Wiring – Why is the Nest thermostat not working with A/C

hvacthermostatthermostat-c-wirewiring

I installed Nest thermostat a few weeks ago, replacing an old Lux programmable thermostat (there was nothing wrong with it). I connected 4 wires (R,W,Y and G) to the Nest. But as soon as I'd connect the Y wire, the Nest would give me an E4 error saying there was no power on R. I could only use the thermostat with the Y disconnected (so only heat, no AC).

When I tested the voltages between R and W, I get a healthy 26 volts at the thermostat and at the control board. But on R and Y, I get 0 volts. The AC works fine with my old Lux thermostat (I verified by re-installing it), however, I also connected the B (blue) and O (beige) wires in the Lux thermostat. From what Nest tech support tells me, I don't need to use B since the Nest can grab power off of the R.

When I connect the Beige wire (connected to O on the control board) to the Y on the Nest, the fan goes on, but no AC.

My workaround for now is to turn on the AC from the Hunter thermostat upstairs (Zone 2) at the same time, so that both dampers are open and the cool air is distributed in both zones. Kind of a hack, at least it keeps the house cool!

So my question is: why is the R to Y voltage zero? And if that indicates some kind of problem in the relay or control board or transformer or condenser, why does the old thermostat work fine?

I put together the following diagram to describe the configuration:

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And here is a picture of the circuit diagram from the control board:

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A picture of the control board:

A picture of the control board

The Nest with the beige wire in Y1:

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Which it happily accepts, and thinks that the AC is connected:

enter image description here

Update: I disconnected all the wires on the thermostat side of the control board and tested with my multimeter again. Same result: R1 to W gives me 28 volts, and R1 to Y is 0 volts.

I just found a technical bulletin for the EWC-ST-2E Control Panel (seems similar to the EWC-ST-2D that I have). Will try to do some more troubleshooting after reading this.

Best Answer

This has been idle a while, but my guess is you need to connect the Nest to the R terminal of thermostat 1 on the controller instead of R1.

It looks like your control system was designed and wired for a heatpump but you no longer have one (there are only 4 wires on the lower left block which I think is the output to the furnace/AC compressor). If so the O and B wires are unneeded and can be removed from the control panel.

So what I would try:

  1. Zone 1 R to Nest Rh (24VAC Hot)
  2. Zone 1 C to Nest C (24VAC Common)
  3. Zone 1 W to Nest W1 (Heat)
  4. Zone 1 Y to Nest Y1 (Cool)
  5. Zone 1 G to Nest G (Fan)

Only thing I don't understand is how the fan gets turned on by the second thermostat in Cool mode. (In cool usually the thermostat has to tie both G and Y to R (24VAC Hot), in heat only W has to be tied to R (+24VAC Hot)... Since it works it must be something the controller does automatically.

By connecting the Nest to the C terminal it gets access to 24VAC without trying to power it self from "leakage current".

It you want to test, jumper Zone 1 R to Y for a minute and see what happens. Does the compressor come on outside? Does the interior blower fan?

Double check this against the manuals..

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