What is an Evaporative Cooler and How Does It Work?

Let’s start with answers to the two questions posed in the title of this page. What is an evaporator cooler? How does an evaporative cooler work? Later in the post, we compare evaporative cooler vs air conditioner, talk about the best and worst climates for an evaporative AC, which is another name they’re given – even though they are not air conditioners – types, brands, costs and more.

What is an Evaporative Cooler?

Simply put, an evaporative cooler lowers the air temperature through the cooling properties of evaporation. They’re designed to promote rapid evaporation of moisture off a material that cools the air passing through it.

The most commonly used example of evaporative cooling is perspiration. The evaporation of sweat cools your skin.

Evaporative coolers are also known as swamp coolers, desert coolers, and wet air coolers.

Fun Fact: An early form of an evaporative cooler, called by some “wind catcher,” was used in Egypt and Persia thousands of years ago. These were shafts on the roof of the building catching and pulling the wind through tunnels and over subterranean water then discharging the cooled air into the building.

How Evaporative Coolers Work

An evaporative AC or cooler includes a fan, a water tank or reservoir, a pump and one or more specially designed pads to draw water from the reservoir. The water is absorbed by the pads or pumped over the pads. These pads have many layers to increase the surface area allowing the water to evaporate as fast as possible.

evaporative cooler works

As the fan draws hot dry air directly across the pads, the evaporation process cools the air and then the fan blows the cooled air into the room.

For large areas, air movement through the area is controlled by opening the windows to provide 1 to 2 square feet of opening for each 1000 CFM of cooling capacity. CFM ratings and how to purchase the correct size will be discussed later in the article. If open windows are a security concern, you can install ducts in the ceiling to allow fresh air in and exhaust the warm air. There are also roof-mounted whole house evaporative coolers which use vents and ductwork.

While evaporative coolers use water to cool, most do not create or blow out mist or water, although they will slightly increase the humidity in the area. In a very dry climate, this is a benefit.

Best Climate for Evaporative Coolers

Do evaporative coolers work? Yes, in many regions of the country, they can be an effective method for cooling yourself, your home or a room in your home. 

Pro Tip: Evaporative coolers are best for dry climates where the relative humidity stays below 50%, water does not evaporate well when the humidity is high, making these coolers inefficient. In dry climates, a large, whole-house evaporative cooler can reduce temperatures by up to 30° F.

Here is a map from the US Geological Survey showing the area of the country where evaporative coolers are the most effective:


And here is a chart showing the effectiveness of evaporative coolers in different temperatures and humidity levels:

effectiveness of evaporative coolers

Note: If you live in a climate with higher humidity levels,  you might be able to use an indirect evaporative cooler. We’ll discuss indirect evaporative coolers later in the article.

Evaporative Coolers vs Air Conditioning – What’s the Difference?

Air conditioners use coils containing refrigerants to remove the heat and humidity. The heat is pumped outside and dispersed; the humidity is condensed onto a cold coil and drained away. They work best in an enclosed space and can cool the air to a specific set temperature regardless of the temperature outdoors.

Evaporative coolers use only water and work best in areas with plenty of fresh air from an open window. A specific temperature cannot be selected as the amount of cooling delivered is based on ambient conditions including outdoor temperature and humidity.

Evaporative coolers are more natural, less expensive and simpler than traditional air conditioning and use much less energy making them much less expensive to operate.

How to Choose the Right Size Cooler

Choosing the correct size swamp cooler/evaporative cooler for your space will allow you to get the best results. Before you shop for an evaporative cooler, find the cubic feet of the area you want to cool and then the CFM (cubic feet per minute) rate that you need. The cubic feet per minute rate indicates the amount of air that is cycled through our space each minute.

Here’s how to find the correct size:

Measure the length, width, and ceiling height in feet for the space you want to cool to determine the cubic feet in the space.

For example, a 20 by 25 foot space with 8 foot ceilings is 4,000 cubic feet. Then divide by 2 giving you the necessary CFM’s for the space.

Here’s the formula based on the above space:

20 X 25 = 500 (sq. ft. room)  X 8 (ceiling height) = 4,000 (cu. ft.)  ÷ 2 = 2,000 CFM’s

So, a 500 sq. ft. room with 8 ft. ceilings will need an evaporative cooler rated at  2,000 CFM.

If you will move your cooler from room to room, use the size of the biggest room you want to cool.

Some evaporative coolers can be used on a deck, patio, or in a garage. For outdoors, you can figure that a cooler will cover about 25% to 35% of the CFM rating in square feet. For instance, a 2000 CFM rated cooler will cover 500 to 650 sq. ft. and a 10,000 CFM rated unit will cool an area of 2,500 to 3,300 sq. ft. of outdoor space.

If you plan to use your cooler outside, make sure it is outdoor rated and has a GFCI cord and plug.

Types of Evaporative Air Coolers

There are several personal, small and large types of evaporator coolers to know about. This will help you choose the right ones for your purposes.

Direct Evaporative Coolers (DEC)

These coolers are available in a variety of styles, some are portable and some are installed into windows, or on a wall or roof.

Portable Evaporative Coolers

Portable evaporative coolers are available in various sizes and with a range of features. Some are small and lightweight and can be carried. Others are larger and include casters on the bottom of the unit so they can be rolled from room to room.

Portable coolers, often called personal coolers or personal ACs, are made for indoor use, outdoor use, or both. Portable coolers usually require you to manually fill the water tank, but some allow you to attach the cooler to a water source, like a hose. Some allow you to add ice to the tank for quick cooling – but studies have shown the ice makes very little difference since these units cool by evaporation, and water temperature is irrelevant.

Mounted Evaporative Coolers

Mounted coolers are meant to be installed permanently, although window units can be removed from the window and stored away when the season is over. All mounted evaporative coolers will require a water source.

Window Mount

A window is a perfect place to mount an evaporative cooler because they need a constant supply of fresh air for the best results.

Wall Mounted

If you are going to mount your evaporative cooler on the wall place it high on the wall. Warm air rises and cool air falls so higher placement will provide better results.

Roof Mount

A roof mounted evaporative cooler is used with ductwork to bring the cooled air into the room or rooms. This is a solution for whole house cooling but you must have, or install, the necessary vents and ductwork.

Evaporative Coolers – Top Brands of Direct Evaporative Coolers

Some of the top brands of evaporative coolers include Hessaire, Cool-Space and Honeywell. Check out these brands and others on the Pick HVAC page for Best Evaporative Cooler – Reviews and Buyer’s Guide 2021

Indirect Evaporative Coolers (IEC)

These whole house units are fairly new to the market. They still cool the air through evaporation but without raising humidity levels in the home and therefore can be used in climates with higher humidity levels.. An IEC uses a heat exchanger and a secondary air source which prevents humidity levels from increasing inside the home.

Inside the heat exchanger are metal tubes that are kept wet. As the hot outside air is drawn across the tubes, the water evaporates and cools the air inside the tubes, which is then blown into the house. The moist air inside the heat exchanger, which is used in the evaporation process, is then blown outside with a second fan.

This unit is installed outside and uses ductwork to bring the cooled air throughout the home. See the photo below:

Swamp Cooler

Pros and Cons of Evaporative Coolers

Evaporative coolers, or swamp coolers, have both advantages and disadvantages.


  • They provide a steady stream of fresh air to the space
  • They use about 75% less energy than air conditioning
  • They are easier and less expensive to install than air conditioning


  • A specific temperature cannot always be set or maintained
  • Large units need open windows or vents – though personal and portable evaporative coolers do not
  • They require more maintenance than air conditioning
  • Air is not cleaned or filtered as well as with air conditioning, but filters can be purchased to reduce dust and allergens
  • They can add humidity to the space
  • Their effectiveness is reduced as humidity increases, so they aren’t useful in humid climates


In addition to CFM rating and the method of installation, there are some useful features you might be interested in:

  • Look for an oscillating fan, directional louvers, and a fan with multiple speeds
  • Check the tank size and determine if it requires manual filling, or can you attach a hose, or use both options
  • Look for optional air filters that attach to the unit to help remove dust and allergens
  • Some units come with a programmable thermostat or timer
  • If you’re looking for a unit for multiple areas, check the size, weight, and portability
  • The unit should have a low water alert

Maintaining an Evaporative Cooler

Clean or have your unit professionally serviced twice a year, once prior to the cooling season, and again midway through the summer. Regular maintenance will extend the life and efficiency of the cooler.

If you clean your unit yourself, following are the steps to take:

  • Check that the water supply is completely wetting the pads
  • Drain, flush, and clean the water reservoir
  • Clean the water distribution system
  • Remove and replace or clean the pads
  • At the end of the season, turn off the cooler, disconnect the electrical source, and drain the reservoir and water lines.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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