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Evaporative Coolers

Whole-house cooling in arid regions at a low first cost

Photo of an evaporative cooler unit.

Have you ever noticed how cool it feels near a waterfall on a hot summer day? That's evaporative cooling: the reduction in air temperature that occurs when water evaporates. Evaporative coolers, commonly called "swamp coolers," use this effect to cool homes. Evaporative coolers have a low first cost, use a lot less electricity than conventional air conditioners, and do not use refrigerants, such as chlorofluorcarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), that can harm the ozone layer.

There are two types of evaporative coolers: direct and indirect (all called two-stage). In a direct evaporative cooler, a blower forces air through a permeable, water-soaked pad. As the air passes through the pad, it is filtered, cooled, and humidified. An indirect evaporative cooler has a secondary heat exchanger which prevents humidity from being added to the airstream which enters the home. Evaporative coolers can be used as a sole cooling system in a home, as an alternative cooling system to a conventional refrigerant air conditioner, or in combination with a refrigeration system. However, conventional air conditioners should not be operated simultaneously with direct evaporative coolers, because air conditioners dehumidify while evaporative coolers humidify, and the two systems will work in opposition.

Evaporative coolers are sized based on cubic feet per minute (cfm) of airflow. Airflow for evaporative coolers is typically higher than conventional air conditioning systems. Two to three cfm per square foot or three to four cfm per square foot in hot desert climates is typical. Improperly sized evaporative coolers will waste water and energy and may cause excess humidity or other comfort problems. Two-speed coolers are available that can handle varying cooling loads.

Unlike air conditioned rooms, windows or ceiling vents need to be open when an evaporative cooling system is operating. The large volume of fresh air added to the home replaces a significant amount of air that exits from the home.

Many systems incorporate bleed-off valve that purges water about every six hours. This leads to an additional five gallons of water used per hour, but may be necessary to avoid mineral build-up. Bleed-off valves are generally recommended.

Indirect, or two-stage, evaporative coolers do not add humidity to the air, but cost more than direct coolers and operate at a lower efficiency. Two stage evaporative coolers combine indirect with direct evaporative cooling. This is accomplished by passing air inside a heat exchanger that is cooled by evaporation on the outside. In the second stage, the pre-cooled air passes through a water-soaked pad and picks up humidity as it cools. Because the air supply to the second stage evaporator is pre-cooled, less humidity is added to the air, whose affinity for moisture is directly related to temperature. The result, according to one manufacturer, is cool air with a relative humidity between 50 and 70 percent, dependent on the regional climate. A traditional system would produce about 80 percent relative humidity air.


Affordability

Direct evaporative coolers cost about $700 to $1000, installed, compared with several thousand dollars for conventional air conditioner and ductwork. In addition, operating costs are about 1/3 that of conventional air conditioning (including the cost of water, depending on electric and water costs). Indirect evaporative coolers cost are much higher.

Energy Efficiency

Evaporative coolers use about ¼ the electricity of conventional air conditioners.

Environmental Performance

Evaporative coolers can improve the indoor air quality inside a home by drawing a large supply of fresh outdoor air through the home. However, they consume between 3.5 and 10.5 gallons of water per hour of operation.


Easy

Evaporative coolers will not work well in humid climates--they will not cool air significantly and cause discomfort by excessive humidity.

Evaporative coolers are readily available through distributors and retailers mainly in arid regions, where they function well. However, they use a large amount of fresh water resources that are usually limited in these regions. Studies have shown that evaporative coolers use between 3.5 and 10.5 gallons of water per hour of run time. Systems that have a bleed-off valve, which reduces mineral build-up on the pads, use the most water.

Two-stage evaporative coolers force large amounts of outdoor air into a home, which may cause discomfort for people with allergies. Regular maintenance is required, although the average homeowner can usually perform the maintenance.


Installed, evaporative coolers cost about $700 to $1,000. A two-stage evaporative cooler with a cooling capacity equivalent to a three-ton conventional system retails for about $1,800, and total installed cost is estimated at about $3,900.


Evaporative coolers use about ¼ the electricity of a conventional air conditioner and add about 19,000 gallons, on average, to a homeowner’s water consumption.


Check with the local building official to determine if code constraints limit the use of evaporative coolers.


Here are the field evaluations linked to this technology:

Community Development Corporation: Magna, UT


Most evaporative coolers are roof mounted, but others are mounted through the wall or on the ground in a shaded area. They can be adapted into existing ductwork, designed with dedicated ductwork, or exhausted directly into a diffuser located in a central area of the home. Dedicated ductwork is preferable, but not usually practical, because it requires higher airflows and larger ductwork than conventional heating and cooling ductwork. Air inlets should be located away from gas flues and plumbing, dryer, and exhaust fan vents, so that exhaust air is not pulled indoors by the evaporative cooler inlet. Access for maintenance should also be considered when installing.

Monthly service and maintenance is highly recommended. Drip pans should be emptied and cleaned if the system is not in operation for an extended period time, because stagnant water can be a source of bacteria and mold growth. Each year, natural fiber (aspen wood) pads should be replaced and synthetic fiber pads should be cleaned.


Warranties vary by manufacturer and product.


Evaporative coolers use between one-tenth and one-third the amount of electricity of conventional air conditioners. They do not use CFCs or other ozone-depleting compounds. They are cheaper to operate than conventional air conditioning.

A 4,500 CFM direct evaporative cooler, adequate for a 1,500 square foot home, would cost about $800, installed. Installation is relatively simple and total investment cost is less than air conditioning. Replacement parts are inexpensive compared with a conventional air conditioning system parts. In addition, most maintenance can be performed by the homeowner.

A two-stage evaporative cooler with a cooling capacity equivalent to a three-ton conventional system retails for about $1,800, and total installed cost is estimated at about $3,900. According to the manufacturer, installed cost is lower than conventional high efficiency air conditioners, and the units can condition a 1,700 square foot house in Sacramento, California during peak cooling season for less than $30 per month (at 8 cents per kilowatt hour).

With evaporative cooling, a complete air change occurs every one to three minutes, offering a great health advantage over traditional refrigerated air conditioning, which recirculates house air repeatedly.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.