How Swamp Coolers Work

By: Chris Landers & Francisco Guzman

working on a rooftop swamp cooler
A worker does preventative maintenance on a commercial rooftop swamp cooler. David Spates/Shutterstock

If you've ever tested the wind by holding a wet finger in the air, you've used evaporative cooling. The same principle cools you off after a swim, and also powers one of the oldest and simplest forms of air conditioning. Known in the U.S. as swamp coolers, modern evaporative coolers can trace their lineage to ancient Egypt. They're cheap, efficient and good for the environment, but they come with some limitations, so don't push your standard air conditioner out the window just yet.

The ancient Egyptians had a great need for air conditioning. They accomplished it by hanging wet blankets across the doors of their homes or, if they happened to be royalty, having servants fan them across jugs of water. When hot dry air passes over water (or better yet, through it), the air cools off. Nowadays, we use electric fans instead of servants, but the principle of cooling the air by evaporation remains the same.

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Unfortunately, evaporative air coolers don't work everywhere. Swamps, for instance, are lousy places for swamp coolers. It's not entirely clear where they got the nickname, but it probably refers to the humidity they add to the air or the swampy smell that can develop when they aren't cleaned often enough.

Swamp coolers are used all over the world. In order to work, swamp coolers need a hot dry climate. In the U.S., they work well in the arid western and southwestern parts of the country. The global evaporative cooler market was valued at $7.6 billion in 2019 and is expected to reach $19.8 billion by the end of 2026.

Swamp coolers are based on a simple, efficient technology that has been around a long time. The principles of evaporative cooling worked for the pharaohs, and they can still work for you.

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Evaporative Cooling Basics: From Ben Franklin's Underwear to Modern A/C

July 1758 was a scorcher in Philadelphia, Pennsylvania, with temperatures reaching 100 degrees Fahrenheit (37 Celsius). As he later wrote in a letter, Benjamin Franklin was in his room, reading and writing with "no other cloaths on than a shirt, and a pair of long linen drawers, the windows all open, and a brisk wind blowing through the house." Even founding fathers sweat, and as he changed to a dry shirt, he noticed something — it felt warm, like it had been near a fire, compared to the damp shirt he had just removed. Franklin theorized that he wasn't being cooled by the hot air blowing through his room, but by the perspiration evaporating off his skin. Later, he tried some experiments — wetting the bulb of a thermometer and watched as the temperature dropped. He discovered that the evaporation of liquids caused heat loss. What he described in his letter is evaporative cooling.

Liquid evaporates by shedding molecules into the air, changing from a liquid state to a gas. As they become suspended in the air, the molecules draw some of the heat from the hotter air, cooling it down as the water and air find equilibrium. The process also cools the remaining liquid, as hotter, faster-moving molecules are the most likely to escape into the air.

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Swamp coolers work by harnessing that cooling reaction — you just need a way to circulate the now-cooler, more humid air through the house.

Picture an air conditioner — just a sheet metal box on the outside of a window, really. In a standard air conditioner, there are some fairly complicated refrigerants inside, but with a swamp cooler, it's much simpler. The main thing inside is a blower — a fan at one end of the box that brings air in from the outside and pushes it into the house at the other end. Before the air goes into the house, it passes through a set of damp pads, where the evaporation takes place. The cooler air goes into the house and the warmer air is pushed out. A small pump keeps the pads moist, so the water doesn't just evaporate away completely. It works just like the Egyptians' woolen blankets or Ben Franklin's sweaty shirt.

So how exactly does a swamp cooler compare to an air conditioner?

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Swamp Coolers vs. Air Conditioners

Standard AC units work by employing the same operating principles as your home refrigerator. The air is cooled by the coils, sent into your house, and then re-circulated over and over through the machine, venting hot air generated in the process to the outside. (For more about standard air conditioners, see How Air Conditioners Work). It's a closed process — leaving a window or door open allows the cool air to escape and makes the air conditioner work harder to supply cold air.

Swamp coolers are an open system. They rely on the flow of air through the building to direct the cool air, and since they always need hot, dry air to evaporate the water, it needs to displace the air already in the house. Both systems can use either a large central unit or small window units, but air from the swamp cooler needs a way out. Opening and closing windows and doors controls the air flow from the swamp cooler to different parts of the house, while central air conditioners use ducts to direct the flow. Swamp coolers can also use ducts in some cases, but they need to be larger than traditional air conditioner ducts to account for a greater flow of air from the swamp cooler.

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Standard air conditioners also dry the air, condensing water vapor from the cooled room as it passes over the cold coils. The water drains outside — that's the distinctive drip you'll feel if you stand under a window air conditioning unit long enough. The result is a drier room, and in humid climates, that can be a good thing. Too much humidity can prevent perspiration, which is how we cool ourselves naturally. Since swamp coolers work by putting water into the dry air, they act as humidifiers. This is great in drier climates because humidity can also be too low for comfort. Under the right conditions the water-laden breeze also can have a secondary effect of helping the skin's perspiration, resulting in an even cooler feel than the swamp cooler would give on its own.

Because of the different ways they work, you can't run a swamp cooler and a standard air conditioner in the same house. They would cancel each other out, just like running a dehumidifier and a humidifier in the same room.

So, if you can only pick one, which is better? The swamp cooler or the air conditioner?

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Swamp Cooler Benefits

swamp cooler
Swamp coolers use the principles of evaporative cooling to cool the air.
John T Takai/Shutterstock

Under the right conditions, swamp coolers look like they have lots of benefits. They're cheap to build and install. The only materials it takes to make them are a blower fan, a pump, an 8- to 12- inch- (20- to 30- centimeter-) thick filter pad (either made of treated cellulose, fiberglass, plastic foam or shredded aspen fibers), some water and a box (usually made of sheet metal). Pumps and fans are widely available. The rest can be turned out in a local shop just about anywhere. Also, a swamp cooler should have at least two speeds and a vent-only option. If you decide to buy one, you can spend anywhere from $40 for a portable model to $3,500 for a roof-mounted or ground unit. By contrast, a central AC unit would cost between $3,000 and $7,000 to install, according to NerdWallet.

The swamp cooler monthly operating costs are considerably lower as well — it costs less than half the price to install and operate of central AC. But the biggest savings is in the electricity — a swamp cooler uses 60-80 percent less electricity than a standard AC. That translates to savings on your energy bill, but also to the environment.

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Swamp coolers have a further environmental benefit, since standard air conditioners have long relied on ozone-depleting chemicals to provide their cooling power. The use of CFCs (chlorofluorocarbons) has been discontinued in developed nations since 1996 by international treaty. The replacement product, HCFCs (hydrochlorofluorocarbon) like R22 (freon) has some negative effects, and its use in new equipment ended in 2010. Since then, AC units have used R410A or puron, but this is being phased out for R-32 which has one-third the global warming potential of R410A. Swamp coolers don't use any refrigerants.

Swamp Cooler Downsides

For all their benefits and cost savings, swamp coolers still only work in the right climate, and that unfortunately doesn't include areas like Philadelphia.

It needs to be not only hot but also dry, because as the dry bulb temperature approaches the wet bulb temperature, the difference between the two gets smaller, and the cooling effect of the evaporating water follows suit. A wet bulb temperature above 70 degrees Fahrenheit (21 degrees Celsius) means the swamp cooler won't be able to adjust the temperature enough to keep it in the comfort zone. (This varies based on humidity, personal preference and activity, but it generally falls in the low 70s.)

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If the air gets too saturated with water, the water condenses. One hundred percent humidity outside the house means it might rain, and while a swamp cooler won't cause a downpour in your home, it won't have any cooling effect either — just the hot, sticky feeling you might associate with swamps. Your perspiration doesn't evaporate into the saturated air.

The Environmental Protection Agency recommends keeping the indoor relative humidity below 60 percent and ideally between 30 and 50 percent to keep down mold and mildew, and your wooden furniture from drying out. Unfortunately, humid air can also cause metal to rust, even in the swamp cooler itself. The problem of excess humidity can be solved in evaporative coolers by using a heat exchanger to heat air inside the house while the humid air vents to the outside, but these systems are considerably less efficient than direct evaporative coolers.

Swamp coolers require maintenance. The pads needs to be cleaned or changed regularly to avoid that swampy smell and associated problems with air quality. Minerals in the water can also build up as water evaporates, requiring a bleed-off of mineral-rich wastewater over time. It's recommended to look at the pads, filters, reservoir and pump at least once a month and replace the pads at least twice during the cooling season, or once a month during continuous operation. The coolers also require a steady supply of water — anywhere from 4.4 to 10.4 gallons (17 to 40 liters) per hour depending on the size. That can be a tall order in the hot, dry climates where swamp coolers function best.

Of course, air conditioners also require maintenance, whether it's changing the filter every three months, or having a yearly service from a technician to keep it working. But swamp coolers that are part of an HVAC unit need a professional too, to disconnect it at the end of the season. The process involves completely draining and flushing the water system so as to prevent a winter freeze from damaging the unit. The technician must also reconnect it in the summer.

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