An evaporative cooler is a cooling system that lowers air temperature by passing warm air through water-soaked pads, causing the water to evaporate and absorb heat in the process. Often called “swamp coolers,” these units use a fraction of the electricity of traditional air conditioning and work best in hot, dry climates. They’re common across the American Southwest, parts of Australia, and anywhere humidity stays low during summer months.
How Evaporative Cooling Works
The physics behind an evaporative cooler is the same reason you feel cold stepping out of a swimming pool on a breezy day. Water molecules naturally stick together in liquid form, and pulling them apart into vapor requires energy. That energy comes from the heat in the surrounding air. As the air gives up its thermal energy to turn water into vapor, the air temperature drops.
Inside the unit, a pump draws water from a reservoir and distributes it across thick cooling pads. A blower fan pulls hot outside air through those wet pads. As the air moves through, some of the water evaporates, and the air that comes out the other side is cooler and more humid. That cooled air is then pushed into your living space through ducts or directly from the unit.
This is fundamentally different from refrigerated air conditioning, which uses chemical refrigerants and a compressor to extract heat. Evaporative coolers rely only on water and airflow, which is why they use so much less energy.
Main Components
Evaporative coolers are mechanically simple, which is part of their appeal. The core parts include:
- Cooling pads: Thick, absorbent media (typically cellulose or synthetic fiber) that hold water and provide surface area for evaporation.
- Water pump: Circulates water from the reservoir up to the top of the pads, keeping them consistently wet.
- Blower fan and motor: Pulls outside air through the pads and pushes cooled air into the home. A pulley system connects the motor to the blower.
- Float valve: Works like the valve inside a toilet tank, automatically refilling the water reservoir as the level drops.
- Water reservoir (pan): Collects water at the bottom of the unit for recirculation.
Whole-house units are typically mounted on the roof or on an exterior wall, with ductwork distributing cooled air through the home. Portable units sit on the floor and cool a single room.
Direct, Indirect, and Two-Stage Systems
Not all evaporative coolers work the same way, and the differences matter for comfort.
Direct evaporative coolers are the most common and least expensive type. Air passes straight through wet pads and into your home. The tradeoff: they increase indoor humidity. In a very dry climate, that added moisture can actually feel pleasant, but in moderately humid conditions it makes the air feel sticky rather than cool.
Indirect evaporative coolers use an air-to-air heat exchanger so the cooled air never touches the water directly. This means they lower the temperature without adding any humidity to your indoor air. They cost more and produce slightly less cooling, but they’re a better fit for climates that hover closer to the humidity threshold.
Two-stage systems combine both approaches. In the first stage, air passes through an indirect heat exchanger, dropping in temperature without gaining moisture. Then it enters a second, direct stage where it flows through wet pads for additional cooling. Because the air was pre-cooled in the first stage, it comes out cooler and less humid than air from a single-stage direct unit. These systems deliver the best performance but carry the highest price tag.
Where They Work (and Where They Don’t)
Evaporative coolers perform well when the outside air is hot and dry. As a general rule, they’re effective when relative humidity stays below about 50%. In places like Phoenix, Albuquerque, or Denver, they can drop indoor temperatures by 15 to 25 degrees Fahrenheit on a hot day.
In humid climates, they struggle. Since the air already holds a lot of moisture, water can’t evaporate efficiently, and the cooling effect shrinks. Worse, they push even more humidity indoors. The Department of Energy advises against using evaporative coolers in humid climates for this reason. If you live in Houston, Atlanta, or anywhere along the Gulf Coast, traditional air conditioning is the better choice.
Ventilation and Open Windows
One of the more surprising aspects of evaporative coolers is that they require you to leave windows partially open while running. Unlike central air conditioning, which recirculates the same indoor air in a sealed house, evaporative coolers push a steady stream of fresh outside air into your home. That air needs somewhere to go. If windows aren’t open enough, humidity builds up indoors and the cooling effect drops off.
This constant air exchange is actually an advantage for indoor air quality. You’re continuously flushing stale indoor air and replacing it with filtered outdoor air. Optional filters on the intake side can remove most dust before it enters the blower. The downside is that in areas with wildfire smoke, pollen, or poor outdoor air quality, you’re pulling all of that inside too.
Water Usage
Evaporative coolers trade electricity savings for water consumption, and in arid regions where water is scarce, that’s worth understanding. A study of residential coolers in Phoenix (sized between 4,500 and 6,500 cubic feet per minute, typical for whole-house units) found that households used an average of 3.3 gallons per hour during operation when coolers were equipped with systems that minimized water waste. Households without those water-saving features averaged around 10.4 gallons per hour.
Over a full cooling season, that adds up. A cooler running 10 hours a day at the higher rate would consume over 100 gallons daily. Many coolers include a bleed-off valve that continuously drains a small amount of recirculating water to prevent mineral concentration from building up. Adjusting this valve properly balances water conservation against mineral buildup.
Cost Compared to Air Conditioning
Evaporative coolers are significantly cheaper to buy, install, and operate than refrigerated AC systems. A whole-house evaporative cooler typically costs between $1,500 and $3,500 installed, depending on size and whether it’s a direct or indirect unit. By comparison, a full central air conditioning system in 2025 runs $7,500 on the low end and $12,000 to $15,000 or more for complex installations involving attic placement or duct repairs.
Operating costs tell a similar story. Evaporative coolers use roughly one-quarter the electricity of a comparably sized AC system because they’re only powering a fan and a small pump, not a compressor. In a climate where they work well, the energy savings over a summer can be substantial.
Maintenance Through the Season
Evaporative coolers need more hands-on attention than air conditioners. The main enemy is mineral buildup. As water evaporates, the dissolved minerals in your tap water stay behind, accumulating on the pads, in the reservoir, and on internal surfaces. Hard water accelerates this problem considerably.
At the start of cooling season, install fresh pads, clean out any debris from the water pan, check that the pump and float valve are working, and make sure the bleed-off valve is adjusted properly. Mid-summer, inspect the pads again. If they have heavy mineral deposits, they restrict airflow and reduce efficiency, so replace them. Check that the bleed-off valve is draining just enough water to slow mineral concentration without wasting excess.
When the cooling season ends, winterizing is essential. Disconnect electricity to the unit, shut off the water supply, and fully drain the reservoir. Stagnant water left sitting through the off-season becomes a breeding ground for bacteria, including Legionella, the organism that causes Legionnaires’ disease. Frozen water in the basin can also crack or destroy components. Disconnect the water supply tubing and drain it from the lowest point to prevent burst lines in freezing weather. Use vinegar to dissolve mineral deposits in the bottom tray before covering or sealing the unit for winter.
Pros and Limitations at a Glance
Evaporative coolers make strong financial and environmental sense in the right climate. They cost less upfront, use far less electricity, and constantly bring fresh air into your home. The added humidity they produce is often welcome in desert environments where dry air cracks skin and irritates sinuses.
The limitations are real, though. They simply don’t work in humid weather. They consume meaningful amounts of water in regions where water is already precious. They require regular maintenance that air conditioners don’t. And because they need open windows, they can’t filter or control indoor air the way a sealed AC system can. For anyone living in a hot, dry climate and comfortable with a bit of seasonal upkeep, they remain one of the most cost-effective ways to stay cool.