An evaporative cooler is a cooling system that lowers air temperature by passing warm air through water-soaked pads, using the natural process of evaporation to pull heat out of the air. Often called a “swamp cooler,” it works best in hot, dry climates where relative humidity stays below about 40%. Unlike traditional air conditioning, which uses a refrigerant and compressor to cool air in a sealed loop, an evaporative cooler relies on fresh airflow and water alone, making it significantly cheaper to run.
How Evaporative Cooling Works
The basic physics are simple: when water changes from a liquid to a vapor, it absorbs a large amount of heat energy from its surroundings. This is the same reason you feel cold stepping out of a pool on a breezy day. Each gram of water that evaporates absorbs roughly 2,430 joules of energy at 30°C, which is a substantial amount of heat pulled directly from the air passing through.
Inside an evaporative cooler, a pump draws water from a reservoir and distributes it across cooling pads. A fan then pulls hot outside air through those wet pads. As the air moves through, some of the water evaporates, absorbing heat and dropping the air temperature. The cooled, now slightly more humid air is pushed into your living space, while the warm indoor air it displaces needs somewhere to go, which is why these systems require open windows or exhaust vents to work properly.
Key Components
Every evaporative cooler has the same core parts: a water reservoir (or sump), a pump, cooling pads, and a fan or blower. The cooling pads are the heart of the system, and the material they’re made from directly affects performance. Cellulose pads, which are rigid, corrugated sheets designed to maximize surface area, typically achieve cooling efficiencies around 37% to 45%. Simpler pad materials like basic shading nets only reach about 24% to 29% efficiency. Aspen wood shavings packed into a frame are another common, budget-friendly option found in many residential units, though they tend to need replacement more often than rigid cellulose media.
The fan size determines how much air the cooler can move, measured in cubic feet per minute (CFM). Larger units designed for whole-house cooling may push several thousand CFM, while portable models for a single room handle a few hundred.
Where They Work Best
Climate makes or breaks an evaporative cooler. When outside humidity is below 40%, the cooler can drop air temperature by 15 to 25°F or more. Once humidity climbs to 50%, the temperature drop shrinks to around 10°F. At 60% humidity, you’re looking at only 5 to 7°F of cooling, which most people would find inadequate.
This makes evaporative coolers ideal for the arid and semi-arid regions of the western United States, the Middle East, parts of Australia, and similar dry climates. In the southeastern U.S. or coastal areas where summer humidity regularly exceeds 50%, a traditional air conditioner is the better choice. Some people in borderline climates use evaporative coolers during the dry early summer months and switch to conventional AC when the humid season arrives.
Sizing a Cooler for Your Space
To figure out what size evaporative cooler you need, you’ll calculate the required airflow in CFM using a straightforward formula from the Building America Solution Center: multiply your floor area (in square feet) by the ceiling height (in feet) to get the room volume, then multiply that volume by the recommended air changes per hour for your region, and divide by 60.
So the formula looks like this: CFM = (floor area × ceiling height × air changes per hour) ÷ 60. For most hot, dry climates, 20 to 40 air changes per hour is typical. A 1,500 square-foot home with 8-foot ceilings and 30 air changes per hour would need a cooler rated at 6,000 CFM.
Ventilation Requirements
Unlike air conditioning, which works in a sealed house, evaporative coolers need an escape route for the air they push in. The U.S. Department of Energy recommends opening windows or vents on the downwind side of the house, providing 1 to 2 square feet of opening for every 1,000 CFM of cooling capacity. A 6,000 CFM cooler, for example, would need 6 to 12 square feet of open window area.
Getting this balance right matters. If you open windows too wide, hot outside air floods in and overwhelms the cooler. If you don’t open them enough, humidity builds up indoors, which reduces the evaporative effect and makes the air feel clammy. A practical strategy is to close windows in rooms you’re not using and open them in occupied areas, directing the cool air where you actually want it. If leaving windows open raises security concerns, ceiling-mounted exhaust vents (called “up-ducts”) can push warm air into the attic instead.
Maintenance Schedule
Evaporative coolers require more regular attention than a conventional AC unit. Because water constantly evaporates, minerals and salts concentrate in the reservoir over time, eventually forming scale deposits on the pads and reducing efficiency. The solution is to continuously bleed off some water from the system. A common starting point is about 1 gallon per hour for each foot of pad length, adjusted up for hard water or down for soft water. You can check your bleed-off rate with a bucket: 75 gallons per hour, for instance, fills a 5-gallon bucket in about 6 minutes.
Beyond bleed-off, the pads should dry out completely at least once every 24 hours. This prevents algae and mold from gaining a foothold. You can accomplish this by setting the cooler to shut off for a few hours during the coolest part of the night, or by adjusting the thermostat so the system only kicks on above nighttime temperatures.
Weekly, check for algae growth. Monthly, inspect for scale buildup and clean any filters. Every three months, drain the entire system, disinfect the reservoir and distribution lines, and remove accumulated debris like dust, insects, and leaves from the sump. If your cooler runs from May through October, plan for a thorough cleaning at the start of the season, again in midsummer, and when you shut it down for winter.
Health Considerations
Evaporative coolers add moisture to indoor air, typically raising relative humidity by 10% to 16%. In very dry climates, this can actually be a benefit, easing dry skin, cracked lips, and irritated sinuses. But if indoor humidity climbs too high, it creates conditions favorable for dust mites and mold growth, both of which are common triggers for allergies and asthma.
Poorly maintained coolers pose additional risks. Stagnant water and dirty pads can harbor bacteria and fungi. Researchers have recovered Legionella bacteria from residential evaporative coolers in some studies, though no cases of Legionnaires’ disease were linked to those units. In one documented case, a severely neglected cooler caused a near-fatal lung inflammation in an 11-year-old girl, traced to bacteria growing in the pads. Regular cleaning and ensuring the pads dry daily go a long way toward preventing these problems.
Evaporative Coolers vs. Air Conditioners
- Energy use: Evaporative coolers use roughly 75% less electricity than comparable central AC systems because they only power a fan and a small water pump, with no compressor.
- Installation cost: A whole-house evaporative cooler typically costs a fraction of a central air system to purchase and install.
- Climate dependency: AC works in any humidity level. Evaporative coolers lose effectiveness above 50% relative humidity.
- Fresh air: Evaporative coolers constantly bring in and circulate outside air, while AC recirculates the same indoor air. This can improve ventilation but also means outdoor pollen, dust, and odors come in with it.
- Water use: Evaporative coolers consume water continuously during operation, which can be a concern in drought-prone areas.
- Maintenance: Evaporative coolers need more frequent hands-on care, including pad replacement, descaling, and seasonal shutdowns.
For homes in dry climates where summers are hot but humidity stays low, an evaporative cooler provides comfortable, energy-efficient cooling at a fraction of the operating cost. In humid regions, or for people who prefer set-it-and-forget-it convenience, traditional air conditioning remains the more practical option.