I have a Nest thermostat currently controlling a LP furnace. The old thermostat didn't have a common wire connected, but there was a an extra wire in the bundle and a common terminal on the furnace control board, so I hooked it up so the Nest would work properly.

I'm getting an evaporative cooler installed soon, and I'd like to be able to control it through my Nest. From some research, it looks like I'll at least need an evaporative cooler transformer relay. One limitation is that this doesn't provide a common wire. I know I currently have a common wire coming from my furnace, but I've read reports that Nest expects it to come from the cooling system and will throw an error if not.

@Tester101 answer about adding a common wire cleared up a lot of things for me, but my question is a little more specific. Since the evaporative cooler transformer relay (see wiring diagram below) does not provide a common wire, I'm wondering if I can pull one off the diode bridge where all the relays connect?

8A18Z-2 White Rodgers Evaporative Cooler Transformer Relay 120 VAC Wiring Diagram

  • Hello, and welcome to Home Improvement. Well-stated question; let's see if one of our pros can answer it. Jul 18, 2019 at 22:54
  • I'm not sure if the Nest can handle switching 24VDC or if it requires AC... Jul 19, 2019 at 2:18
  • Are you required to use this all-in-one control box, by the way, or would it be possible to field-assemble these controls from more standard parts? Jul 19, 2019 at 2:19
  • @ThreePhaseEel I hadn't considered that aspect, I just assumed it would work since ecobee users had reported success, but maybe that's a poor assumption. You could definitely assemble your own, in fact this post tries to do just that (diy.stackexchange.com/q/84680/104028)
    – Matt
    Jul 19, 2019 at 3:05

2 Answers 2


I'm concerned about @ThreePhaseEel's comment on how the Nest will respond to the switching 24VDC that the diode bridge will provide. Although I'm confident that would be an appropriate location to pull a common wire from, I'm going to construct my own circuit from more standard parts.


Your smart thermostat requires a common wire so it can get power. It doesn't "use" the common wire for anything and your furnace or the new cooler will not need anything to happen with the common wire. This is a simplification, but think of the R wire as positive and the C wire as negative. The thermostat sends the positive voltage from R to the other wires for control, but it needs a negative to complete the circuit and use voltage to power itself.

So, what do you need? Well what you will end up having is a furnace that has a transformer and a cooler that has a separate transformer. Most thermostats are designed for this by having two separate R connectors for each, and those are normally labeled Rc and Rh. It's common for there to be a jumper wire between Rc and Rh on single transformer systems.

You will have an R wire and C wire and control wires coming from your furnace (R to Rh). Then you will have an R wire and control wire coming from the cooler (that R goes to Rc). You don't need a second C wire for anything.

Just a note - the diagram you posted expects a pump control signal on the W wire. I'm not sure how you're going to handle that, but a "standard" thermostat uses the W terminal to call for heat.

  • People report success running the C wire from the furnace, but all they're really doing is running off the Nest battery. This becomes an issue when the battery fails, and it starts short cycling your system to draw power, so I'd rather avoid that by installing a correct C wire. I'm not proposing installing two common wires; I believe Nest expects Rc/C, so I'd like to run it that way if possible. To your note: R->Rc, W->Y1, Y->Y2, G->G perhaps this is a better method
    – Matt
    Jul 19, 2019 at 17:56
  • Ok, since you had the nest working now, I didn't realize the C coming from the furnace would be an issue for it, but I see what you're saying about expecting Rc and C rather than Rh and C. If the cooler uses 24vac for signaling like a normal HVAC system, it will have a C wire. There might not be a terminal for it, but the C wire is just the side of the transformer that R is not connected to.
    – JPhi1618
    Jul 19, 2019 at 18:01
  • Just as a confirmation, the Nest Pro Installer Guide, page 20 " In dual transformer installations...The Rc wire will also charge the thermostat using the C terminal." Typically, swamp coolers are controlled with a 120v wall switch, so to use a 24v thermostat you need to construct your own transformer/control similar to the linked method in my previous comment, or use a pre-fab system like the question links.
    – Matt
    Jul 19, 2019 at 18:19
  • @ThreePhaseEel brings up an interesting point, though, that this creates switched DC when the Nest will expect AC, so maybe creating my own is best.
    – Matt
    Jul 19, 2019 at 18:19
  • @Matt, Thanks a lot for that link and clarification. Looks like I don't know much about swamp coolers.
    – JPhi1618
    Jul 19, 2019 at 18:28

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