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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:

Without Mister

96 degrees outside, 76 to 74 degrees inside took ~3-4 hours

With Mister

99 degrees outside, 76 to 74 degrees inside took ~1.25 hours

<p>It's a great idea of which I learned decades ago.</p><p>These misters will always save a lot of money depending on how much water can be evaporated before air saturation occurs.</p>
<p>Are you using distilled water or soft water for this? The hard water deposits might eventually build up on the outside of the unit decreasing its efficiency and or clogging the misters. I would hate to having to descale the fins on that unit.</p><p>I always thought it would be a good idea to utilize the condensate water from the unit, and the condesnate water is already distilled. Maybe this could be done instead of using connecting to your water supply.</p><p>I liked how you wired the solenoid directly to the unit, and your right be careful in there.</p>
<p>I also considered using condensate water. Only two extra things needed would be redirection and (possibly) a small pump to generate enough pressure to mist.</p><p>Alternative to the pump could be some absorbent wicking thing similar to the type used in furnace humidifiers.</p>
No, it's just water from the Utah mains which is very hard. Currently the unit is hit with the sprinklers and hasn't had any buildup but I could just be lucky so far.
<p>I see all 5 colored wires from my A/C thermostat coming into my A/C unit outside... I chose the Common and the 24VAC lead to use. The other wires were heat/cool, compressor/ defrost heat etc. But my valve won't shut off after the unit shuts down... Any ideas?</p>
<p>You probably need to select Common and Yellow, but you probably already know this by now.</p>
I would try the cool and ground. I believe those should activate when the compressor turns on.
<p>This is really incredible. Such an simple ingenious mod. I bet it would really save on power costs especially for people in dry climates.</p>
mspine45@gmail.com
Great solution! That Mistbox is $449.00! I wonder if you can qualify for the tax credits that Mistbox users get?
Really nice diy solution! I contemplated doing this but my air conditioning is inverter type and packed with sensitive electronics. The high moisture content would damage my inverter unit. For standard air conditioning units, sure could be worth a try once the condenser fins are treated for corrosion.
<p>Can you let us know if you have noticed a considerable change to your AC Bill? The mistbox claims a significant savings up to 30%?</p>
This month was far hotter than previous months and last year around this time so it's hard to tell but I think it's saved us some money on cooling costs?
when connecting to the thermostat wires, I noticed mine has 5 wires instead of 2. Which 2 do you connect the solenoid to to get the 24 volts. I don't want to fry my ac. thanks for the great instructable.
<p>There are usually 5 in the central HVAC unit but only two that actually go to the a/c unit (a/c and 24v). The others are for heat, fan, and ground. You'll need to check for your specific unit but usually there's a schematic printed inside one of the doors or you can check which wires are actually out at the A/C unit outside. Here's a link to a site explaining common wire colors: https://wiki.xtronics.com/index.php/Thermostat_signals_and_wiring</p>
thanks I got it set up. it works great we will see how the next power bill is. I appreciate your instructable!
This is awesome, now if my wife will let me tinker on the AC unit this would save so much money and its clever you my friend truly think outside the box.
<p>I recently saw the article on the Gizmag site as well, and I was trying to come up with a way to have the water come on like you have done here. Now I just need to go to Home Depot and pick up some stuff. Thanks for the great article.</p>
No problem at all! Glad I could help :)
<p>The name brand units add a water filter to help soften the water a little. You change the filter once a year. A home water softener helps as well. Obvious.</p>
<p>Not sure if its a typo or misunderstanding but they are called swamp coolers.</p>
<p>Complete typographical error on my part. Thanks for catching that</p>
Although the concept is great (cooling towers depend on water evaporation as well) I would caution if your water supply is high in dissolved minerals (hard water) since that will scale up your coil.
<p>Currently the unit is hit with the sprinklers and hasn't had any buildup but I could just be lucky so far.</p>
That's a really good idea! The only down side I can see to this is that unit can get clogged with wet debri. That's not good for your unit. So you would have to do weekly cleaning or as needed.
Thanks for the heads up, I'll watch for debris buildup and clean the unit if I see any. Thankfully it's not too dusty here in Utah so it shouldn't be too bad.

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