I read about a device called a Mistbox from Gizmag and decided to attempt to make one without all the fancy control circuitry that Mistbox said they put on theirs.
Misting an A/C unit uses the technique as swamp coolers (evaporative cooling) where water droplets are used to cool the air. The A/C unit is then able to transfer the heat from inside the house to outside much more efficiently. I figured this was a win if I could get the mist to turn on with the A/C unit and turn off again when the A/C turned off. Thankfully both the A/C unit and sprinkler solenoid valves use 24 volts AC to signal which makes this hack incredibly easy to pull off.
This should go without saying but working with an A/C unit or your HVAC system can be dangerous! There are fan blades that can start suddenly and without warning, high voltage that can electrocute or burn down the house or destroy delicate control electronics, and a myriad of other things that can go wrong. Please use extreme caution and get help if you are not comfortable or familiar with a task at hand.
I assume no liability for anything you do to your own system nor for any harm that befalls you. Please check all voltages, users manuals, and consult or hire a professional if you are not familiar with the work detailed in these instructions.
Step 1: Gather Required Materials
Some stuff I had around the house but most of the supplies I had to buy. Here's where I got everything and prices for what I bought.
Solenoid Sprinkler Valve - Home Depot - $12.97
Hose to 3/4" PVC Threaded Pipe - Home Depot - $1.86
Two Way Hose Splitter - Home Depot - $6.47
50' Garden Hose - Home Depot - $9.97
20' Misting Kit - Lowes - $24.97
3/4" PVC Threaded Pipe Male to Hose Female - Lowes - $4.19
Hose Male to Hose Male Connector - Lowes - $4.29
Wire Nuts - On Hand - Free
Screwdrivers - On Hand - Free
Strap Wrench - On Hand - Free
Pipe Thread Tape - On Hand - Free
Twist Ties - Hose Packaging - Free
There is low power circuitry wire in the picture but I ended up not having to use it since I could just tap off the A/C unit directly.
Step 2: Assemble Hose Side of Solenoid Valve
Wrap pipe thread tape around the threads of the hose to 3/4" converter and screw it into the solenoid valve. Note the arrows that show the direction water should flow through the valve and connect this to the in side. This valve had a 3/4" to 1" adapter that I had to tape and screw in before the hose connector but yours may not. Make sure to get these connections as tight as possible so they don't leak. The strap wrench definitely helps here.
Step 3: Assemble Mister Side of Solenoid
This side of the solenoid also needed to be stepped down from 1" to 3/4" with the included adapter. Then I used two more adapters to get back to a hose side threads and from female to male connections. Be careful not to overtighten this side as the metal connectors could cause the plastic to crack but get it tight enough that it won't leak and use pipe thread tape on all the connections. Note that I used the metal connectors here as they do not freely spin and it makes it easier to connect the mister kit later.
Step 4: Assemble and Attach Mister Kit
Follow the included mister kit instructions to get the hoses, misters, filter, etc ready for use. The kit I used had fairly easy to follow instructions and since every kit is potentially different, I'll leave it to the manufacturers to show you how to assemble their product. Note that you should use rubber washers between all connections and that you shouldn't need pipe thread tape but it never hurts either.
Step 5: Connect Wires in A/C Unit
Use extreme caution! High voltages exist inside A/C units that could kill or injure you. If you're not comfortable doing this work yourself, find someone who is qualified to make the connections for you.
Turn off the thermostat so that the HVAC fans and A/C unit won't start up while you're working on the unit. Then, locate the circuit breaker for the A/C unit and cut power there as well. Once you're sure all power has been removed from the unit, find the side panel that hides the electrical components. It shouldn't be difficult but if you're having trouble, look for the panel where all the wires are going to.
Once you open this panel, you should see some larger wires connecting power from the house as well as a few smaller wires that carry the signal to turn the unit on from the central HVAC system. On most systems these carry 24 volts AC which is the same as what's used by sprinkler systems. You should be able to simply connect one wire from the solenoid to each of the signal wires so that the AC unit and the solenoid are on a parallel circuit to each other. Secure each signal wire, solenoid wire, and A/C unit wire with a wire nut and make sure no wires are exposed as that could cause a short circuit. I also attached a zip tie so that the wires for the solenoid could not be easily pulled out the bottom of the A/C unit.
Step 6: Secure the Misters to the A/C Unit
I used twist ties but it's a pretty janky solution to attach the misters to the outside of the A/C unit. Find a way to attach the Misters to the outside ideally at the top and pointed down at about a 45 degree angle away from the unit. Let me know if you come up with a better solution so I can upgrade my setup.
The idea is to mist the air surrounding the unit and have that sucked up past the coils where the heat exchange is taking place. So try to point the misters in such a way as to cover the whole side of the unit when they spray. That way you get the cooling effect on as much of the coils as possible.
Step 7: Add Water and Test
Connect a garden hose to the in side of the solenoid valve, turn back on the breaker for the A/C unit, and set the thermostat to cool the house to a temperature lower than what it thinks it currently is. Watch the A/C unit to make sure it turns on like normal and that the misters turn on and are spraying in the correct directions.
It was 96 degrees here a few days ago and getting home from work it was 76 degrees inside the house. It took our A/C unit ~3-4 hours to cool the house down to 74 degrees. After installing the mister on the A/C unit, it was a happy coincidence that the house was again at 76 degrees but today it's 99 degrees outside. It took the A/C unit 1 hour and 15 minutes to cool the house. A significant decrease in the time taken and the energy spent.
For the tl;dr:
96 degrees outside, 76 to 74 degrees inside took ~3-4 hours
99 degrees outside, 76 to 74 degrees inside took ~1.25 hours