat, 16 Apr 2022 03:48:16 GMT

I did measure the voltage. R to W gives the expected ~25 VAC but R to C shows only ~12 VAC (and similarly C to W shows ~12 VAC). Is that why the Nest is unhappy? Is this a problem with the Taco ZVC?

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

Patrick_Caezza — Sat, 16 Apr 2022 04:46:11 GMT

Yes, that is why the Nest isn't happy and can be the problem

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

MelbaDT — Thu, 21 Apr 2022 07:27:35 GMT

Hey folks,

Thanks for responding to this, Patrick_Caezza.

Khatva, I'm jumping in to check if everything is covered here. Let us know if you have more questions in mind.

Best,

Melba

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

MelbaDT — Tue, 26 Apr 2022 06:51:40 GMT

Hey Khatva,

Checking back in — still need our help here?

Best,

Melba

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

Khatva — Wed, 27 Apr 2022 00:31:10 GMT

My HVAC person is going to replace the ZVC in the hope that that fixes the 25v R to C problem.

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

MelbaDT — Wed, 27 Apr 2022 04:07:24 GMT

Hey there,

Got it ― thanks for getting back to us. Keep us posted once the HVAC technician is done with the replacement.

Best,

Melba

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

MelbaDT — Sat, 30 Apr 2022 06:19:06 GMT

Hi,

It's me again ― how's your thermostat now?

Best,

Melba

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

Khatva — Sat, 30 Apr 2022 16:11:17 GMT

The controller didn't fix it. Further investigation seems to imply some issue with the C wires, which doesn't make much sense as there are 4 different runs of cable and all show the same behavior (so it's not a wire break or other problem somewhere), but for the time being we added bleed resistors per Taco and Nest tech support to help charging in the 2-wire configuration. We will see if that helps the Nests charge more consistently.

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

Ryan_G — Wed, 13 Jul 2022 00:47:34 GMT

Hey Khatva,

Thanks for reaching out here in the Google Nest Community.

I'd like to jump in here and check on how your Nest Thermostat is doing now. I hope it is getting a stable power from the 2-wire configuration.

Let us know some updates and we'll be glad to talk about it.

Best regards,

Ryan

Re: Nest Wiring with Taco ZVC-405-4, common doesn't work?

Ryan_G — Mon, 18 Jul 2022 20:40:30 GMT

Hey Khatva,

I just wanted to follow up and see if you're still in need of help. Please let us know as we would be happy to answer any questions you may have.

Best regards,

Ryan