Controlling 110v swamp cooler using Nest thermostat

Question

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:

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

Answer 1

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.

Answer 2

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.

Answer 3

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.