Evaporative coolers, also called swamp coolers, are the most efficient cooling appliances available for residential use, when they work. In hot dry climates (the American Southwest, parts of Australia, the Middle East, much of the Mediterranean), an evaporative cooler drops indoor temperature 10 to 25 degrees Celsius below outdoor temperature at one fifth the electricity cost of an air conditioner. In humid climates, the same unit adds moisture without cooling and makes the house worse. This guide explains the physics, the climate thresholds, the sizing math, and the real ownership costs so you can determine whether your climate makes a swamp cooler a smart choice.
How evaporative coolers work
The principle is the same as sweat evaporating off your skin. When water changes from liquid to vapor, it absorbs a large amount of energy (the latent heat of vaporization, about 2260 kJ per kg of water). That energy comes from the surrounding air. Air that gives up energy to evaporate water gets cooler in the process.
An evaporative cooler pulls outdoor air through wet pads. The water in the pads evaporates into the moving air stream. The air loses heat to the evaporating water and exits the pads cooler than it entered. A fan pushes that cooled air into the house.
The maximum possible temperature drop is set by the wet-bulb temperature of the incoming air, which depends on the dry-bulb temperature and the relative humidity. In hot dry air (35 degrees Celsius at 15 percent humidity), the wet-bulb temperature is about 17 degrees Celsius, so the cooler can deliver air at 17 to 22 degrees Celsius. In hot humid air (32 degrees Celsius at 70 percent humidity), the wet-bulb temperature is about 28 degrees, and the cooler can only drop air temperature by 2 to 4 degrees. The latter is useless for cooling.
The humidity threshold
The practical rule: evaporative cooling works well at 40 percent relative humidity or below, drops in effectiveness between 40 and 60 percent, and becomes useless above 60 percent. Daily and seasonal humidity in your climate determines whether a swamp cooler is the right choice.
Areas where swamp coolers excel year-round: Phoenix, Las Vegas, Albuquerque, Denver, Salt Lake City, El Paso, much of inland California, the Mediterranean basin, central Australia, the Middle East.
Areas where swamp coolers work in spring and fall only: Texas inland areas (Dallas, Austin), parts of the lower Midwest, much of inland Mexico.
Areas where swamp coolers do not work: Florida, the Gulf Coast, the Eastern Seaboard, most of the UK and Northern Europe, Southeast Asia, India during monsoon, the US Pacific Northwest in summer humidity.
Check your local annual average relative humidity (NOAA publishes this for US locations, Met Office for UK). If summer humidity averages over 50 percent, a swamp cooler is the wrong choice regardless of how dry it occasionally gets.
Sizing the cooler
Evaporative coolers are sized in CFM (cubic feet per minute), not BTU. The required CFM depends on the volume of the house and the desired air change rate.
For most homes, two air changes per minute is the target during cooling. A 1500 square foot house with 8 foot ceilings has a volume of 12000 cubic feet, requiring 6000 CFM of cooler capacity. A 2500 square foot house at the same ceiling height needs about 10000 CFM.
Whole-house coolers (5000 to 12000 CFM) are roof-mounted or side-wall-mounted and ducted to the interior with a fixed duct system or a single down-discharge through a central ceiling opening. Window-mounted evaporative coolers (200 to 1500 CFM) are smaller units that cool one or two rooms. Portable evaporative coolers (200 to 800 CFM) are essentially fans with a wet pad and a water reservoir.
For homes in arid climates, whole-house units are the most common and most cost-effective per CFM. For single-room cooling in semi-arid climates, window-mounted or portable units work for spot cooling but cannot replace a whole-house system.
The open window requirement
Evaporative coolers must have open windows or roof vents on the side of the house opposite the cooler. The cooler pushes air in at 5000 to 12000 CFM, and that same volume of air must exit somewhere. With closed windows, the cooler pressurizes the house, indoor humidity climbs rapidly to 80 plus percent, and cooling stops because the indoor air is too humid to accept more moisture.
The typical setup: cooler mounted on one side of the house, windows cracked 2 to 4 inches in rooms on the opposite side of the house. As the cooler pushes humid cool air in, the warm dry air inside exits through the cracked windows. The result is a continuous air flush rather than a recirculating cycle.
This is the structural difference from air conditioning. An AC cools indoor air and recirculates it. A swamp cooler pulls in outdoor air, conditions it once, and pushes the conditioned air through and out of the house. The cooling effect is real but it requires air movement that some users find drafty.
Energy comparison
A typical whole-house evaporative cooler:
5000 to 12000 CFM capacity. Power consumption 600 to 1200 W on the blower motor, plus the small water circulation pump (40 to 80 W). Total electrical load: 700 to 1300 W.
A central AC of equivalent cooling capacity for the same house:
3 to 5 ton compressor. Power consumption 3000 to 5000 W on the compressor, plus 500 to 700 W on the indoor blower. Total electrical load: 3500 to 5700 W.
Running both for 8 hours per day in a hot summer:
Evaporative cooler: 5.6 to 10.4 kWh per day, costing 0.84 to 1.56 dollars per day at 0.15 per kWh.
Central AC: 28 to 45.6 kWh per day, costing 4.20 to 6.84 dollars per day.
Over a 120 day cooling season, the swamp cooler saves 400 to 630 dollars compared to central AC. In arid climates that saving more than offsets the additional water cost (15 to 200 dollars per season).
Water consumption
Water use depends on the difference between incoming outdoor humidity and the saturated indoor air leaving the cooler.
Typical whole-house unit: 3 to 8 gallons of water per hour in dry conditions, 1 to 3 gallons per hour in moderate conditions.
Over a 120 day cooling season at 8 hours per day operation, total water consumption: 3000 to 8000 gallons.
In arid regions where water costs 0.005 to 0.015 dollars per gallon, seasonal water cost: 15 to 120 dollars.
In water-restricted regions (parts of Australia, much of the western US in drought years), evaporative cooling may be limited or prohibited during severe drought even where the climate would otherwise favor it.
Maintenance
Evaporative coolers require more maintenance than AC units.
Pads: aspen pads need annual replacement, rigid media pads last 2 to 5 years. Cost: 30 to 100 dollars per replacement.
Water reservoir: needs cleaning every 2 to 4 weeks during operation to prevent mineral buildup and biological growth. Drain, scrub, refill.
Bleed-off line: a small drain that constantly purges some water to prevent mineral concentration. Verify it is flowing properly.
End-of-season shutdown: drain the entire system, clean and dry the pads, cover the unit. Skipping this step is the most common cause of mold and bacterial problems the following season.
Belt and pump: inspect annually, replace as needed. Belt life 3 to 5 years, pump life 5 to 8 years.
Total annual maintenance cost: 50 to 150 dollars in parts plus 1 to 2 hours of labor.
When swamp coolers are worth it
Climates with summer average relative humidity below 40 percent. Homes in arid regions where summer electricity bills are otherwise dominated by AC. Owners who do not mind the open-window operation and the slightly higher maintenance. Homes with budgets for 1500 to 4000 dollars install (whole-house roof unit) and 100 to 200 dollars per year in maintenance.
When they are not
Climates with humid summers. Homes in dense urban areas where outdoor air quality (smoke, smog, pollen) is a problem (the cooler pulls all of that into the house). Households with severe allergies or asthma triggered by mold spores. Water-restricted regions during drought. Homes built airtight (modern passive house construction) where the necessary outflow path does not exist without major retrofit.
For more on whole-house cooling strategy see our AC types guide and methodology at /methodology.
Frequently asked questions
At what humidity does an evaporative cooler stop working?+
Evaporative coolers work well below 40 percent relative humidity, drop in effectiveness between 40 and 60 percent, and become useless above 60 percent humidity. In high humidity, the outdoor air already holds significant moisture and adding more from the cooler does not lower the dry-bulb temperature meaningfully. The cooler just adds moisture to the air without producing cooling, making the room feel clammy and warmer.
How much electricity does an evaporative cooler save versus AC?+
An evaporative cooler uses roughly one fifth to one quarter the electricity of an equivalently sized air conditioner. A 5000 CFM whole-house swamp cooler draws 700 to 1000 W. A central AC that cools the same area draws 3000 to 5000 W on the compressor plus 500 to 700 W on the indoor fan. Over a 120 day cooling season in a dry climate, the savings can total 200 to 600 dollars on the electricity bill.
Can I run an evaporative cooler with the windows closed?+
No. Evaporative coolers must have open windows or vents on the opposite side of the house to create the airflow path. The cooler pushes humidified cool air into the house, and the warm dry air inside the house has to exit through opened windows to make room. With windows closed, the humidity rises rapidly to saturation and cooling stops. The correct setup is to crack windows in the rooms farthest from the cooler by 2 to 4 inches each.
How much water does a swamp cooler use?+
A typical whole-house unit consumes 3 to 15 gallons of water per hour depending on outdoor humidity and unit size. Over an 8 hour summer day in a dry climate, that is 25 to 120 gallons per day. In a 120 day cooling season, total water use is 3000 to 14000 gallons. In arid regions where water rates are 0.005 to 0.015 dollars per gallon, the seasonal water cost runs 15 to 200 dollars. The water-for-electricity tradeoff usually favors the cooler in dry climates with cheap water.
Do evaporative coolers cause mold problems?+
Yes, if maintained poorly. The water reservoir, pads, and ducting are warm and damp during the cooling season, which favors mold and bacterial growth. Regular pad replacement (annually for aspen pads, every 2 to 5 years for rigid media), reservoir cleaning (every 2 to 4 weeks during operation), and full system flush at end of season prevents most problems. Units installed in humid climates or run with stagnant water for weeks at a time develop mold within one season.
