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Musings of an Energy Nerd

Saving Energy With an Evaporative Cooler

In a dry climate, lowering the temperature of your indoor air with a swamp cooler uses much less electricity than an air conditioner

Most evaporative coolers are roof-mounted. These cooling appliances must be wired for power (usually 120 volts AC) and connected to a cold-water plumbing line.
Image Credit: Image #1: Public domain

Evaporative coolers are appliances used to cool indoor air. Evaporative coolers use much less energy than air conditioners, but they can’t cool indoor air effectively in all weather conditions.

Sometimes called swamp coolers, evaporative coolers lower the temperature of an airstream by passing the air through a moistened pad. The moving air causes water to evaporate off the pad. Evaporation requires energy (heat); in other words, the process of evaporation (a phase change process) removes heat from the air. The air exits the appliance at a lower temperature, but with more moisture, than when it entered.

To keep the pads in an evaporative cooler damp, tubing with nozzles delivers water to the top of the pads. The pads are usually made of a material called excelsior — basically, aspen wood shavings. Water trickles down the pads and drips off the bottom of the pads; the base of the metal cabinet includes a sump where the water collects. A recirculating pump pulls water from the sump and delivers it to back the top of the pads. The sump is equipped with a float valve similar to the valve in a toilet tank; the float valve opens and adds water to the sump when the water level drops due to evaporation.

These devices only work in a dry climate

Evaporative coolers work best when the outdoor relative humidity (the wet-bulb temperature) is low. If the outdoor temperature is 90ºF at 10% RH, the wet-bulb temperature is only 58ºF — good conditions for operation of an evaporative cooler. (For more information on wet-bulb temperatures, see How to Use the Psychrometric Chart.)

The bigger the difference between the dry-bulb and wet-bulb temperatures, the more effectively an evaporative cooler will operate. Most evaporative coolers can’t lower the temperature of the air below…

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  1. kevin_in_denver | | #1

    Minisplits will Kill the Residential Evaporative Cooler Market

    Maybe this article would have generated more discussion if you called it "Evaporative Cooling is Dead".

    When I designed my current house in 2001, I went with radiant heat because of all its perceived benefits and minisplits were not yet very common. Since central air would require costly ductwork, I felt an evaporative cooler was the perfect solution for air conditioning.

    So I'd like to add a few points about swamp coolers for dry climates:

    1. For most locations, cheap coolers are a bad choice. The $354 cooler won't do the job when the temperatures rise above 95F, just as the chart above shows.

    2. A better cooler CAN get the job done all year (up to about 102F) as I have proven in my own house. The secret to better performance is the new 8" and 12" cardboard media. The extra first cost of the cooler is recouped because the media doesn't need replacing nearly as often.

    Here is one report that documents how much better the thicker media works:

    3. The "tight house problem" you cite isn't really that hard to solve... just give the cooler its own little room inside the house with a large operable window. That also solves the maintenance cost issues because nobody has to climb on the roof, and it never needs to be winterized.

    4. The cost of the water needed is NOT significant:

    John Proctor (the AC expert) said "I’m befuddled by the fact that more people don’t use evaporative coolers.” Once he reads your article, he should understand.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Kevin Dickson
    Yes, I considered the "Evaporative Cooling Is Dead" title, but I decided that it might lead to eye-rolling. And in any case, like John Proctor, I retain affection for these quirky devices, even though their time has passed.

    You are I are in substantial agreement. My extensive references to Ryan Randazzo's article, “Once-Common Evaporative Coolers Are Disappearing from Phoenix-Area Homes,” is due to the fact that I think Randazzo nailed it.

    I'm trying to visualize your "just give the cooler its own little room inside the house" suggestion. I'm thinking that the room is airtight; the evaporative cooler is in the middle of the room with a duct leading from the cooler to a grille mounted in your exterior wall; and that the room has an operable window in a partition, so that the window opens to an interior room (perhaps a hallway). When you want to turn on the cooler, you open the window between the hallway and the mechanical room. Is that how it works? If so -- I like it.

    I appreciate your point about water usage; however, some swamp coolers use more water than others, and water restrictions are increasing in many Western states, at the same time that water rates are rising.

  3. kevin_in_denver | | #3

    Indoor Swamp Cooler
    Here are a couple photos of my indoor swamp cooler. It works perfectly but I've never heard of it being done before or since. The awning window is wide open all summer, and since it's on the third floor and under an overhang, rain is not a problem.

    The 8" media MasterCool unit performs well with a thermostat 100% of the time. I previously had a cheaper cooler which didn't. We made it work by overcooling the high mass well-insulated house at night, then sealing the house up tight until 8pm.

    Phoenix has a monsoon season each summer that has high humidity with high temperatures. That fact has also helped kill evaporative cooling there. Denver and the high mountain west remains perfect for it.

  4. GBA Editor
    Martin Holladay | | #4

    Response to Kevin Dickson
    A very nice (and ingenious) installation. Well done.

  5. kevin_in_denver | | #5

    OK, just a couple more points
    1. This installation allows me to keep the house at 66-70F all summer for $20/month.
    2. The summer air quality can't be beat, about 2000cfm of filtered, scrubbed, and humidified air.
    3. Low speed is very quiet and usually is enough. Hi speed is a little noisy and windy in the room shown.
    4. Cooler components are super cheap, I just replaced my 7 year old pump for $16.
    5. We use it as the humidifier in the winter on a humidistat (with the window closed). The single biggest complaint about Denver's climate is the dry indoor air in the winter.

  6. Dennis_Miller | | #6

    Maybe our swamp cooler was a lousy brand or the units are better nowadays, but when we lived in California around 1990 we only used the swamp cooler a couple times and found that although the indoor temperature went down, the increased humidity made it uncomfortable as well. So we never used it again. And then we moved out of California...

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