Nest Thermostats Install Power Issue

[prafalow]

I am trying to install 2 Nest Thermostats which only have 2-wire connections, so I purchased 2 Nest Power Connectors designed for my situation.  Because I have 2 older zone units for different floors, I must use these power connectors with each of the zone controllers.  I finally found a C, or Com connector to use with the Nest Power Connector but it isn't working with the thermostat.  I have confirmed 24v on the C wire I'm using.  If I bypass the Nest Power Connector with the 2 thermostats, the connection works fine but the readout shows I must provide the C wire connection.  My only alternative at the moment seems to be rewiring with new 3-conductor wires which I would have to try to fish up through my walls, which I'm trying to avoid.  Has anyone had this issue before?  Many thanks in advance!!!

[ABUS]

Are your thermostats Nest Learning or e thermostats?

[prafalow]

They are not Nest Learning or E thermostats; just called Google Nest Thermostats, Model G4CVZ.

It definitely needs the C wire, but my problem is that the Nest Power Connector doesn't seem to correctly provide power to the thermostat when I have wired it according to the directions.  The C wire of the power connector is attached to a 24V C wire which I have tested.  I have no idea why.

[ABUS]

I found that Learning Thermostats need a C wire while e thermostats only need the two, red and white.

[dhx227]

Unfortunately, you haven't provided quite enough information to help you out.   You say you have two Nests because you have multiple zones.   I'm assuming this means you have a single furnace unit that runs the two zones?  Or do you have two independent furnace units, one for each zone?   Are these simply furnaces or are these HVAC's (furnaces with A/C units)?  That would be a much more complicated setup.

Second, the power is 24vac.  With AC current, you need to measure both sides.   Typically, this would be between C and R (for heating).   When you say you measured 24v on "C", what was the other terminal you were measuring that with?

You say you bought 2 power connectors, that is good, but you did not provide any schematic of how you wired them in.   So, it's really hard to troubleshoot how you might have mis-wired them.

Here would be a complete stab in the dark:

First, some terminology to help:

• T1 = Thermostat 1, T2 = Thermostat 2.
• PC1 = Power Connector 1, PC2 = Power Connector 2
• The following refer to the connections on the Power Connector):
  • Rt = Red to thermostat
  • Wt = White to thermostat
  • Rf = Red to furnace
  • Wf = White to furnace
  • Cf = Common on furnace
• R1 = zone 1 power (R or Rh) on furnace
• R2 = zone 2 power (R or Rh) on furnace. Note: you may only have a single R on the HVAC, in which case both R1 & R2 can be connected to the  single R)
• W1 = zone 1 call for heat on HVAC
• W2 = zone 2 call for heat on HVAC.

This next part should be obvious, but worth double checking:

• T1-R -> PC1-Rt &  T1-W -> PC1-Wt
• T2-R -> PC2-Rt & T2-W -> PC2-Wt

Here's the tricky part:

• PC1->Rf -> R1 (or R)
• PC1->Wf -> W1
• PC1->Cf -> C
• PC2->Rf -> R2 (or R)
• PC2->Wf -> W2
• PC2->Cf -> C

[prafalow]

Thanks for taking the time to give me the details you provided. I’ve been able to get one of the power connectors to work for one of the zones, but the wiring on the other zone I need to connect are so convoluted by past HVAC vendors that I’m going to have to have someone come in. I was hoping to avoid the $500 expense that this will probably cost, but my zone controllers are a schematic nightmare with no labels for me to solve it. But thanks for trying to help me!