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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:

enter image description here

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

enter image description here

A picture of the control board:

A picture of the control board

The Nest with the beige wire in Y1:

enter image description here

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.

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Are you taking measurements with the thermostat connected? If so, you would measure 0 Volts from R to Y if the thermostat switch is closed (thermostat is calling for cool). Doesn't the Nest require the C wire to be connected? Do you have the Nest configured correctly? –  Tester101 Jun 5 at 12:04
@Tester101: I took the measurements with the thermostat removed -- directly on the wires at both ends. The fact that it is 0v at the control board tells me that the problem might be at that end. I found a long thread on what appears to be a similar issue here: doityourself.com/forum/thermostatic-controls/… Something about condensates, float switches and evaporator coils -- I'm clearly out of my depth there! –  mcaddu Jun 5 at 12:33
A rejected edit asked how you have the wires hooked up to the Nest. –  BMitch Jun 5 at 21:15
@BMitch: As the picture above indicates, I have the Beige wire in the Y1 slot of the Nest. This simply starts the fan in Zone 1. –  mcaddu Jun 6 at 12:23
Thanks mcaddu, I think you did a great job explaining the wiring, just wanted to pass along the question since I was rejecting the edit request. –  BMitch Jun 6 at 12:30
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2 Answers

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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AC unit outside requires Yellow and the Common leg of the 24 volt control circuits power. Common is like the negative terminal on a cars battery, EVERY single low voltage circuit terminates on the Common leg,thus the Common designation Common is the unswitched "leg" of the 24 volt control circuit power. Most Tstats that are parasitically powered by the heaters transformer require the Common leg to be physically wired to the thermostats sub base to power the thermostat. The Red 24 volt Hot leg of the 24 volt power enters the thermostat on the Red terminal. RH is Red Heat RC is REd Cool RC & RH are designated as older GE units used 2 transformers, 1 for cooling and another for the heat mode. The transformer for heat feeds it's hot leg of 24 volts to RH terminal on the thermostats sub base, the Cool does the same but to RC. On units with 1 transformer we run the hot leg into either RC or RH and install a jumper wire from RC to RH. The nest instructions say the thermostat will sense the Common side of the 24 volt power via the assorted circuits as cool or heat or fan, if the thermostat does not stay charged the Common leg must be ran to the stat to it's C or Common terminal.

The Common leg is ran to the AC unit from the heater not the thermostat. You are using the wrong colors on the wrong terminals , this makes it harder to recall requiring making a note, such as OK at the furnace white is on yellow, etc. Red is the 24 v hot leg,Common is the 24 v neutral, White is heat, Yellow is cool and Green is fan, Common is usually blue however you must go to the air handler to see whats on what. You state you removed a 4 wire stat and replaced with the Nest with the same 4 wires? were the terminals wired as shown? Rewd is the 24 v hot leg that the thermostat sends out on white for heat, Yellow for Cool and Green for Fan on and Cool speed, the Heat speed is low speed and controlled via time or temperature delay, so for AC all you need to do is test for 24 volts going outdoors with the refrigerant line set from the heater. Its just 2 wires Yellow and Common, the color really does not matter it makes it easier to recall though when troubleshooting You have jumpers in the wrong terminals w to y I think which does nothing and nothing is wired or jumpered to RC on the stat enter image description here

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