This article was originally published in the May/June 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online May/June 1996

Installing and Maintaining Evaporative Coolers

by Roy Otterbein

Roy Otterbein is president of Otterbein Engineering in Phoenix, Arizona. He holds three patents in indirect evaporative cooling and is a member of the ASHRAE Standards Committees on direct and indirect evaporative coolers.

As warm weather comes upon us, many people in the western United States will be starting up or replacing evaporative coolers, or buying them for the first time. Proper installation and maintenance of these systems is very important, and recent improvements in the technology change how to best handle these tasks.


Evaporative coolers cost only one-tenth to one-fourth as much to operate as refrigeration air conditioning and are much cheaper to buy ($400-$800). This makes them an excellent option, particularly in hot, dry areas of the country.

An evaporative cooler is a simple device consisting of a fan and a water-wetted pad. A small pump recirculates water from a sump (which is part of the cooler cabinet) to keep the pad wet. The fan draws outside air through the wet pad, making the air more humid but colder. This air is blown into the house, forcing the warmer air in the house to be exhausted out open windows or through a vent into the attic (see Figure 1). This is quite different from refrigeration air conditioning, which cools inside air and returns it to the house.

Although the use of outside air is one major benefit of evaporative coolers, it also complicates their installation and operation. The evaporative cooler must be installed outside the house, ducted into the house, and freeze protected and isolated from the house during the winter months.


Figure 1. Evaporative coolers bring outside air into a house and exhaust it through open windows or ceiling-mounted barometric dampers.


Cooler Performance

Evaporative coolers make air cold by forcing dry air through a wet pad. The wet-bulb temperature (the temperature of air leaving a 100% effective pad) is a function of the entering-air temperature and relative humidity. In heat exchanger theory, evaporative cooler pads are considered constant temperature heat exchangers, whose surface is at the wet-bulb temperature of the air passing through them. How close it gets to that temperature is called the saturation effectiveness of the pad, which improves at lower air-flow rates. Most pads have a saturation effectiveness of 60% to 90%.

Evaporative coolers are most popular in areas with the coldest summer wet-bulb temperatures, which tend to be in the western United States. Figure 2 shows 1% wet bulb isolines--the wet bulb temperature that is exceeded only 1% of the time during the summer months in a given area.


Figure 2. Wet bulb temperature isolines at the 1% level. Summer wet bulb temperatures indicate where evaporative coolers will work best. If an area has a 1% summer wet bulb temperature of 70°F or below, an evaporative cooler should be able to provide most of a house's cooling needs. However, they are still very popular in areas with wet bulbs between 70°F and 75°F.


A quick check for evaporative pad performance is to compare the temperature of the water in the cooler sump (approximately the wet-bulb temperature of outside air) to that of the air entering and leaving the cooler (see Figure 3). The following equation can be used:

Saturation effectiveness = ( TOSA-TLA)/( TOSA -TSUMP ) x 100

where TOSA = Temperature of outside air entering cooler

TLA = Temperature of air leaving pad (air inside cooler)

TSUMP = Temperature of sump water


Figure 3. The saturation effectiveness of evaporative cooler pads can be checked with a thermometer. In this example, a cooler has 70°F air leaving the pad during a 100°F day, and has a sump water temperature of 60°F. Saturation effectiveness = (100-70) / (100-60) x 100 = 75%.


An alternate way to determine pad performance is to check the temperature of the air leaving the cooler against the cooler index (see Table 1). The cooler index is the anticipated temperature of the air leaving an aspen pad cooler (described below) and accounts for heat added to the air by the pump and fan motors and a cabinet exposed to the sun. The cooler index has the advantage of enabling the homeowner to check cooler performance by watching the news and checking the temperature of the air leaving the diffuser in the house. If this temperature is 3°F or more higher than the cooler index indicates as normal, the cooler is not operating as well as it should be, probably due to a malfunctioning water distribution system, sagging pads, or poorly manufactured aspen pads.


Table 1. Evaporative cooler index for standard aspen-pad coolers. Enter the table from the left with outside air temperature and the top with outdoor humidity. Where the row and column meet is the temperature of air a typical evaporative cooler will produce. The cooler index can also be used to check the performance of cooler pads.


Homeowners can also use the cooler index to decide when to switch between evaporative cooling and air conditioning. As a general rule, if a cooler produces air colder than 70°F, it will create a comfortable environment; if it produces air hotter than 75°F, it will not. Between 70°F and 75°F is a gray range in which some people are comfortable and some are not.

Evaporative Cooler Types

There are two main categories of evaporative cooler: single-stage and two-stage. Single-stage or direct evaporative coolers are by far the most common and are categorized primarily by pad style.

Fiber Pad Coolers

The most common pads are shredded aspen wood fibers packed in a plastic net. This material (also known as excelsior) was once widely used to ship delicate items like glassware. There are a number of synthetic-fiber pads; however, few perform as well as high-quality aspen pads, which have a naturally wettable surface. These pads are 1 to 2 inches thick. Quality and cost vary substantially; the least expensive pads are usually the thinnest. If thin pads are used, each pad frame should be double-packed, using two pads to improve saturation effectiveness.

Fiber pads must operate at low air velocities to prevent water from being pulled off the pad by the airstream. They are therefore used on coolers that have air inlets on many sides. The pads are simply discarded every year or two and replaced with new ones. Fiber pad coolers usually cost the least and require the most maintenance.

Rigid-Sheet Pad Coolers

The other main type of cooler uses a rigid-sheet pad--a stack of corrugated sheet material that allows air to move through at higher velocities than is possible with aspen pads. These pads are usually 8 or 12 inches thick. Twenty years ago they were found only in large expensive commercial coolers, but they are now common in residential coolers as well.

Coolers using rigid-sheet pads usually have a single air inlet (and are often referred to as single-inlet coolers). The pads have a corrugation pattern that forces water to flood the pad's air inlet side where most of the evaporation of water (and scaling) occurs (see Figure 4). These pads are substantially more expensive than aspen pads, but they can last for many years if water quality is properly maintained with a bleed-off or sump dump system (discussed below). Therefore the life cycle cost for these pads can equal the cost of aspen pads (not to mention the labor savings from not having to change the pads every year or two).


Figure 4. A rigid sheet pad has alternating corrugated layers at 45° and 15° angles. The front (air-inlet face) of the pad is at the left on the diagram and is where most of the water evaporation and air filtration occurs. Water shoots upward from the water inlet pipe and is distributed by the water distribution cap. Although the air flow tends to push the water toward the back of the pad, the 45°/15° corrugation pattern forces the water to flow predominantly to the air-inlet face.


Two-Stage Coolers

Two-stage (also called indirect/ direct) evaporative coolers usually use a rigid pad and have an indirect evaporative precooler. The indirect coolers precool the air without adding humidity to the air going into the house. To understand this concept, imagine blowing air through the core of a pipe. Then sprinkle water on the outside of the pipe, and blow air across the pipe. The air inside the pipe is cooled by contact with the cool pipe, but it is not in contact with the water, so its humidity does not increase. Since this precooling adds no humidity to the air, it can still be subsequently direct-evaporatively cooled. However, because the precooled air cannot hold as much moisture, the result is both colder and drier air.

A rather startling feature of two-stage evaporative coolers is that they can produce air colder than outside wetbulb. Two-stage coolers are the highest priced and best-performing evaporative coolers. They are at their best during extremely hot (110°F-plus), dry days.

Cooler Maintenance

Rigid pads should be washed down every fall at winterization, when the scale on the pad is still soft and can be removed with the least damage to the pad. Aspen pad coolers, on the other hand, should have major maintenance in spring, when the pads should be replaced. (See Maintenance Guidelines for both cooler types.)

Since evaporative cooler pads are designed to provide wet-surface contact with all the air moving through them, they are also remarkably good air filters (hence the term air washer or scrubber). Many rigid-sheet pads can filter out 90% of particles 10 microns (µ) and larger, including most pollens and dust. Fewer data are available on aspen pad filtration; however, in my personal experience, these pads also perform well as filters.

Maintaining Water Quality

As the water in an evaporative cooler evaporates, fresh water (makeup water) is brought into the cooler. A float valve controls delivery. However, the minerals (salts) brought into the cooler with the makeup water do not evaporate, and the water in the sump becomes brackish. Eventually the water becomes saturated with minerals and the minerals precipitate out (usually at the air inlet side of the pad). During operation, most of the water evaporation and filtration occur at the air inlet side, leaving a combination of scale and previously airborne dirt on that surface. When a pad has failed, the inlet face is usually clogged while the downstream face can appear brand new. A trick to lengthen the life of rigid sheet pads is to rotate and turn the pad upside down, so that the previously downstream face becomes the upstream face.

To prevent the water from becoming saturated with minerals, a bleed-off or sump dump system should be installed.

A bleed-off system is simply a tee installed in the water distribution discharge, with a hose to a nearby drain or to the ground. Whenever the pump turns on, a small amount of the water is diverted.

A sump dump system (referred to as a blow-down in cooling tower jargon) evacuates the water from the sump every six hours or so while the cooler is operating. The dumping is done by a second pump (most commonly) or by a power-activated sump drain valve.

Sump dump systems are better than bleed-off systems because they discharge not only brackish water but also some of the enormous amount of filtered dirt that collects on the bottom of the sump. Some coolers have sloped bottoms so that minerals and dirt will gravitate toward the sump dump.

A water treatment system is a good idea for fiber pad coolers. Often it enables the user to keep a set of pads for two years. For rigid-sheet coolers, a water treatment system is essential, because rigid pads cost up to $100 to replace.

In reality, it is rare for water in a cooler not to become saturated with minerals in most desert environments. Hard water is very common in areas where evaporative cooling is used, and maintaining ideal water conditions in a cooler would consume too much water. Bleed-off systems can use as much as 5 gallons of water per hour, but if water is particularly expensive in the area, even 1 gallon of water discharge is substantially better than trapping all the minerals in the cooler.

A technique to minimize the effects of water waste from bleed-off and sump dump systems is to send the discarded water to a consumer of potentially low-quality water. Sending this water to a garden is ideal, because the cooler discharges more water when the weather is hottest and the watering needs of the garden are greatest. (Mineral-sensitive plants could be harmed, but I have watered a standard vegetable garden with no trouble.) Someone should develop a system for using this water to flush toilets.

Sizing Evaporative Coolers

Evaporative coolers generally provide warmer air than refrigeration air conditioning and therefore must deliver more air to do the same job. A basic rule for sizing evaporative coolers is to use the largest cooler (within reason) that one can afford. A large evaporative cooler with a big blower and a low-horsepower motor will perform better than a small cooler with a high-horsepower motor. (This is different from air conditioners, for which the most efficient unit is the smallest one.)

Evaporative coolers are assigned an Industry Standard CFM by the manufacturer. This CFM (cubic feet per minute air flow), which is usually a number between 2,000 and 6,500, is approximately 50% higher than the highest air flow the cooler can actually produce with no ductwork restriction. Although the Industry Standard CFM claims much more air flow than a cooler can deliver, this approach to defining cooler sizes has been used for years by many manufacturers and shocks only the novice specifier. It is not unlike the technique used to define lumber sizes; most people know that a 2 x 4 is actually smaller than 2 inches x 4 inches. Most manufacturers also provide the actual air flow the cooler produces at various duct resistances.

The ideal evaporative cooler installation is an engineered system--a room-by-room heat load is calculated, a cooler is selected, and a corresponding duct system is designed. In reality, this is rarely done, because coolers are so inexpensive. Here's an alternate way to size three basic systems common in residences.

A/C Add-On

An add-on evaporative cooler blowing into the refrigeration cooling duct system is most common in low elevation desert areas that have high cooling needs. It has a refrigeration cooling system and ductwork sized to meet the needs of that system. The ductwork is smaller than the ideal size for an evaporative cooler, but the system offers many advantages over straight refrigeration air conditioning:
  • Overall cooling costs are reduced by using evaporative cooling when feasible. (A rule of thumb is that overall cooling costs can be cut in half without loss of human comfort. Rugged individuals can save substantially more by using the evaporative cooler only.)
  • More outside air is introduced into the residence, producing better indoor air quality.
  • The A/C compressor life is extended by eliminating swing season A/C usage with its short compressor cycling.
  • A second cooling system is available if the main system breaks down.
A guide for sizing the evaporative cooler for such a system is to use 1000 CFM (Industry Standard) per ton of refrigeration.

Independent Ducted System

Another type is an evaporative cooler blowing into a single diffuser in the hall ceiling or into a dedicated duct system in the ceiling space. This is most common in areas with modest cooling needs and in houses that have floor-based heating-duct systems entirely too small for evaporative-cooling ductwork.

The sizing guide for these systems is to use 2-3 CFM (Industry Standard) per ft2 of floor space in most climates. Use 3-4 CFM per ft2 in hot desert areas.

Window-Mounted Coolers

The third type is a window-mounted evaporative cooler. This is a low-cost installation and is found wherever coolers are common. These coolers should be sized in the same way as an independent ducted system.

Blower Orientation
and Cooler Location

Most evaporative coolers are mounted on the roof and have a blower that discharges out of the cooler bottom (called a down-discharge cooler). Rooftop installations are usually the least expensive and represent a reasonable compromise between first-cost and maintenance considerations. However, problems with rooftop installations include
  • Roof deterioration, due to foot traffic and water exposure from leaking coolers.
  • Slightly (about 1°F) warmer air produced by a sunlit cooler.
  • The nuisance of requiring a ladder for maintenance.
Fiber pad down-discharge coolers have four pad frames (instead of the three pad frames of a side-discharge cooler), which produces higher pad effectiveness. Rooftop installations can also use side-discharge coolers. These require an additional sheet-metal elbow but can often be located further below the roof ridge line than a down-discharge cooler can be.

Other less common installations are

  • Side discharge through the wall to an interior duct. These installations can often be difficult to maintain, because maintenance must be done while standing on a ladder.
  • Ground mounted up discharge (or side discharge with an upward elbow). These installations are the easiest to maintain and are often naturally shaded. They are my personal favorite but be careful--dogs can sometimes mistake the coolers for fire hydrants.
See Installation Guidelines for tips to help ease maintenance and improve cooler performance.


Figure 5. Many evaporative coolers have adjustable-speed belt-driven blower wheels which allow airflow to be changed.


Increasing Air Flow

Most evaporative coolers that are installed with ducts have a belt drive system with an adjustable pulley (formally called a sheave) on the motor. This sheave has two bolts--one to secure the sheave to the motor shaft and the other to allow the effective diameter of the sheave to be changed. The motor-belt-blower system is similar to a bicycle drive system: the larger the motor sheave is made, the faster the blower wheel will rotate and the more air the cooler will deliver (see Figure 5).

The motor will run at roughly the same speed no matter what its work load. If it is overworked, it will draw excessive current and overheat, and the thermal circuit breaker in the motor will turn the motor off; as the motor cools down, it will automatically restart and repeat this process. If this occurs, the effective diameter of the motor sheave should probably be made smaller (although failed flex duct or tight bearings could also be contributing to the problem).

By checking the motor current and readjusting the sheave diameter and belt tension, the installer can maximize the cooler air output. This is often not done. In fact, I have never visited a cooler installation (except field test sites) in which the motor was putting out its potential.

If a motor is being replaced, this might be a good time to increase the horsepower, blower speed, and airflow. Whenever blower speed is increased, it is important to make sure that the cooler blower can withstand the new speed, that the increased air flow does not cause water to be pulled into the blower, that the duct air noise is acceptable, and that the circuit and wiring to the cooler are adequate for the increased current draw. Although increasing motor speed can improve cooler performance, the higher operating temperatures will decrease the life of the motor. So increase the blower speed only if the cooler's performance is inadequate.



Maintenance Guidelines

  • Always disconnect power to a cooler before maintaining it.
  • Never use light oils for bearings. Use lubricants recommended by the manufacturer or 30 SAE nondetergent. Light oils act as solvents and wash out the heavier lubricant.
  • Use only listed (UL, UR, or ETL) pumps.
  • Frozen blower motors can sometimes be salvaged to provide years more service. Sometimes a rap with a hammer can free up a stuck motor.
  • Submersible pumps should have a float switch lockout. These pumps can be ruined by running dry.
  • Some residential coolers use a thicker B belt instead of an A belt. Carefully check the replacement belt (especially for 3/4- and 1-horsepower coolers).
  • Do not use asphaltum sump liners on modern powder-painted coolers. Powder paints are superior to liquid paints; manufacturers have adopted them to meet EPA guidelines. The liner will not bond to these new paints, and today's modern coolers last much longer than the older coolers that were painted with solvent-based paint. Consult a cooler supply store if a patching material is needed for a particular cooler.
  • As a simple check, use a 12-ounce soda pop can to test the discharge rate through a bleed-off system. The can should fill up in one minute at a flow rate of 5 gallons per hour.
  • A trick for removing debris from the sump of a flat-bottom cooler is to use two wet towels to trap the debris and force it to the drain.
  • If the performance of the cooler pad is in question, check the leaving air temperature against the cooler index. If a cooler is not providing cold air, the water distribution system is probably clogged, or there may be a sparse area in an aspen pad. Be aware, however, that weather conditions (temperature and relative humidity) may simply not permit the cooler to provide air cold enough to satisfy the occupant.
  • The very prudent user will store the pump and blower motor in the house during the winter. The nighttime radiation cooling of a rooftop cooler leads to condensation inside the cooler, causing rust-seized shafts. As a result, it is fairly common for a pump or blower motor to fail at spring start-up.
  • Check the barometric damper blades. These devices are notorious for sticking open, causing heated or air-conditioned air to be lost to the outside.

Problems Not Covered in the Owner's Manuals

  • A thud when the blower starts. This is caused by a loose motor sheave and/or a loose blower pulley.
  • Blower pulley continually falls off shaft. Use a thread-locking compound on a clean bolt and pulley thread when reinstalling.
  • High-frequency humming noise. This usually occurs only on rigid pad coolers at the float valve. A resonating float valve can cause copper water lines to break. The solution is to provide additional float valve support by securing the water line to the cooler.



Installation Guidelines

  • Use a two-speed blower motor. About 60%-80% of the time, a cooler operates in low speed, which is its more efficient mode (low watts per CFM and higher saturation effectiveness).
  • Use a low-voltage thermostat. High-voltage thermostats permit greater temperature swings, although they are better than no thermostat at all. Manual control wastes energy and can make the house uncomfortably cold at night. (Many people trade in their evaporative coolers for air conditioners when all they really need is a $50 thermostat.)
  • In an add-on system, use a barometric damper at the fan discharge of the evaporative cooler. These dampers make it much easier to switch between heating/air conditioning and evaporative cooling. While barometric dampers tend to leak air more than a standard slide-in damper, the convenience of having a barometric damper tends to increase the use of the evaporative cooling mode.
  • Check the float valve setting after one pump cycle. The water held in a pad drains into the sump and can cause the sump to overflow if the float is set too high.
  • Provide an easily accessible water shutoff for rooftop installations. A leaking cooler should not require a ladder for an emergency shutoff.
  • Use closed-eye hooks in chain-hung coolers. Inadvertently lifting the cooler while removing the pad frame can cause an open-eye hook to lift out of the chain and leave the cooler unsupported. (I have first-hand experience of this!)
  • Following are installation guidelines required by code. It is pretty rare for a cooler installation not to have some code violation.
  • Provide an electrical disconnect near the cooler to facilitate safe maintenance. This is particularly important for rooftop installations, because lack of a disconnect encourages people to work on live coolers. Higher quality units now come with a disconnect.
  • Provide a minimum of 3 ft of clearance to any side of the cooler that requires access for maintenance. This is a code requirement for sides with electrical parts.
  • Be sure the cooler inlet is 10 ft away from, or 3 ft below, plumbing vents, gas flues, clothes dryer vents, or bathroom, kitchen, or laundry exhaust fan vents. The installer may find it easier to relocate the troublesome vent than the cooler.

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