A Guide to Evaporative Coolers

an evaporative cooler system on a roof

Splash some water on your face on a hot day in the summer and you will immediately feel cooler. That's evaporative cooling working. An evaporative cooler works on the same principle—evaporating water provides cooling—but combines water evaporation with air movement to cool interior space.

Evaporative coolers are sometimes called swamp coolers, probably because they add humidity to the air, making it more swamp like. The concept of evaporative cooling has been around for years—people living in arid climates used to hang damp clothes in their windows at night, knowing the breeze blowing through the window would help cool the room.

How Do Evaporative Coolers Work?

While they've moved beyond the "hang a cloth in the window" stage, the principle of evaporative cooling remains the same. Nowadays such systems usually consist of a large box containing a fan surrounded by a wet pad, mounted on the roof or side of a house. A pump circulates water to keep the pad wet as the fan blows air through. The damp, cooler air blows into the house, driving the hotter inside air out through open windows or vents.

According the US Department of Energy, the difference between the inside air temperature and the cooled air can be expected to be in the range of 15 to 20 degrees, but the difference can be as high as 40 degrees in certain conditions. As an added bonus, evaporative coolers provide incremental cooling effect, since the constant air movement caused by the cooler's fan actually makes a room's temperature feel even cooler.

evaporative cooler on a stucco roof

Coolers vs Air Conditioners

While both evaporative coolers and air conditioners aim to do the same thing—cool our homes—evaporative coolers are definitely low tech and unlike air conditioners in many ways. To start with, evaporative coolers add humidity to the air, while air conditioners remove humidity. They don't use any refrigerants (like Freon) so they aren't potentially damaging to the atmosphere.

Evaporative coolers provide a constant supply of fresh air, while an air conditioner operates best in a closed environment and re-circulates the same air over and over. Air conditioners also cost substantially more to install and maintain than evaporative coolers, and use three to four more times electricity to operate.

Sizing an Evaporative Cooler

The size of an evaporative cooler is measured in cubic feet per minute (cfm), or the number of cubic feet of air a cooler can move in a minute. Household units are commonly available in sizes from 3000 cfm to 25000 cfm. Manufacturers recommend that a properly sized unit should be able to change the air in a house 20 to 40 times in an hour.

You can determine the proper size unit for your home by calculating the cubic feet in your home (i.e., 1500 square foot house with eight foot ceilings is 12,000 cubic feet) and dividing that number by two. So in this case, the 12,000 cubic foot house will require a 6000 cfm evaporative cooler.

bright blue portable evaporative cooler

Benefits of Evaporative Cooling

There are a number of economic and environmental benefits to using an evaporative cooler. For one thing, evaporative coolers use substantially less electricity than an air conditioner for the same sized house (some estimates are as much as 75 percent less).

Evaporative coolers also rely on relatively simple technology and as a result are much cheaper to buy, install and maintain. Plus, they operate on regular household current so they don't require a dedicated circuit.

In dry climates, adding moisture to the air can help keep both furniture and fabrics from drying out, and the moist pads in the cooler act as air filters, helping keep dust and pollen out of the home while providing a constant supply of fresh air.

Evaporative coolers are even available in small sized window and portable units that can be used to cool an individual room.

Why Don't We Use Them Everywhere?

Evaporative coolers work best when the air they are pulling in is dry. As the humidity in the outside air increases, the effectiveness of swamp cooling goes down. For practical purposes, evaporative coolers work best only in certain areas, such as the American Southwest, where the climate is arid and the air is dry. In areas with higher humidity, they'd be adding moisture to saturated air and there would be little to no cooling effect.

Another drawback to evaporative coolers is they use a substantial amount of water (up to 15 gallons per day) to provide their cooling effect. In dry areas, this consumption can put a real strain on water supplies.

In spite of the fact that evaporative coolers may not be a cooling option for all of us, they do provide energy savings and reduce the risk of damage to the environment in the right climates. So even if we all can't use this ancient method, we all benefit from the energy conservation and environmental protection it provides.