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. The only catch is that they can be used effectively only in drier climates.
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
- 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
- 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.
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.
Temperature Delivered by Evaporative Cooler
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
- 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.
- 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
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