Understanding How Swamp Coolers Function

If you've ever gauged the wind by wetting your finger and holding it up, you've experienced evaporative cooling. This same process refreshes you after a swim, and it powers one of the oldest and most basic methods of air conditioning. Known in the U.S. as swamp coolers, modern evaporative coolers have roots that go back to ancient Egypt. They're affordable, efficient, and environmentally friendly, but they do have certain drawbacks, so don’t rush to replace your traditional air conditioner just yet.

The ancient Egyptians were pioneers in the need for cooling systems. They achieved this by draping wet blankets across their doorways or, for the elite, having servants fan them across water-filled jugs. When dry, hot air moves over (or through) water, it cools. Today, we use electric fans in place of servants, but the fundamental cooling mechanism through evaporation still holds true.

Unfortunately, evaporative coolers aren't suitable for every environment. For example, swamps are not ideal places for swamp coolers. The origin of the term is somewhat mysterious, but it likely comes from the extra humidity they introduce into the air or the musty scent that arises if they're not cleaned regularly.

Swamp coolers are used worldwide. For them to function effectively, swamp coolers require a hot, dry climate. In the U.S., they are particularly effective in the dry regions of the West and Southwest. The global evaporative cooler market was valued at $7.6 billion in 2019 and is projected to grow to $19.8 billion by the end of 2026.

Swamp coolers rely on a straightforward and efficient technology that has existed for a long time. The evaporative cooling methods that worked for the ancient pharaohs are still effective today.

Evaporative Cooling Explained: From Ben Franklin's Underwear to Today’s A/C

In July 1758, Philadelphia, Pennsylvania, sweltered under a heatwave with temperatures reaching 100 degrees Fahrenheit (37 Celsius). As he later described in a letter, Benjamin Franklin was in his room, reading and writing, wearing "no other clothes than a shirt and a pair of long linen drawers, with the windows wide open and a strong breeze blowing through the house." Even the founding fathers perspired, and when he changed into a dry shirt, he noticed it felt warm, almost like it had been near a fire, in contrast to the damp shirt he had just removed. Franklin realized that the coolness he felt wasn’t due to the hot air circulating in the room but rather because of the sweat evaporating off his skin. Later, he performed experiments — he wet the bulb of a thermometer and saw the temperature drop. He discovered that the evaporation of liquids leads to heat loss. This, as described in his letter, is evaporative cooling.

When a liquid evaporates, it releases molecules into the air, transitioning from liquid to gas. These molecules then disperse in the air, absorbing heat from the warmer air, which leads to cooling as the water and air reach balance. The process also cools the remaining liquid, as the hotter, faster molecules are more likely to escape into the air.

Swamp coolers take advantage of this cooling effect — all you need is a way to circulate the cooler, more humid air throughout the house.

Imagine an air conditioner — essentially a sheet metal box mounted on a window. Inside a typical air conditioner, there are complex refrigerants, but with a swamp cooler, the design is much simpler. The key component is a blower — a fan located at one end of the unit that draws in outside air and pushes it into the home from the other end. Before entering the house, the air flows through a set of damp pads, where the evaporation occurs. The cooled air enters the home, while the warmer air is expelled. A small pump keeps the pads moist, preventing them from drying out completely. It functions just like the ancient Egyptians' wet blankets or Ben Franklin’s sweat-soaked shirt.

So, how does a swamp cooler measure up to an air conditioner?

Comparing Swamp Coolers and Air Conditioners

Standard air conditioning units function similarly to your refrigerator at home. The air is chilled by the coils, directed into your house, and continuously circulated through the system. Hot air produced in the process is vented outside. (For more details on how air conditioners operate, see How Air Conditioners Work). It’s a closed system — leaving a window or door open allows the cool air to escape, forcing the air conditioner to work harder to keep the space cool.

Swamp coolers, on the other hand, operate as an open system. They depend on airflow through the building to distribute cool air, and since they need hot, dry air to evaporate water, that air must replace the air already present in the house. Both swamp coolers and air conditioners can be either large central units or smaller window units, but the air from a swamp cooler needs to be vented out. Adjusting windows and doors regulates the airflow from the swamp cooler to different areas of the house, while central air conditioning uses ducts to direct air. Swamp coolers can also utilize ducts, but these need to be larger than typical air conditioning ducts to handle the higher volume of air from the cooler.

Standard air conditioners also dehumidify the air by condensing water vapor as it passes over cold coils, draining the water outside — which is why you can often feel a drip under a window AC unit. This results in a drier room, which is beneficial in humid climates. Too much humidity can hinder the body’s natural cooling mechanism — sweating. In contrast, swamp coolers add moisture to the air, acting as humidifiers. This is beneficial in dry climates where the air may lack sufficient humidity for comfort. Under the right conditions, the moist air can also enhance the cooling effect by helping sweat evaporate from the skin, making the environment feel even cooler.

Due to the differing ways they operate, it’s not possible to run both a swamp cooler and a standard air conditioner in the same home. They would counteract each other, much like running a dehumidifier and a humidifier in the same room at the same time.

If you had to choose between them, which would you pick? The swamp cooler or the air conditioner?

Advantages of Swamp Coolers

When conditions are favorable, swamp coolers offer numerous advantages. They’re inexpensive to build and install. The necessary components include a blower fan, a pump, a filter pad (typically 8 to 12 inches thick, made from materials like treated cellulose, fiberglass, plastic foam, or shredded aspen fibers), some water, and a metal box. Pumps and fans are readily available, and other parts can be locally sourced. A swamp cooler should have at least two speed settings and an option for ventilation-only mode. If you’re in the market for one, prices range from $40 for a portable model to $3,500 for a roof-mounted or ground unit. In contrast, installing a central AC system generally costs between $3,000 and $7,000, according to NerdWallet.

The monthly operating costs for a swamp cooler are much lower — it costs less than half the price to install and operate compared to central AC. The most significant savings come from the reduced electricity usage — a swamp cooler consumes 60-80 percent less electricity than a conventional air conditioner. This results in lower energy bills and a positive impact on the environment.

In addition to being more affordable, swamp coolers offer an environmental advantage. Standard air conditioners have historically relied on chemicals that harm the ozone layer. Although the use of CFCs (chlorofluorocarbons) was banned in developed nations in 1996 by international treaty, the replacement, HCFCs (hydrochlorofluorocarbons) like R22 (freon), has its own issues, and its use in new equipment ceased in 2010. Since then, AC systems have switched to R410A or puron, but this is being phased out in favor of R-32, which has a third of the global warming potential of R410A. Swamp coolers, however, don’t use any refrigerants at all.

Challenges of Swamp Coolers

Despite their advantages and cost-effectiveness, swamp coolers are only suitable for certain climates, which unfortunately excludes places like Philadelphia.

For swamp coolers to work effectively, it needs to be hot and dry. As the dry bulb temperature gets closer to the wet bulb temperature, the difference between the two decreases, and so does the cooling effect of the evaporating water. When the wet bulb temperature rises above 70 degrees Fahrenheit (21 degrees Celsius), a swamp cooler won’t be able to cool the air enough to maintain comfort. (This can vary based on humidity, personal preferences, and activity, but it typically hovers in the low 70s.)

If the air becomes overly saturated with moisture, it will condense. A humidity level of 100 percent outside usually signals rain, and while a swamp cooler won’t cause a downpour inside, it won’t cool the air either — it will just create that hot, muggy feeling you typically associate with swamps. In this scenario, your sweat doesn't evaporate into the humid air.

The Environmental Protection Agency recommends that indoor relative humidity be kept under 60 percent, ideally ranging between 30 and 50 percent, to prevent mold, mildew, and to protect your wooden furniture from drying out. Unfortunately, high humidity can also lead to rust on metal components, including parts of the swamp cooler itself. Excess moisture can be managed in evaporative coolers by using a heat exchanger, which heats indoor air while venting the humid air outside. However, these systems are far less efficient than direct evaporative coolers.

Swamp coolers require regular maintenance. The pads need to be cleaned or replaced frequently to avoid unpleasant odors and maintain good air quality. Minerals in the water can accumulate as it evaporates, necessitating a discharge of mineral-rich wastewater over time. It’s advised to inspect the pads, filters, reservoir, and pump at least once a month and replace the pads at least twice per cooling season or monthly during continuous use. Additionally, swamp coolers require a consistent supply of water — ranging from 4.4 to 10.4 gallons (17 to 40 liters) per hour, depending on the unit's size. This can be a challenge in hot, dry climates where swamp coolers are most effective.

Like swamp coolers, air conditioners also need upkeep, whether it’s changing filters every three months or having a technician perform an annual service. But swamp coolers integrated into an HVAC system also require a professional to disconnect them at the end of the season. This involves draining and flushing the water system to prevent freezing damage over the winter, and reconnecting it once summer arrives.