I currently have a Nest controlling my LP furnace. I'm getting an evaporative cooler installed and I'd like to also control it with my Nest. This post gave me a good starting point, but I'd like to expand on it some to

  1. Be able to control 1 stage heating, 2 stage cooling
  2. Keep manual fan control of both furnace and evap. cooler
  3. Automate pre-wet stage (pump runs for specified time before fan kicks on)

Since I'll have a dual transformer system, I know I'll need to use both the Rh and Rc terminals, and run the C from the cooling side. Here's is my first WAG at a schematic (sorry if it's confusing, first time drawing a schematic).

Please let me know if I've made any boneheaded mistakes, or if you see any way I could improve it. The only thing I'm missing is fan only in high speed, but I'm not sure if I actually want that (I rarely run the fans, I more just like things installed to work properly/as intended), and pump only (but the only point of that setting is to pre-wet, which I'll accomplish with the delay).

I'm willing to lose manual fan control if you guys think I should just drop that (remove heat fan relay and cool fan relay).

1 stage heating, 2 stage cooling


I don't think my heat fan relay works as wired (if I called for just a fan on my furnace, the W wire wouldn't energize, otherwise I'd be getting heat too). Also, I think the G terminal on the Nest is only powered by Rc, so I'm not sure I could get manual fan control of my furnace (maybe with a latch relay as @JPhi1618 suggests, but that would require user logic that I'd rather avoid). So I'm dropping fan control.

I also wanted to make sure the fan was delayed if the Nest started in high speed, so I moved the location of the delay.

Finally, I was having trouble finding appropriate DPST relays, so I changed the cooling relay to a SPST.

If I don't hear any objections/comments, I'll order some parts, breadboard it up, and check back in with this post to update how it goes.

updated 1 stage heating, 2 stage cooling

  • Looks good (with my limited swamp cooler knowledge). When designing the delay, be sure you know what happens if the call for cooling is canceled before the delay is over and then cooling is called for again. I had to make a similar "manual" control scheme to get a heat pump working with a zone controller that wasn't totally meant for my system.
    – JPhi1618
    Commented Jul 23, 2019 at 13:57
  • I plan to breadboard all the components and make sure they're working as intended before I wire it all up. I need to confirm what wires Nest will energize during each mode. I don't think I can run the furnace fan independent of heat with the current relay, so maybe I'll just have to lose that functionality.
    – Matt
    Commented Jul 23, 2019 at 15:06
  • Maybe use a latching relay so that the "fan only" call would run the fan of the last-used system? Probably a slight improvement from what you have now, but not a big deal.
    – JPhi1618
    Commented Jul 23, 2019 at 15:09
  • If you're running fan-only, does it really matter which fan you run? Just run the swamp cooler fan.
    – longneck
    Commented Jul 23, 2019 at 19:59
  • Just wanted to be able to run either fan if desired, since independently they both have that functionality. I've resigned myself to losing that, though.
    – Matt
    Commented Jul 23, 2019 at 20:29

1 Answer 1


I purchased some relays and everything seems to be responding as intended. When I get the cooler installed, I'll run the 120V lines too and hopefully we'll be in business.

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