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Experience:  15yrs in the HVAC industry with certificates and degrees.
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Trying to install a Nest Gen 3 thermostat. It will replace

Customer Question

Trying to install a Nest Gen 3 thermostat. It will replace the slave thermostat on a two-zone (two-damper) gas heat/cooling system using forced air. Both the master and slave thermostats hookup to a Durozone SMZ-2SW controller. The slave thermostat has just the three wires: R, W and Y since the master thermostat controls the heat/cooling mode switch with two separate wires (B and O) and a fan switch (G wire) that go to the Zone 1 of the Durozone controller board.
With just the R, W and Y wires connected, the Nest complains that there is no power going to the R wire (which is connected to the RH terminal in the Nest).
I think that I may need a C connection to power the Nest, but the Durozone SMZ-2SW manual states I need a separate transformer to power 24 V powered thermostats. Any experience with this type of system? (Current system works OK with the three wires but uses batteries).
Submitted: 10 months ago.
Category: HVAC
Expert:  HVAC Guru replied 10 months ago.

Hello, one minute while I get some information together.

Expert:  HVAC Guru replied 10 months ago.

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You need a C wire to power the NEST, you should have a 24 volt transformer that sends power to controller. If you do not then yes you would need to hook up a separate thermostat. If you do you can go directly to the COMMON side of the thermostat with a thermostat wire then connect it to the C wire at the NEST. You almost have to have a 24 volt transformer powering the control it may just mean that you can not hook it to the control itself.

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Customer: replied 10 months ago.
There is a 24 volt transformer that goes to the controller and I have two spare wires on the cable that go to the Next thermostat.The 24 volt transformer is current hooked up to the Durazone "equipment" section on the SMZ-2SW board. Do you think that it would be OK to use this transformer to power the Nest thermostat? If so, would I hook up the two wires to the Nest terminal RC and C? Again, the R wire for the Zone 2 system is hooked up to the Nest RH terminal.
Customer: replied 10 months ago.
I had tried to connect just the white connection to the equipment transformer to the Next C terminal and that did not work. I think that was because there was a separate 24 volt transformer somewhere that was providing the 24 volts to the Zone 2 terminal R wire. Therefore, no current would flow from the Zone to R wire to the C wire going to the "equipment" transformer. That is why I am asking about using two wires directly. But the question is really will the Nest power off the RC and C wires when there is already a RH wire connected.
Customer: replied 10 months ago.
By white connection, I mean the equipment transformer has two wires, a R and a W going to the equipment section of the DuroZone controller board. This board uses relays to switch connections.
Expert:  HVAC Guru replied 10 months ago.

Yes I would try to use the transformer to power it, that transformer should be able to handle powering the Nest

Customer: replied 10 months ago.
OK. I will, but if I use the two spare wires that go to the Nest, should I hook them up to the Nest "C" and "RC" terminals (the Zone 2 Red wire is using the RH terminal).
Expert:  HVAC Guru replied 10 months ago.

You should hook them up to RH and C not RC

Customer: replied 10 months ago.
I traced out the circuitry on the DuroZone SMZ-SW wiring diagram to figure out how this board works and it appears as if the voltage path is different than in conventional systems. I've attached two diagrams I made showing how the 24 volt power gets to the Zone 2 relay when a call for heating or cooling is being made. The problem is that this system uses the Zone 1 thermostat to put 24 volts on either a Blue (heat mode) or Orange (cool mode) wires. In the Durazone controller, the B terminal in Zone 1 is hardwired to the W terminal in the Zone 2 section and the O terminal in Zone 1 is hardwired to the Y terminal in the Zone 2 section. Thus, when the Zone 2 stat calls for heat and connects "R" to "W", 24 volts is sent to the Zone 2 relay and that opens the Zone 2 damper and starts the furnace. However, when there is no heat call, the "R" terminal in Zone 2 does not have 24 volts (its the "W" terminal that is powered), thus, the Nest thermostat can't pull power from the RH terminal. Any suggestions? (I'm not sure if my files were attached since there is no indication that the files were, in fact, attached).
Expert:  HVAC Guru replied 10 months ago.

Just connect it from the equipment R and C 24 volts connections at the board.