The Prepper Journal

How to Build an Evaporative Cooler – Grid Down Air Conditioning

Portable Swamp Cooler. Click for larger image.
Portable Swamp Cooler. Click for larger image.

How to build a low-cost 12 volt evaporative cooler.  My husband and I built two swamp coolers that we use to air condition our tent when we camp in hot, dry weather at Burning Man.  These compact units blow cool air like an air conditioner, but they use just water for evaporation instead of using a compressor or toxic Freon. There are two great designs on the internet for making evaporative coolers for camping.  This blog post enhances one of those designs by providing lots of detail on sourcing parts, comparing options for parts, and a variety of power options.  We built ours for under $40 each and they worked great for cooling a tent and letting us nap in the Nevada desert.  After building one, we hosted a workshop where we helped neighbors build 5 coolers, which let us buy parts in batches and save on shipping costs.

Reference designs:

We used the following materials:

  • 5 gallon bucket from the local hardware store or recycled, $0 – $2.30
  • 12v water pump from, $18.50 + shipping = $20
  • Swamp cooler pad 33.5″ x 24″, $2 – $7
  • Drip irrigation tube 48″ long, Tee connector, diameter should fit your water pump, $5 – $10
  • Recycled PC fan, 12VDC and around 1 to 2 Amps, $0.50 – $4
  • Elbow duct 4″ diameter, $3
  • 12 volt battery, $10 – $20
  • Screws, 4 Short sheet metal screws, $0.50 – $1
  • Quick connect female crimp connectors, sized for your battery
  • Four to eight feet of small gauge wire (20 – 26 gauge)
  • Butt splice connectors, to connect the wires butt to butt, in the gauge of your wire
  • Optional: window screen 33.5″ x 13″, $0 – $6
  • Total:  $40 to $73

Tools that you will need to buy or borrow:

  • Drill
  • Hole saw drill bit, 2″ to 3″ , $13
  • Wire cutters and very small gauge wire strippers, 20 – 26 gage
  • Tin snips for cutting your elbow duct
  • Optional: silicone caulk

How do I choose a good recycled PC fan?

Recycled PC fan screwed to lid of bucket. Note the edges of the elbow duct. Click for larger image.

You can get a great deal if your town has a store where you can get recycled computer parts, or perhaps your local Goodwill has electronic stuff.  We got a bunch of fans for 50 cents each at PC Recycle in Seattle.  Or for a bit more money you can buy recycled PC fans off the internet from places like or  Or you could pay the full price for a new fan from your local computer store or  Shame on you for paying full price.  Retail is for chumps!

You want to read all those letters and numbers and find “12 VDC” which means 12 volts of direct current.  Direct current is what you get from a battery or solar panel.  Next you want to find a fan that will move a lot of air by drawing a lot of current, so you look at the number preceding the “A” for amps.  For example, “0.11A” or “0.20 Amps” are pathetically weak, and a “1.2A” or “1.6A” fan will blow a lot of air!    The bigger the number, the cooler you will be, and the faster it will drain your battery.  My fan was 1.6A and our little $10 battery ran the fan and pump for 5 hours before we noticed the battery getting low.

Online the listings will also describe the dimensions of the fan, such as 80mm or 120mm.  The size doesn’t really matter as long as it is in the ballpark of about 80 – 120 millimeters.  You’re going to drill your own mounting holes in the lid of your bucket so the exact size doesn’t matter.  Bring the fan with you to the store when you buy your elbow duct.  Your elbow duct diameter must be smaller than the distance between the corners of the fan’s mounting holes.

The number of wires coming out of the fan also don’t matter for this application.  You will only use power and ground, typically red and black.  The other wires control the speed of the fan if it were connected to a computer, but we don’t care.

Which Swamp Cooler Pad to Buy?  Synthetic or Organic?

You can buy swamp cooler pad that is made of organic or synthetic fibers.  You can read the marketing stuff about which one is better, but here’s my experience from hosting a workshop where a bunch of people built 5 swamp coolers in my yard and some of them used organic and others used synthetic pad.  When you cut the pad, little bits will get everywhere.  The organic pad that got all over our garage and yard was quickly used by birds and squirrels to make energy-efficient nests in the trees.  The synthetic blue bits seemed to get ground into the floor and dirt by our feet.  The synthetic bits were difficult to remove with a shop vac or broom, perhaps because they had some static charge making them cling?  The organic fibers smell like wood chips or hay, and the synthetic pad has a slightly plastic smell.  Neither smell was offensive to me, but some folks didn’t like the  smell of hay when the swamp cooler was operating.

I recommend purchasing a roll of the organic Aspen Cooler Pad, and cut rectangles to share it with friends and neighbors.    We cut one piece of pad into a  33.5″ x 24″ rectangle and then folded it in half, and rolled it into a cylinder that fit inside our bucket.  The instructions on the ePlaya link explain how to cut two perfectly fitted rectangles using the synthetic pad by Duracool.

Battery Options and Solar Energy

For maximum portability, we used a 12 volt sealed lead acid battery for about ten to twenty dollars.  This type of battery is often used to run a computer backup uninterruptible power supply, or UPS.  You will want a battery which can give enough current to both the fan and the water pump, so 1.2 AH rating on the battery is quite sufficient.  AH stands for Amp Hours.  We ran our swamp cooler with the big 1.60 Amp fan for 6 hours and it ran down our battery enough that it wasn’t blowing very hard anymore. and sell batteries like this, however you’ll save money on shipping if you buy from a local hobby store, electronics store, or computer store.

If you’re traveling with a car or boat, then you can run your swamp cooler on your 12V vehicle battery.  When we go to Burning Man, we take a deep cycle marine battery that weighs about fifty pounds.  That huge battery runs our lights, a stereo, and our swamp coolers for 9 days.  The marine battery is easily recharged with a small (12″ x 4″) solar panel and a charge controller.

You could try using this solar-powered water pump for $25.  However you would still need a power source for the fan.

Steps to build your Evaporative Cooler

Generally, we followed the instructions for “Cooling your Tent or Van” by FigJam on the ePlaya website.  Here I will reference his instructions and add my own tips and alternatives.

Step 1 Drill Holes in your 5 gallon bucket

FigJam drilled one row of big holes, we drilled two rows of 2″ holes because that’s the size of hole saw that we already owned.  This step is messy and fun.  Use a shop vac to cleanup the mess.  The goal is to maximize air flow through the damp pad, and leave enough water in the bottom of the bucket to cover your water pump for the entire duration of the runtime (e.g. six hours of sleep).

Key considerations on where to drill the holes:

  • Your water pump must be completely submerged or it will run dry and break.  Mark your water line that gives you at least 2 inches of water above your pump intake.  Cut your ventilation holes above that water line.
  • The holes should be at least 4 or 5 inches from the lid of the bucket.  You want the air to come in through the holes, pass through the damp pad, then come out through the fan at the top.   If the air passes through more damp pad, then that’s great!  See pictures.

Step 2 Cut your Screen and Cooler Pad

Two rows of 2″ holes above the water line.

The optional window screen helps to keep the cooler pad and the drippy water inside of your bucket.  I tried gluing the screen to the inside of my bucket, but that didn’t work well, so don’t bother.  The screen and cooler pad should cover all of your ventilation holes.  Measure from the inside bottom of your bucket to at least 3 inches above your ventilation holes.  Leave an inch or two at the top section of the bucket for your PC fan and wires.

You want to force ALL the air through the damp pad.  If air can sneak in the air holes and out through the fan without going through the damp pad, then it won’t be cool!

Option A exact dimensions:  Measure and cut your screen in a trapezoid shape because the bottom circumference of your bucket is closer to 32″ and the top diameter of the bucket is about 33.5″ if you are using the same bucket that I did.    Measure the diameters of the inside of your bucket near the lid and near the bottom, and then do some math (diameter * 3.14 = circumference).  Now measure and cut your window screen and test the fit before measuring and cutting your cooler pad.  For the cooler pad, cut your trapezoid slightly smaller to allow for the thickness of the pad.

Option B Close Enough Engineering:  Cut your screen to a rectangle about 34″ x 13″ and it will overlap some when you stick it in your bucket.  Cut your swamp cooler pad about 33″ by 24″, fold it in half and shove it in your bucket.  Trim it if you have to.

Step 3 Drip Irrigation

FigJam used clear poly tube, we used black drip irrigation tube and a little bit of clear poly tube.    Measure the dimension of your water pump output spout and purchase the appropriate size hose.  Take your pump with you to the store.  Buy whatever is cheap and available by the foot from your local store.  Buy the appropriate size Tee connector for your hose and any connector that you might need to connect the hose to your water pump.

Swamp cooler pad with drip irrigation tube.

You’re going to make a halo shape with the irrigation tube. The diameter of your drip tube halo should be about one inch smaller than the dimension of your bucket, so that your tube rests in the middle of your swamp pad ring.  If your halo is too close to the edge of the bucket, then the water will drip out of your ventilation holes.  Again, circumference = diameter * 3.14.  Adjust your circumfrence result to include the amount of the Tee connector that will complete the halo.  Now cut the irrigation tube.  Use the Tee to make the circle first, and then attach a pice of irrigation tube that will go down the middle of your bucket to the pump on the bottom.   Place your halo inside of your bucket with the cooler pad and make any adjustments to the size and shape of your irrigation tube halo.

Next you poke holes in the halo to let the water rain down on the cooler pad.  Carefully place your halo on a flat surface to make the holes on the bottom of the halo so it rains down on the cooler pad, and not up on your fan!  Space the holes equal distant from the next hole, about the same size.  We tried three methods of poking holes in the irrigation tube.  You can use a drill with a very small bit, and be gentle to not drill all the way through the tube!  For more control, you could use a small nail and manually poke the tube, or use a gentle tap from a hammer to poke the hole.  My preference was to use a light weight hammer and two gentle taps for each hole.  A fourth option that we did not try was heating a piece of wire and melting holes into the irrigation tube.

After you poke holes, test your irrigation system.  Put your cooler pad in the bucket.  Pour water in the bucket.  Attach your water pump to the irrigation tube halo and connect the battery.  How much water is coming out?  Enough?  Is the water coming out of all the holes, or just the holes nearest to the Tee connector?  Decide if you want to make your holes larger or make more holes, or scrap your halo and start over with new tube.

Step 4: Install duct on and fan on the Lid

Fan and duct screwed to the bucket lid

Wear gloves so you don’t cut your hand on the sharp metal.  Cut a hole in the lid so the duct just fits through snugly.  Mark a 1 inch line on one end of the duct.   Use the tin snips to cut narrow triangle-shaped notches out of the elbow duct, to form little flaps.   Stick the duct through the lid and bend the flaps you just made.  The flaps should be on the inside (bottom) of the lid, and the elbow duct should be on the outside (top) of the bucket’s lid.

Hold your fan against the bottom of the lid so that at least 3 of the four mounting holes on the fan will go through metal flaps and lid.  You want your four screws to securely sandwich the metal flaps between the plastic lid and the fan.

You want to buy sheet metal screws.  They have very coarse thread, which means fewer turns per inch.  You also want them to be fairly short screws, shorter than an inch.

First use a drill to make a tiny hole in the bucket lid.  Then just use a manual screwdriver to push the sheet metal screw from the top of the bucket lid, through the bucket, the metal flap and the fan.  alternatively, you could use a nut and a bolt, but if you try this option then you must use a lock washer to prevent the nut from shaking lose.

Step 5: Wire everything together

Connect all the wires.  First strip the wires to expose a good 1/3″ (1 cm) of copper core of the wires.  We used butt splice connectors that look something like this (click for example).  Decide how long a wire you want from your swamp cooler to your battery.  The wires from the PC fan will likely be really short, so use the butt splice connectors to make them much longer.  Use Red wire for the positive wires, or use black wire and mark it with a flag of red tape.  You’re going to make a Y shape when you connect the positive wire from the fan and the positive wire from the pump to a single wire using the butt connector.  The singe wire will then connect to your battery.  This is called wiring in parallel.  Do the same thing for the ground wires.

Our battery used Quick connect female crimp connectors.  Your battery may use a different shape of connector.

You’re done!  Test it a few times before you make any significant modifications.  Optionally, seal the gaps around the elbow duct with caulk.


If water gets outside your bucket and onto the floor: Perhaps you need more swamp cooler pad to absorb the water, or perhaps your pump is squirting too hard in a direction that aims the stream directly at the ventilation holes.

Air doesn’t feel very cool: touch your swamp cooler pad and check for uniform wetness.  Verify that the pump is working  and the water is dripping uniformly around the halo.


How much water does it use?  Answer:  I add about a gallon of cold water that we drain from our drink cooler to the bottom.  Then each day we top it off with another pint or quart, depending on how much we run the cooler during the morning sleep or afternoon nap.

How long do the batteries last?  Answer: the batteries really depend on a lot of factors, such as wattage of your fan & pump, age of your battery, and temperature of the battery.  We topped ours off with a tiny solar panel (the size of an ipad).  If you plan to go completely nocturnal at Burning Man, then you will want a small, deep cycle marine battery to run your coolers for 6 – 8 hours every day while you sleep.  Keep the battery in the shade and off the ground with two wood blocks or a milk crate to allow air circulation.

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