What Is an Evaporative Cooler?

Updated: May 23, 2023

More environmentally friendly and affordable than central air conditioning, evaporative coolers may be a better way to beat the heat in your home.

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Evaporative coolers — commonly known as “swamp coolers” but also called evaporative air conditioners, desert coolers, wet air coolers and even swamp boxes — have been around in their modern form since the 1930s. But the basic concept can be traced to the ancient Egyptians, who discovered hot, dry breezes could be made pleasantly cool and moist when blown through damp reed mats, or past porous clay pots filled with water.

“Installing an evaporative cooler is a great way to keep indoor air fresh and cool, especially with the windows open,” says Scott Jenschke, an air circulation and accessories merchant for The Home Depot. Here’s what you need to know if you are considering this type of cooler for your home.

What Is an Evaporative Cooler?

An evaporative cooler uses an electric-powered fan to draw hot, dry air through a dampened pad and blow the cooled air out into the room. This relatively simple process consumes much less energy than a standard air conditioner and doesn’t require ozone-damaging refrigerants to work. Evaporative coolers are also less expensive to install and maintain than central air conditioning.

How do evaporative coolers work?

Jenschke explains evaporative coolers utilize evaporation to cool air temperature. “The liquid turns into gas when water evaporates, releasing the highest energy particles and causing the temperature to drop,” he says. “This is why your forehead feels cooler after you put a moist cloth on it on a hot day.” This same scientific principle can be applied to the evaporative cooling process for the air inside your home.

Evaporative coolers are made up of four essential parts: a fan, a water tank, filter pads (sometimes called media) and an internal motor. The motor pulls warm air through a wet internal filter pad, where the evaporation process quickly lowers its temperature before it is circulated back into the room. The internal pads also act as a filter, removing dust and allergens from the air.

“Evaporative coolers work best in arid climates, where you can see as much as a 30 degree reduction in temperature,” says Jenschke. “In less arid climates, they’re not as effective, but can still cool the surrounding air by five to 15 degrees.” Jenschke also recommends opening nearby windows to help renew the air and keep the breeze fresh.

How Is an Evaporative Cooler Different From an A/C Unit?

Evaporative coolers and A/C units work with different technologies.

“Air conditioners use chemical refrigerants in tubes and coils to remove hot air from your home and leave behind drier, cooler air,” Jenschke says. “In many situations, an air conditioner is connected to the outdoors because the tubes and coils absorb heat from indoors and release it outdoors. You can feel warm air coming from an air conditioner outside for this reason.”

Evaporative coolers, on the other hand, harness the power of evaporation to cool the air naturally. “The evaporative cooler is a more sustainable option as it uses one-fourth of the energy and costs about half as much to install than an air conditioner,” says Jenschke. “It also doesn’t require ductwork to move the cooler air throughout the house like a central air conditioning unit does.”

What Are the Different Types of Evaporative Coolers?

There are four main types:

  1. Lightweight and easy to move around, portable coolers are best for cooling a single room, outdoor patio spaces, garages or even sports field sidelines. Larger models can cool spaces from 500 to up to 3,000 square feet, according to Jenschke.

  2. Indoor evaporative coolers designed to fit a window or wall opening are ideal for cooling a single room, garage or open-plan living space.

  3. Outdoor evaporative coolers called down discharge (or down draft) coolers are designed to be installed on the roof and can cool an entire home.

  4. Side discharge (or side draft) coolers are designed to cool the whole house. They are typically installed on the side of a building, although some models can also be installed on the roof.

Do Evaporative Coolers Work in Humid Climates?

Evaporative coolers are most efficient in drier climates where the humidity levels stays under 60% most of the time. However, they can still be somewhat useful in the more humid climates of the South and Midwest, working effectively in fan mode to circulate air throughout a building. And, of course, you could always use them on days where the humidity levels are low.

How Much Do Evaporative Coolers Cost?

Evaporative coolers can cost less than $100 for small portable models to around $1,500 for down or side discharge models powerful enough to cool an entire home. “Some municipalities offer rebates of up to $250 for the purchase of a whole-house unit through your utility provider,” Jenschke says.

What Size Evaporative Cooler Should I Buy?

To figure out how large an evaporative cooler you need, check the cubic feet per minute (CFM) rating. This is the cubic feet per minute the unit can cool. Calculate the minimum CFM you need by determining the cubic feet of space you want to cool (length in feet x width in feet x height in feet = cubic feet) and multiply that total by the number of times you want the air to turn over in an hour.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) suggests from five to nine air changes an hour for residential rooms. Then divide that total by 60 (minutes). The result tells you the CFM rating you need. Some models also list a square foot rating, which is less precise but still helpful when choosing the right cooler for your space.

Evaporative Cooler Accessories

Evaporative coolers generally come with all their necessary parts except possibly the motor (see below), but you’ll need to purchase replacement pads over time. You can buy these and other accessories from the same supplier from whom you purchased your cooler, home centers or even Amazon.

  • Fiber pads: “These pads are made from shredded Aspen fibers woven together and come in two thicknesses, one inch and two inches,” Jenschke says. “The thicker the pad, the more moisture it can hold, which improves cooling.” Fiber pads deteriorate over time and need to be changed annually.

  • Synthetic pads: Synthetic pads last longer than fiber but don’t retain as much moisture, reducing their effectiveness.

  • Brand-compatible pads: Some brands use custom pads, including the popular Mastercool line. Their type of pad is made from a sturdy honeycomb-shaped cardboard. It offers the best cooling ability and can last up to five years with proper care.

  • Evaporative cooler covers: These protect outdoor evaporative cooler units from the elements when not in use.

  • Motors: Some large down- or side-discharge units are sold separately from the motor. Most can accommodate a one-half, three-quarters or one horsepower motor, based on your cooling needs.