I have an evaporative cooler with 120VAC control that I want to switch on and off using a relay that is activated when the cool call comes from my thermostat. I plan on just using 1 relay to activate the whole cooler control (rather than multiple relays to control the different aspects of the cooler), so this is a simple on/off when cooling call comes from the thermostat.

All of the wiring for the thermostat goes to the air handler and then heat, fan, etc. kick on from there. I have a C wire running to the thermostat. My thought was that I just put the relay in line with the Y wire (run Y from thermostat to relay, then out from relay to the air handler). This puts 24VAC across the relay (which is what the relay is rated for), but does not throw the switch.

  • So you've already tried wiring this up but it's not working? Could be the power supply for the 24vac cannot provide enough current to pull the relay.
    – bcworkz
    Jul 14, 2013 at 18:53
  • I've wondered about this. Any ideas on how to wire if I need to put in a 2nd 24VAC source?
    – jmg229
    Jul 14, 2013 at 20:40
  • Could you please include a wiring diagram of what you're trying to do? This will help both us, and you understand the problem a bit better.
    – Tester101
    Jul 16, 2013 at 13:00
  • Does this diagram represent what you're trying to do?
    – Tester101
    Jul 16, 2013 at 13:30
  • Tester101, that diagram does represent what I'm trying to do
    – jmg229
    Aug 11, 2013 at 22:12

4 Answers 4


You'll want to connect the Y and C wires to the coil of the relay, so you end up with something like this.

enter image description here

When the temperature switch in the thermostat closes (calling for cool), 24 volts will be applied to the relay coil.This should pull the contact in the relay to the normally open (NO) position, allowing electricity to flow to the evaporative cooler (or whatever 120V device needs to be powered).

enter image description here


It actually sounds like you don't have it wired correctly. You should have a wire from the C terminal connected to one side of the relay, and the Y terminal to the other.

To verify, use your multimeter to test that you measure 24V AC across the relay coil when the thermostat calls for cooling.

  • This was actually my 2nd guess. Tried that as well, no luck. I tried it both with the Y from the thermostat and the line to the relay attached only to each other and with them both attached to the Y on the air handler as well.
    – jmg229
    Jul 14, 2013 at 20:40
  • 1
    What does your multimeter measure? If you don't have one, go buy one, you need it to diagnose what's happening.
    – gregmac
    Jul 14, 2013 at 20:47

I would look for the green wire (fan) at the air handler. You might as well use this control voltage since you would want the evap cooler active when the fan is on. Measure the voltage at the fan relay to ensure it's 24 VAC (remember this value). Run wires from the green wire to your relay and also connect the common. Does your relay activate when the fan turns on? If the 24 VAC transformer can't handle the extra load, it may not turn on the fan. What is the voltage at the fan relay now? Alternately, you could replace the fan relay with a 2-pole version to give you the extra contacts for the evap cooler while only using a single relay.

Hope this helps.

Thinking about this again, use the cool control since you do not want the evap cooler on during heating mode. Or stay with the fan signal and place a normally closed relay in series wired to the heat signal. That way you would have evap cooling any time the fan is on (which you want for cooling) and the evap cooler is off during heating operation.


Late answer, but I did this on my cooler. Since my thermostat requires batteries, it doesn't use the C connection. Run one lead of 24ac to the thermo (R) then take (Y) from the thermo to one input of relay coil. Take other lead of 24ac and connect to the other input of relay coil.

My wiring

The black wire goes the the 24vac transformer. The grey wire going down goes to the relay inside cooler. The grey wire going up/left goes to the theromstat, hooked to R and Y. It's setup like this because my cooler is in a window. In winter I just disconnect the wires, shove inside the window and close it. Since the polarity doesn't matter, you hook it back up in Y configuration, always works. Yes, that's Cat5E cable, I'm using a pair for each run.

Relay is in inside the cooler, outside. Thermostat is upstairs in the hallway. 24VAC transformer is inside house, plugged into outlet.

One problem I have is the current draw on this huge cooler welds the contacts. I would suggest a 24ac contacter, or a 40 amp relay.

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