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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

Matt
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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.

Matt
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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.

JPhi1618
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So I tried this. But then I thought I burnt out the transformer. It turned out to be a broken belt. I tapped into the center of the diode bridge, and I think I measured around 29 Volts between the common and the red wire at the time. In case its not obvious, this converts to 24 V DC. Somewhere else in the circuit I used a SS Relay to operate the furnace blower, since that's the lowest power relay. Then the old furnace fan stopped going, replaced the capacitor, but probably needs a new motor. It lasted around two months so far. It was the last one available on Amazon at the time.

user243145
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