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

[Khatva]

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?

[Patrick_Caezza]

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?

[Khatva]

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?

[Patrick_Caezza]

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

[MelbaDT]

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

[MelbaDT]

Hey Khatva, 

Checking back in — still need our help here?

Best, 

Melba

[Khatva]

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

[MelbaDT]

Hey there, 

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

Best, 

Melba

[MelbaDT]

Hi, 

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

Best, 

Melba

[Khatva]

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.

[Ryan_G]

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

[Ryan_G]

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