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Two-stage Evaporative Cooler

Two-stage evaporative coolers improve efficiency and reduce the amount of moisture added to the air.

Drawing of a two-stage evaporative cooler. It includes intake, blower, direct cooling module, indirect cooling module, exhaust, and cool air to house.

Getting out of a pool and standing in a breeze will help you feel cool, even on a hot day. This is the principle behind evaporative cooling. Evaporative coolers, often called "swamp coolers", are cooling systems that use only water and a blower to circulate air. In the system, warm, dry air is pulled through a water-soaked pad. As the water evaporates, a cooling effect on the surrounding air occurs. Evaporative coolers use only a fraction of the energy of traditional air conditioning systems. Unfortunately, except for in very dry climates, they may increase humidity to a level that makes occupants uncomfortable. Two-stage evaporative coolers do not produce humidity levels as high as that produced by traditional single-stage evaporative coolers.

In the first stage of a two-stage cooler, warm air is pre-cooled indirectly without adding humidity (by passing inside a heat exchanger that is cooled by evaporation on the outside). In the direct stage, the precooled 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 (because cooler air can’t hold as much moisture as warmer air). The result, according to the manufacturer, is cool air with a relative humidity between 50 and 70 percent, depending on the climate, compared to a traditional system that produces about 80 percent relative humidity air.

An advanced two-stage evaporative cooler uses 100 percent outdoor air and a variable speed blower to circulate cool air. Two-stage evaporative coolers can reduce energy consumption by 60 to 75 percent over conventional air conditioning systems, according to the American Society of Heating and Engineers (ASHRAE). Yet this relative improvement depends on location and application. Evaporative coolers work best in very dry climates and are not suitable for much of the East Coast, Midwest, and Coastal U.S.

Energy Efficiency

Evaporative cooling can reduce energy consumption by 60 to 75 percent over conventional air conditioning systems.


There are two systems currently available. One system is an add-on module to an existing direct evaporative cooler that is available from select evaporative cooler distributors. The other is an all-in-one unit that is expected to be commercially available March 2006.

The packaged two-stage module is anticipated to retail for about $2,500 for a 2.5 to 3-ton equivalent capacity. Installation is expected to cost about $1,000.

According to one manufacturer, a commercially-available two-stage cooler 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 kWh. Energy costs can be reduced over conventional compressor cooling systems by 60 to 75 percent.

One manufacturer reports that the system uses about 3,000 to 6,000 gallons of water per year. Using one of the most expensive California water rates from another locale, Santa Barbara, water costs are less than $16 to $32 per year respectively, using $3.92 per hundred cubic feet rates.

Check with your local building official to determine if codes limit application of evaporative coolers in your area.

Not Applicable

Like all evaporative coolers, two-stage evaporative coolers bring outdoor air into the home at a relatively high rate and do not recycle indoor air. Therefore, an outlet is needed for the system to work effectively. A duct can be used to vent the air through the attic space.

Units can be mounted on an exterior wall, in an attic, or on an exterior concrete pad. The units require a 115 VAC electrical connection, a supply water connection and, if they have a water bleed-off mechanism to reduce mineral deposits, a method for water disposal. A heating source can also be integrated.

Check with the manufacturer for details.

When ducted, evaporative coolers are quieter and more energy efficient than traditional air conditioners.

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.