How to Build an Evaporative Cooler


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 Amazon.com, $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?

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 www.vetco.com or www.allelectronics.com.  Or you could pay the full price for a new fan from your local computer store or www.FrozenCPU.com.  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.

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

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.  Frys.com and AllElectronics.com 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.

Troubleshooting:

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.

FAQ:

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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About Christin

I build robots as a hobby and I work with software as my career. Last year I mentored a FIRST Robotics team, Team XBot. I worked at Microsoft for 11 years in such roles as developer tech support, Developer Evangelist, and Program Manager in Visual Studio.
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27 Responses to How to Build an Evaporative Cooler

  1. FIGJAM says:

    The only thing you did’nt mention was to make sure the pad material goes clear to the top of the bucket.

    This will create a seal that will force ALL the air through the damp pad.

    If air can get around the pad at any point, the cooler will not be as affective!

    Otherwise….Great job!!!!

    FIGJAM

  2. Bill Woodring says:

    I made one following your directions but used a tall Coleman ice chest with handle and ducted it to a window in my small camping trailer. The ice chest does not sweat and the water really stays cold.

  3. Robert Rhu says:

    How much daily water did you go through when using this on the playa?

    • Christin says:

      On the playa, we used about a gallon or two to start with, then topped it off with a quart or less each day. We used the slightly dirty water that we drained from our food/drink cooler. Before you turn on the swamp cooler, you can use a cup to manually pour water from the bottom of the bucket over the evaporation pad. A pre-soaked pad will start blowing cool air immediately.

  4. misterbubble says:

    Do you run your cooler inside or outside of your tent/trailer?

  5. Leah says:

    This is a great tutorial! I’m starting to prepare for my first burn and am so excited to use my hands a bit! Could you tell me how large of a space did you cool using the 5 gallon model? I’m building an 8′ hexayurt and I’m wondering if it will be this swamp cooler will be sufficient.

    • Christin says:

      If you’re building your yurt with the insulation board, then yes this will be sufficient. In the hottest part of the day, it helps to have it blowing directly on your legs while you sleep. I find that blowing on my face is annoying, but the legs can handle the feeling of the breeze and it cools your body well. If there’s 2 of you in the bed, then you can aim it to blow across your legs & torso from the side of the bed/air mattress/futon.

  6. Leah says:

    There seems to be a disparity between the amount of water your cooler uses and the amount of water that figjam’s uses. is this because he uses a double layer of the cooler pad or a more powerful fan? or something else? trying to figure out how much water to bring to the playa.

  7. Joyce says:

    We live in a very humid climate, and I can’t imagine this set-up not turning into one big moldy mess. Any solutions?

  8. sharpstick says:

    it wasn’t clear in your writeup, but the incoming air has to come from outside the tent, otherwise humidity builds up and cooling stops.

  9. Bob says:

    In Arizona the water usage depends on the dew point and other factors as water is “lost” to the out put air. The drier the incoming air (<50* dew point) the better it works and more water you use.I used over 1 gallon an hour on 1,000 square inches of pad with a much bigger fan. Also recycling air back through does not help cool as the air has already absorbed all the moisture it can, It needs fresh air to work..

  10. PapaWhisky says:

    I’m late to the party–but thought I’d add a couple of things. My house has no ducting, and so I installed a swamp cooler to cool off in the summer. Probably one of the best investments I’ve made in my home.

    I’m sure you all know how and why it works, but I noticed all the advice is to place the swamp cooler in the room/tent/dwelling with you. It would certainly be more efficient and effective to place the cooler outside of the tent with some short ducting to an opening. Having the cooler in the same room will make it gradually lose it’s effectiveness as it adds humidity to the air. This is why swamp coolers are used in arid parts of the world and not jungles or, ironically, swamps. The whole process relies on evaporation, and the less humid the air being pulled into the unit the more effective the cooling. This will obviously result in an increase in the water needed, though.

    With the larger home units, when you turn on that fan you’re basically pressurizing your home–and you cool off certain rooms in the house by opening a window in it to “steer” the airflow. The unit itself is outside of the building in the drier air.

  11. Richard Amirault says:

    Running a battery until the fan slows (or the light dims) is a VERY POOR way to determine when to recharge. If you run a rechargable 12v battery below 10.5 volts you will have damaged it. Yes you can recharge it, but it will die an early death.

  12. Ryan says:

    I would just like to mention that I got a perfect submersible water pump at lowes for $18 that was more than strong enough. Also, I didn’t have time to drive there but there is a ‘grow shop’ about 30 min from my house that sells the same pump for $8. I used one in my aquaponics setup for a while.

  13. Gordon says:

    If more coolness is wanted, a taller bucket could be used so another row or two of holes could be made, and therefore more cooler pad. A 6 gal. bucket can be found at a homebrew supply shop.

  14. Boab says:

    >> Vetco.Com is an unresolved/broken link. I bet you meant Vetco.NET in Bellevue – and I like them for all sorts of cool stuff too :o)

  15. pooter says:

    Did you leave the battery hooked to the solar the entire time? Thanks!

  16. Old Harry says:

    50 years ago in El Paso we had a shop-built swamp cooler mounted in the top half of a door. It did well in coolong, the fan noise drounded out noise from the airport, and the cooler pads removed some of the dust. Real up-town folks had a roof-mount job and ducring to each room. But as has been said, a window must be ajar to let the air out or it just doesn’t work. A neat trick is to have a swamp cooler and a half ton A/C – that cools and dehumidifies.

  17. eksine says:

    I have to say a few things about this. the original has larger holes, single row, starting a little bit higher than where your holes start. yours is more so their is more surface area for the fan to suck in. less surface area (holes) might create a vacuum and increase air pressure. also using 2 rows will restrict you from being able to put more water in the bucket (not really important but, I think you put water 2″ above the pump, the level of the water might make a difference). I think the height of where the intake holes are drilled would contribute to the optimum airflow too. just like in a pc case the case fans are designed to flow air in and out a certain way. another thing that concerns me is the drip irrigation looks like it’s spilling half of the water to the side of the bucket, instead of over the filter. also the filter material is much bigger than the original and the empty area in the middle looks tiny. there may be a airflow constriction . aspen filter may be denser than the original synthetic filter. i can’t see it clearly but it looks like theres an airleak around the fan because its sitting on top of the metal ducting instead of flat on the lid then having the ducting on the other side with caulk to seal it from leaks. I’m not sure what motor is being used but if the specs are way different it could change the efficiency a lot. the original was a 1.6 amp 3700rpm with 151 cfm flow. I know that the motor selection is important and should be chosen carefully. I would have liked it better if you could have done some testing compared to the original plans and improve on the design. or give insights on what changed make the most differences.

    eksine

  18. sharpstick says:

    I plan to make one for next year’s burning man trip. Living in Florida, it is difficult to transfer bulky stuff across the country. I can possibly do some of the building in Reno, but time is also limited.
    One possible drawback of this standard design is that the area of the drilled holes is fairly small and the airflow is restricted. To avoid this, an open frame(stiff wire?) would allow more of the filter material to be exposed to airflow. or an airspace between the bucket walls and the filter.
    Another variation that would improve overall efficiency is to enclose your bed inside of a smaller tent within a tent. We’ve been doing that for years to keep dust and cold out. Now I just suspend a draped piece of thin dropcloth over the bed with two bamboo spreaders. the whole thing is suspended from the tent frame with loops of string threaded through the ceiling and over the frame tubes. the edges are tucked under the air mattress to close it up, and the sides can be clipped up to open both sides during the day. with a swamp cooler, the enclosed space that needs to be cooled is about 70 cu ft, as opposed to 700+ for a cabin tent.

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