Routine Maintenance of an Evaporative Cooler

A rooftop evaporative cooler.
credit: Robbins Heating & Air Conditioning Inc.
Rooftop swamp coolers provide whole-house comfort in arid climates.

A little science talk helps you appreciate how an evaporative cooler works and for which climates it's best suited. Evaporation is a endothermic process, which means it absorbs heat from the surroundings in order to effect a change of state from liquid to vapor. Particles in a vapor are in a higher energy state than those in a liquid, so energy is required for evaporation to occur. That energy comes in the form of heat from the surroundings. In other words, when a liquid evaporates, the immediate environment become cooler. The process doesn't create cool air—it creates air that is less warm. It's a subtle distinction, but an important one.

Air conditioners rely on this principle, but the substance that evaporates is a specially formulated refrigerant sealed in a system of coils. A compressor circulates this refrigerant as a liquid through the condenser coils and forces it through a tiny aperture into the evaporative coils. As it vaporizes, it draws heat from the air around the coils.

Evaporative coolers—also known as swamp coolers—employ the same principle, but instead of a refrigerant in a sealed system, they stream plain old water onto an absorbent pad, from where it evaporates. Anyone who has ever sweated knows that evaporating water cools the surrounding air. The same principle applies with an evaporative cooler A fan inside the unit blows the cool air through a grid or duct system and into the living space. This process is especially suited for hot arid climates, where swamp coolers are de rigeur. They require more maintenance than air conditioners, but not much.

Evaporative Cooling vs. Air Conditioning

Like an air conditioner, an evaporative cooler has a pump, but since its only function is to circulate water, it requires far less energy than a refrigerant compressor. Whereas air conditioners typically run on 240-volt, 30-amp circuits, a swamp cooler needs only conventional 120-volt, 20-amp power. A swamp cooler also has a fan, but this also runs on 120-volt power, and both the fan and the pump can operate from a single power source.

The trade-off for the power savings is that the evaporation of water from an internal pad does not provide the same amount of cooling as the evaporation of refrigerant in air conditioner coils. Another characteristic of evaporative coolers is that they humidify the surrounding air. This could be a major drawback in hot, humid weather, but in arid, desert-like places where these coolers are best suited, it's an advantage. In an arid environment, a properly sized evaporative cooler is all you need to keep your house cool and comfortable.

Evaporative coolers come as rooftop units that provide whole-house cooling through a system of ducts. These units are usually connected to plumbing pipes, so you don't need to worry about replenishing the water supply or draining the unit. You can also buy window-mounted units that resemble air conditioners from indoors, and standalone units that you can move from room to room. Both of these types usually require manual filling and draining.

An Overview of the Internal Parts

Internal view of a swamp cooler.
credit: U.S. Department of Energy
A swamp cooler consists of a water pump, a evaporation pad and a fan.

To function properly, an evaporative cooler needs an internal water pump that provides a steady stream of water. The water may be recycled from a reservoir, which is common in standalone units; or it may come from a pipe connected to the home's water supply, which is more common in rooftop units.

The water circulates through a filter medium specially designed to absorb as much water as possible and therefore to provide maximum cooling. Manufacturers typically supply filters made from aspen fibers, which swell as they absorb water to provide maximum water retention and cooling capacity. Filter pads made from cardboard, coconut fibers and synthetic materials are also common.

Finally, every evaporative cooler has a fan to circulate air through the filter medium and promote evaporation and to blow the air through the grid or duct system into the living space. The fan is typically protected by a second fiber pad to filter out dust.

Weekly Maintenance

Evaporative cooler pads.
credit: Indoor Comfort Supply
Corrugated and mesh pads are both common and need regular care.

An evaporative cooler is not a sealed system, and the water that circulates through the filter pad gets dirty. It may get contaminated by bacteria, which become airborne as the fan blows through the pad. To keep your system clean, you should drain the water reservoir at least once a week. Standalone units have a drain plug, usually located on the bottom of the housing. Turn off the unit, open the drain plug and let all the water drain, then turn on the fan to dry out the filter pad. This helps discourage mold growth in the pad.

Standalone units need to be replenished with fresh water regularly. The amount you need to add depends on how often you use the cooler. It's a good idea to check the reservoir level every day if you use the cooler frequently.

You don't have to worry about water contamination if you have a rooftop unit connected to a supply pipe and a drain, because fresh water is always circulating through the pads.

Monthly Maintenance

A window-mount swamp cooler.
credit: Sylvane
Window units look like air conditioners from inside.

Although the air circulating through the filter pads is a natural mold and mildew deterrent, some mold is bound to get a foothold during the course of a hot summer. Allowing the filter to dry out completely when you change the water is a good way to keep it under control, but to be on the safe side, it's best to remove the filter and clean it at least once a month. Let the filter dry, then saturate it with a 50-50 solution of vinegar and water and let it dry out again. Vinegar is natural disinfectant that doesn't create any toxic byproducts, which makes it better than bleach for this purpose.

When you have the filter out, clean the inside of the filter compartment, as well as the water reservoir, with soap and water.

Beginning and End of Season

Cool Space evaporative cooler.
credit: Cool Space
Portable swamp coolers usually have a drain plug on the bottom.

You probably don't need your evaporative cooler during the winter, so before you put it into hibernation, drain it completely and remove the filter pads. Wipe down the interior of the unit or wash it with soap and water, then replace the pads when everything has dried out. Unplug the unit or switch off the circuit breaker to ensure it doesn't start up when it's empty of water.

When you start up the unit in the spring, it's a good idea to remove the pad again and inspect it. This is the best time of year to replace the pad, if necessary. Some pros recommend replacing the pads at least twice a year, and you should heed that advice if you keep your unit running constantly. High-quality pads may not need replacing quite that frequently, but if inspection reveals damage or cracking to any part of the pad, then it should be replaced.

Fan and Water System Maintenance

The fan rotors and motor need periodic lubrication to keep them in good working order. The best time to do this is in the spring when you're restarting the unit. Clean the fan blades and the motor housing with a damp rag, and spray lubricant into the motor shaft. This is also a good time to replace the fan filter.

Every few years it's a good idea to disassemble the fan blades—following instructions in the owner's manual—and give them a deep cleaning, along with the fan housing. While you're at it, disassemble the water tubes inside the unit and soak them in a vinegar and water solution to keep them free of mold and other debris.