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?