I currently have an HVAC system that is connected to a Nest thermostat (2nd generation) and I'm planning to use the same Nest thermostat to control a 110 swamp cooler that is currently not connected.

Before dealing with the trouble to connect both systems to the same thermostat (maybe using switches/relays to shut down one system while the other is on) I want to know if it possible to control the swamp cooler using the Nest thermostat. I don't have too much electrical knowledge, but I've been doing some research and I think it is possible if I buy some additional things.

The swamp cooler has a 2-speed motor, therefore I would like the Nest thermostat to control both speeds using Y1 and Y2 terminals. As far as I know 2 stage thermostats will apply power from the R terminal to the Y2 if more cooling power is required at the same time power is applied from R terminal to Y1. I also want the water pump to work everytime the thermostat calls for cooling (for both speeds), but I want the water pump to run alone for some minutes before the motor is started.

I have found that the Honeywell RC840T-120 might allow me to use the 24v Nest thermostat with the 110v swamp cooler and using a SPDT relay I could connect Y1 and Y2 to the 2 terminals of the 2-speed motor. Honeywell RC840T-120 was created to connect line-voltage controlled heaters to 24v thermostats but I think it will also work with the swamp cooler. This device will be the first relay that will decide if the pump and motor are going to receive power. This first relay will be connected to a PTD102 Delay that will delay the power to the SPDT relay, this last realy is the one that will control the power to the motor based on Y2 terminal from Nest thermostat. The thermostat will send power to Y1 only for 1st speed and in case it needs more cooling power it will start sending power to Y1 and Y2 at the same time causing the SPDT relay to switch power to High through terminal 3 as the following diagram shows:

Nest thermostat connected to swamp cooler

Please let me know if you think this way to connect the thermostat will allow me to control the swamp cooler and if it will also control both speeds

  • I doubt this is on topic here...
    – cde
    Commented Feb 18, 2016 at 23:28
  • 2
    Are you sure the nest is connecting Y2 to C and not R? Commented Feb 19, 2016 at 0:40
  • thank you, I was not understanding the flow for Y2 Commented Feb 19, 2016 at 18:48
  • Did you get this to work. Is there a final diagram of how you got it to work if so.
    – user58377
    Commented Aug 5, 2016 at 21:05
  • Yes @Eric, please see my answer below Final Diagram Commented Aug 6, 2016 at 23:53

3 Answers 3


Yes, it is easy to control a swamp cooler with any thermostat. The problem is that in order to use a swamp cooler effectively, you need strategic openings (typically open windows) to let the air out. This is the part which is hard to automate. The best way to use a swamp cooler is to open windows in the rooms that need the cooling the most. You want as much windowspace opened as possible without any outdoor air coming back in through the windows. There are some automated devices I've seen (such as UP-DUX that vent into the attic but they cause additional resistance so they are much less effective.

Your drawing has a couple of problems...

  1. You need to use the Rc terminal
  2. The coil on your R8222B1062 needs to connect to the Y2 and the C (not the R)
  3. I don't think you want to connect to the C terminal on the nest because your heating system is already providing the C to power the thermostat.
  4. I think you're right about this one but I wouldn't rely on the thermostat powering Y1 and Y2 simultaneously when it called for high. I would rather to design it so that if the Y1 is not powered but the Y2 was, it would still run on high. This gives you a little more redundancy in case of a relay or wire failure.

Below is a drawing that would fix these problems but it is much more generic, the only specific part is that Packard contact delay because I am unfamiliar with a generic part name for that. If you fix 1 - 3, your design should work flawlessly. Again, the exhaust ventilation is your biggest problem.


I would put a nice 24v DPDT relay and a nice 24v 3PDT relay on the appropriate mounting sockets and mount them to a DIN rail. This will make it easy and clean. Put it all in a box with the transformer and the Packard and voila.

  • The Packard delay relay does not need a neutral, from what I can tell. Commented Feb 19, 2016 at 4:42
  • @ThreePhaseEel Thank-you, I just removed that. Commented Feb 19, 2016 at 4:44
  • @MaxfieldSolar Thank you. For now I won't be automating the exhaust ventilation and the heating system is going to be completely disconnected when summer comes. This new diagram looks very good to me, and now I won't need the RC840T-120 which is not easy to get in my local stores. I guess I could move the Packard delay to the neutral wire of the motor if there is the case the thermostat starts with Y2 (maybe if the room temp is very hot and I set a very low temp in the thermostat) just to make sure the motor start is delayed no matter the initial speed (Y1/Y2). I'll be trying your suggestions Commented Feb 19, 2016 at 19:00
  • Update... The drawing below by Javier is better than mine. The only thing I would change would be to try to incorporate a time delay on the blower, but as Dale said, it's not totally necesary. Additionally, I've come to realize that in some cases UP-Dux may be more effective because they can blow hot air out of the attic. I still think they reduce airflow to some extent but a cooler attic may outweigh the airflow restriction. Commented Jan 24, 2017 at 5:20

This is how I ended up connecting the Nest thermostat. I did not connect the C terminal to the transformer since the Nest gives me an error "No power to Rc wire detected" if I do that. So far I haven't had any low battery issues since Nest is able to recharge itself from Rc wire.


  • This design is better and simpler than mine, it avoids the more expensive three pole relay, and it is a cleaner drawing! Good Job! If you still wanted the Packard Delay, just put it on the low wire between the final relay and the motor. Commented Jan 24, 2017 at 4:58
  • I know it's been a few years, but how's the control going? I have a similar circuit designed, but I'm thinking I'll add fan control too so I can tell the Nest y2 is 2nd fan and not just 2nd stage cooling.
    – Matt
    Commented Aug 4, 2019 at 23:42
  • @Matt I still use this circuit for one of my Nest controls and it's still working fine. Though sometimes when it starts the motor does it abruptly Commented Aug 6, 2019 at 0:38
  • Thanks for the response! What did you mount all your relays in? I'm looking at something like this
    – Matt
    Commented Aug 12, 2019 at 4:17
  • I also used a metallic box. This a photo I have but it is the inside of it Commented Aug 15, 2019 at 23:53

I wouldn't follow that recommendation by Mr. Solar due to safety and it's overkill. You don't want the potential to power both High and Low speeds at the same time plus the 3PDT relay is not required for anything, keep the DPDT. The standard method is to use a SPDT contact to change speeds so that voltage is only applied to either the Low or the High but never both. So the proper way is to have the DPDT relay (Y1 first stage cooling) turn on pump and the time delay relay which controls power through the common leg of a second SPDT relay. This second SPDT relay is energized by the Y2 for high speed. Basically what I am saying is that the original poster had the control logic exactly correct (except for R8222B1067 coil needs C) on his diagram and that it follows standard control procedures and I respectfully recommend to not follow the advice given later by Mr. Solar. Nest has intelligence and doesn't care if you terminate R or Rc since it automatically will jump these two connectors. Your problem is two transformers. I suggest replacing the RC840T-120 with a second R822B1067. The transformer power will be supplied by your existing furnace transformer already connected. You didn't show your heating wiring so I don't know if you had to use the C terminal currently or not. Many times you don't need the Common wire terminated at the Nest. Wire everything like you originally show except the R822B1067 coil goes to C not R.

Also UP-Dux "are much less effective" is not my experience at all. They are great and don't cause any significant reduction in effectiveness. Plus it's secure since windows are not left open.

FYI: The time delay isn't critical, you could leave it out and the minute or so it takes your pads to get wet may not be worth it to you. I have two coolers in my house and do it both ways and the time delay wasn't worth the extra cost. But if maximum effectiveness is your desire then I would put it in for the value of pre-wetting the pads to prevent warm air from entering.

  • Thanks. Since I was not able to get a 3PDT relay I followed your recommendation to use SPDT relay energized by Y2 for high speed Commented Jun 24, 2016 at 18:22
  • You're right, If the furnace is already powering the nest, do not to connect this transformer to the C terminal on the nest. If you look at the upper right corner of the drawing, you'll notice that it already said that. Also, if you look carefully, my design specifically makes it impossible for the high and low to be simultaneously powered. Thank-you though, for the idea on how to use SPDT relays as that helped Javier to come up with a much better drawing than mine. Commented Jan 24, 2017 at 5:15
  • If the furnace is powering the nest it may not be necessary to provide an extra transformer, just tap into the C and Rc terminals at the nest.
    – Jasen
    Commented Mar 15, 2020 at 23:41

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