Evaporative Coolers: An energy-saving way to beat the heat

Suppose you've already taken care of the most energy efficient ways of keeping cool: using fans, nighttime ventilation, natural shade and exterior shades, and insulating and venting your attic. On those few summer days in Western Oregon when the temperature climbs above the mid-90s, an evaporative cooler offers an energy-efficient, ozone-friendly way to further cool off your house. Energy-saving evaporative cooling works great in Oregon's dry, Mediterrean-style summers.

Evaporative Cooler vs. Air Conditioner

Evaporative coolers compare very favorably to air conditioners. Air conditioners are noisy, consume lots of electricity, require ozone-eating refrigerants, and are difficult for homeowners to repair. Evaporative coolers are relatively quiet, simple appliances which use less than a quarter as much electricity as an air conditioner.

• An evaporative cooler basically consists of a large fan and water-wetted pads. Fresh outside air is cooled by about 20 degrees as it is drawn through the wet pads and blown into the house. The cooler slightly increases the humidity of the entering air. This contrasts with air conditioners, which reduce humidity as they recycle the air in the house.
• The wetted pads on an evaporative cooler are fairly efficient air filters, trapping particles on their wet surfaces. The continuous wetting of the pads flushes the trapped particulates into the sump, where they are contained.
• The low power requirement means you can just plug an evaporative cooler into a nearby 120 volt wall outlet on all but the largest units. Usually no special wiring is needed. Many air conditioners require their own high amperage power circuit.
• You will need to provide a water supply to the cooler; some small coolers are filled manually, while for the larger ones a ball-valve "Y" hose fitting on a outside hose bibb will do the job. Water consumption can be from 3 to 15 gallons a day.
• A small evaporative cooler can sometimes be temporarily placed in a window. Any evaporative cooler must have immediate access to outside air. Larger units usually require some ducting, the complexity of which will be dependent on the location of the cooler. Like air conditioners, larger units can be installed in window-mount, wall-mount, and roof mount locations, and can be interconnected with the forced air duct system in the house. Coolers can also be placed on the ground. Any ducting will need to be large enough to handle the large amount of air delivered by the cooler. Keep in mind that since the cooled air supplied by an evaporative cooler is warmer than the air delivered by an air conditioner, it will take more air flow to cool the house.
• It is important to leave at least one window open when an evaporative cooler is running! You can steer the cooled air blown into the house to various rooms by opening the appropriate windows. It may also be appropriate to open an attic access hatch to allow air to escape into the attic, but only if sufficient passive attic venting is installed.
• Like any air conditioner, an evaporative cooler will need to be winterized or taken inside at the onset of cooler weather. Openings into the house will need to be sealed and insulated for the winter.
• Evaporative coolers cost approximately half as much to buy as an air conditioner of equivalent cooling capacity. Installation costs will be similar. Operating costs will be less than a fourth as much as an air conditioner.

Cooling Performance

The cooling performance of a single stage evaporative cooler is determined primarily by the temperature and relative humidity of the incoming air, as you can see by the chart below.The yellow boxes identify the optimum conditions. Typically, the relative humidity falls to around 30% on a 100 degree day in the Willamette Valley, yielding a cooled air temperature of about 82 degrees.

Types of Evaporative Coolers

There are two basic types of evaporative coolers: two stage or direct/indirect coolers, and single stage or direct coolers. Two stage, or direct/indirect coolers use an air-to-water heat exchanger/ precooler which reduces the incoming air temperature without raising the relative humidity, then puts the incoming air through a direct evaporation stage, further reducing its temperature. Because of their expense, direct/indirect units are typically only used where daytime temperatures consistently exceed 100 degrees. Single stage or direct type coolers are the most common, and come in three different pad types:

• Fixed fiber pad coolers, where the pads are made of shredded aspen wood fibers packaged in a plastic netting. The pads may also be called aspen or excelsior. There are a few synthetic substitutes, which reportedly do not work quite as well as the real material. A small pump moves water from a catch sump at the bottom of the cooler up to the top of the pads. The pads are wetted by water dripping onto them from the top. Natural fiber pads may require replacement every year or two if the cooler is used frequently. A new set of pads typically costs $20-$40.
• Rotating pad coolers, where the pads are made of a rough polyester material sewn into a wide belt shape. The belts are rotated by pad motors through a trough of water at the bottom of the cooler. No pump is required in this type of cooler, and the pads should last longer than aspen.
• Rigid-sheet pad coolers, where the pad is a stack of corrugated sheet material that allows air to flow through. The pads are wetted by a pump distributing water to the top of the pads, similar to the fiber pad types. This type of pad is far more expensive than fiber pads, but can last much longer if water quality is maintained.

Water Quality and Pad Life

Water quality is important to the longevity and performance of any evaporative cooler. Minerals in the supplied water will concentrate in the sump and eventually begin to create scale or deposits on the pads. These deposits can severely degrade the efficiency of the pads, and shorten their useful life. If you have hard water, there are three ways to handle the problem:

• Install the appropriate water treatment system. Supply the cooler with water of at least the same quality as your drinking water.
• Install a bleed-off system that continuously leaks a small quantity of water from the upper distribution trough, diluting mineral concentrations.
• Install a sump dump, or blow-down system that periodically dumps the water from the sump while the cooler is being operated. A blow-down system is preferred, especially in dusty areas, because it will clean the sump of filtered dirt and particulates. The discarded cooler sump water can be used on trees and plants, or to flush toilets. Bleed-off or blow-down systems are typically available as options with the cooler, if they are needed.

Sizing an Evaporative Cooler

Correctly sizing an evaporative cooler to the cooling load of your home is much less critical than it is with an air conditioner. Coolers usually have 2 or 3 fan speeds, and they actually cool more efficiently at lower speeds*, so oversizing is not a real concern. There are, however, two sizing methods, both based on an Industry Standard CFM rating which is assigned to the evaporative cooler by the manufacturer. The first sizing method is to figure in 2 to 3 Industry Standard CFM per square foot of floor area. (The 2-3 IS CFM per sq. ft. is appropriate for the Willamette Valley. Eastern Oregon may require more like 3-4 IS CFM per sq. ft.) The second method is most appropriate for coolers interconnected to a forced air system, and assumes that a Manual J-type cooling load calculation has been performed; simply figure 1000 Industry Standard CFM per ton of cooling. (A ton of cooling is 12,000 Btu.) Select an evaporative cooler that is rated for at least the industry Standard CFM arrived at by the sizing calculation.

*This is because of increased fan efficiency at slower blower speeds, and higher saturation effectiveness as the wetted pad temperature drops closer to the wet-bulb temperature of the ambient air at lower air velocities.

Installation Tips

• Buy a cooler with a 2 or 3 speed blower motor, and the correct blower control.
• Install a thermostat to turn the cooler on and off automatically. Low voltage thermostats are more accurate than line voltage thermostats. Evaporative cooler control systems are available that include a low voltage thermostat and fan delay that allows the pads to get wet before the blower turns on.
• Since the cooler will be taking in fresh air from outside, put it at least 10 feet away from or 3 feet below plumbing vents, combustion appliance flues, clothes dryer vents, or exhaust fan vents. Keep animals, yard chemicals, and fuels and solvents away from the cooler vicinity.
• Forced air system interconnected coolers should have a damper between the cooler and the ducts which allows the cooler to be isolated from the house system. The damper may be a barometric, motorized, or manual type.
• Provide an electrical disconnect near and in a direct line of sight from the cooler, preferably reachable from ground level. Allow a minimum 3 foot clearance in front of any panel with electrical parts. Turn the cooler power off when you work on it!
• Provide a cooler water shutoff reachable from ground level.
• Be sure to check the float valve setting when you test the new cooler to prevent overflows.

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