Evaporative coolers can be an effective and less expensive alternative to compressor-based air conditioners for cooling your home if you live in a dry climate. Evaporative coolers (known in the past as swamp coolers) use evaporation and blowing air to cool. They can use about one-fourth as much energy as central air conditioners and cost less than half the price to install and operate. Evaporative coolers work best in dry climates where the outdoor relative humidity during the cooling season is less than 50%.

The evaporative cooler is a fairly simple device consisting of a box-like housing that contains a large blower fan and an 8- to 12-inch-thick sponge-like filter made of treated cellulose, fiberglass, plastic foam, or shredded aspen fibers, that is constantly soaked with water from a sprayer and recirculating pump located in the unit. The blower fan pulls in outside air and blows it through the sponge-like medium and into the building.

The flowing air evaporates some of the water, giving up heat in the process, which can reduce the temperature of the air entering the home by 15°F to 40°F.

The incoming air is not recirculated inside the house but is blown through the house and must have a way to exit. When operating, evaporative coolers bring in enough fresh air to completely replace house air every 2 to 3 minutes. For the system to work correctly, doors or windows must be opened to provide a path for the air to exit. Alternatively, ceiling exhaust vents can be installed. These typically exhaust directly into the attic, which must be a vented attic. The exhaust vents usually come with a barometric damper that automatically opens when the cooler is on. They may also have a mechanically-controlled damper so they can be closed during the heating season or if the home is also equipped with refrigerant air conditioning. For every 1,000 CFM of air delivery, 2 ft2 of unrestricted air relief is required (PG&E 2004). Because the air will tend to flow to open outlets, some zoning of cooling can be achieved by opening windows or vents in those rooms where cooling is desired.

An evaporative cooler should have at least two speeds and a vent-only option. The vent-only operation allows the unit to be used as a house fan during mild weather. Unlike refrigerant-cycle-based air conditioning, evaporative coolers increase rather than decrease the humidity in the air.

The cooler can be installed to blow air directly into a central location in the home or the cooler can be connected to ductwork to distribute the air to different rooms. Central-location installations work well for compact homes with open floor plans. Ducted systems are required for larger houses with hallways and multiple bedrooms. Down-flow systems, which are mounted on the roof and connected to a duct system in the attic, are the most common installation. However, ground- or wall-mounted units are preferred because they eliminate the risk of roof leaks and they are much easier to access for maintenance.

Evaporative coolers require regular maintenance to keep the water reservoir clean and odor-free. Maintenance should be done at least twice a year, when the unit is first started up in the spring and when it is shut down in the winter. In the spring, the filter media should be replaced or cleaned, the belt inspected for proper tension, and the cabinet cleaned and inspected for damage. In the winter, the water line should be shut off, the pan should be drained and cleaned, and a winter cover should be installed to prevent dirt and dust from getting inside the unit. During the cooling season, the drain pan should be emptied whenever the cooler will be idle for more than a few days as standing water left in the pan can become a breeding ground for mosquitos and bacteria, including Legionnaire’s Disease. 

If the local water has a high mineral content, a bleed-off valve can be installed in the recirculating line to drain off water and reduce mineral concentrations; however, this can increase water usage by 10% to 50%. A more efficient alternative is to install an autoflush system (also called a dump pump) that empties the cooler pan and brings in clean water at regular intervals (e.g., every 8 to 12 hours of operation) to reduce mineral buildup.
Although evaporative coolers are typically less expensive to purchase, install, and operate; their water usage and water costs must be taken into account. Annual water use varies based on the number of hours run and the dryness of the air. For example for a 1,500 ft2 home, annual water use for an evaporative cooler without a bleed-off valve could range from 2,980 gallons in Santa Fe, to 7,261 gallons in Albuquerque, to 12,457 gallons in Las Cruces, New Mexico. Increase these numbers 50% if the cooler has a bleed-off valve (SWEEP 2007).

The single-inlet direct evaporative coolers of today are more efficient than their "swamp cooler" counterparts of years ago. Their cooling effect is dependent on the temperature difference between the dry-bulb and wet-bulb temperature (wet-bulb depression) of the entering air, the velocity of the air moving through the medium, and the quality and condition of the medium itself.

Newer evaporative coolers have thick media with efficiencies (cooling effectiveness) of 75% to 93%. For example, a 12-inch-thick medium can have an efficiency of 90% or better. The cooling effectiveness or efficiency of the evaporative coolers is a measure of how closely the supply air temperature leaving the evaporative cooler approaches the outdoor wet-bulb temperature.

When we think of outdoor temperature, we are usually thinking of dry-bulb temperature, which is the temperature of the air measured using a common, ordinary thermometer. Wet-bulb temperature is measured with a thermometer that has a wet sock or cloth wick wrapped around the bulb (Figure 4). As the water in the sock evaporates, the sock loses heat and cools the sensing bulb, bringing the thermometer reading down to the wet-bulb temperature. The wet-bulb temperature is the lowest temperature that can be reached through evaporation; the more humid the air, the higher the wet-bulb temperature. The local wet-bulb temperature can also be determined from local weather information (the dry-bulb temperature and relative humidity) with the help of a psychrometric chart, which can be found at various education sites on line. (See for example ASHRAE 2013.) The temperature difference between the dry-bulb thermometer and wet-bulb thermometer is dependent on the amount of water vapor in the ambient air. 

When the wet-bulb temperature is equal to the dry-bulb temperature, the relative humidity will be 100%. If the relative humidity is lower than 100%, the wet-bulb temperature will be lower than the dry-bulb temperature. The difference between the wet-bulb temperature and the dry-bulb temperature is known as the wet-bulb depression. The greater the wet-bulb depression, the better an evaporative cooler works.

For this reason, evaporative coolers work well in dry areas of the country such as the Southwest. For example, according to ACCA Manual J Table 1, Phoenix, Arizona, has an outdoor dry-bulb (1% cooling design) temperature of 108°F and a coincidental wet-bulb temperature of 70°F. The wet-bulb depression would be 38°F (108 – 70 = 38). In these outdoor conditions, evaporative coolers would work very well. However, in Sarasota, Florida, where the outdoor temperature is 92°F and the coincidental wet-bulb temperature is 79°F, the wet-bulb depression is only 13°F (92 – 79 = 13); here an evaporative cooler would not work very well.

If you know the dry-bulb and wet-bulb temperature and the medium manufacturer’s efficiency rating for the medium, you can calculate what the incoming air temperature is likely to be. For example, if the outside air’s dry-bulb and wet-bulb temperatures are 105°F and 60°F, the wet-bulb depression would be 45°F (105 - 60 = 45). If the single-inlet evaporative cooler is using a medium that is 90% efficient, the discharge air temperature should be 105 - (45 x 0.9) = 64.5°F.

Evaporative Coolers - Cooling Effectiveness

Evaporative coolers are not measured in SEER (seasonal energy efficiency ratio, which is the efficiency measurement used for compression cooling systems) and there is no national or international testing organization for rating evaporative coolers, although some guidance for testing can be found in ANSI/ASHRAE Standard 133-2008. Instead, manufacturers refer to the cooling effectiveness of evaporative coolers, which is a measure of how close the supply (leaving) air temperature is to the outdoor (entering) wet-bulb temperature. If the supply air temperature were the same as the outdoor wet-bulb temperature, the cooling effectiveness would be 100%.

The formula used to calculate cooler effectiveness is:

e = (TDB - SAT) / (TDB - TWB)

Where:
e = the cooling effectiveness of the cooler
T_DB = the outdoor dry-bulb temperature
T_WB = the outdoor wet-bulb temperature
SAT = the supply air temperature leaving the evaporative cooler
Note. T_DB - T_WB = the wet-bulb depression

Example:
An evaporative cooler that is delivering conditioned air into the home at 74°F when the outdoor conditions are 108°F dry-bulb and 70°F wet-bulb would have the following cooling efficiency:

e = (TDB - SAT) / (TDB - TWB)

e = (108 - 74) / (108 - 70)

e = (34) / (38) = .89 OR 89%

In the example, the cooler’s efficiency is 89%, which would be considered an efficient evaporative cooler.

The chart in Figure 6 shows the cooling temperatures that an evaporative cooler can provide at outdoor temperatures ranging from 75°F to 125°F at 2% to 80% relative humidity. The numbers in blue represent the ideal cooling temperatures achievable across these conditions. 

Here is an example of how wet-bulb (WB) temperature impacts an evaporative cooler’s cooling ability. On a day when the outdoor temperature is 100°F dry bulb (DB) at 10% relative humidity (RH) (63°F WB according to a psychrometric chart), the supply air entering the home will be 73°F DB, which is a 27°F DB temperature drop. However, if the outdoor temperature is at 95°F DB at 50% RH (79°F WB), the supply air temperature will be 84°F DB (only an 11°F DB temperature drop).

System Sizing

Sizing an evaporative cooler is not an exact science. Evaporative coolers are sized by airflow not by "tons of cooling." Common evaporative cooler sizes range from 3,000 to 6,500 cfm (cubic feet per minute).

Most manufacturers call for something like 20 to 40 air changes per hour (ACH) through the conditioned space, depending on location and outdoor conditions. The map in Figure 7 is a simplified version of the map in Figure 5 and shows the air changes per hour that an evaporative cooler would have to produce to provide comfort at that region’s average wet-bulb temperature at 1% summer design conditions (i.e., the outdoor temperature that will be exceeded only 1% of the time in a given year). 

To determine your evaporative cooler system size:

1. Consult the ACH Zone Map shown in Figure 7.
2. Determine the cubic feet (CU FT) of the space to be conditioned.
3. Multiply house volume by regional ACH and divide by 60 to get evaporative cooler size in cfm. Cooler size in cfm = (CU FT x ACH) ÷ 60.

For example, if we have a 2,000-ft2 home with 8-ft ceilings located near Salt Lake City, Utah, according to the zone map, we should size the evaporative cooler so that it will provide 20 ACH. The volume of the conditioned space of the home is 16,000 ft3 (2000 x 8). So the formula would look like this: (16,000 x 20) ÷ 60 =  5,333 cfm.

Newer Technologies

Indirect coolers use an air-to-air heat exchanger so they don’t add humidity to the cool air (Figure 8). The main blower fan supplies outside air that is cooled by passing through the heat exchanger as it is drawn into the home. A secondary fan draws exhaust air from the home and/or outside air through wet pads, providing cool air, which takes heat from the incoming supply air as the two air flows cross paths in the heat exchanger.

Indirect/direct coolers cool in two stages. In the first stage, the air passes through an indirect heat exchanger, which lowers the temperature without adding humidity. The air then enters the second, direct cooling stage where it flows through wet pads just like a direct evaporative cooler to be further cooled and humidified before flowing into the home. Because the air has been pre-cooled in the first stage, when it leaves the second stage it is cooler and has less moisture than the air leaving the single-stage unit. This technology achieves evaporative cooling efficiency of 95% or greater (PG&E 2004). One model, developed by Building America partner Davis Energy, reportedly provides up to 5 tons of cooling while using less than 1600 watts. The model is eligible for utility rebates in several southwestern states.

Another innovative evaporative technology is the water-cooled evaporative condenser, which is a scaled-down residential version of the 250-ton chillers used on commercial buildings. Inside the housing, a mist of water is continually sprayed on the condenser coils to remove heat from the refrigerant and at the same time reduce the work of the compressor. Unlike traditional air conditioners, which use 10% more power for each 10°F increase in temperature above 95°F, an evaporative-cooled condenser draws the same power over a wide range of outdoor temperatures. It uses about half the energy of conventional “air-cooled” condensing units. One model has an EER of 17 at 95°F (German et al. 2012).

Evaporative coolers have been developed that use photovoltaic (PV) panels to create the electricity used to run the blower and the water pump. For hot desert areas, the combination of evaporative cooling and solar power is a perfect match: the afternoon, when the most solar energy is available, is also the hottest part of the day, when cooling is most needed. And, because evaporative coolers use a fraction of the energy of air conditioners, PV cells can provide enough electricity to run the system effectively (SWEEP 2007).

How to install an Evaporative Cooler

1. Select the most efficient model the project can afford. Consider newer types of evaporative coolers including indirect/direct evaporative coolers.
2. Select a model with a cover to minimize maintenance during the off-season.
3. Install on the ground or wall rather than roof to make maintenance easier for the homeowner.
4. Install in a central location for best air distribution. If there is ductwork, size as per ACCA Manual D.
5. Install ceiling vents with barometric and mechanical dampers.