A swamp cooler, also called an evaporative cooler, cools your house by blowing hot outdoor air through wet pads. Water evaporating off the pads pulls heat out of the air, so the air leaving the cooler can be 15 to 30 degrees cooler than the air going in, using only a fan and a small pump. The catch is humidity: in dry air a swamp cooler can replace air conditioning for much of the summer, and in humid air it mostly makes the house damp.
How Does a Swamp Cooler Work?
Every swamp cooler has a water reservoir in the bottom of the cabinet, a small pump, absorbent pads, and a large blower. The pump lifts water to a trough at the top of the pads, the water soaks down through them, and the blower pulls outdoor air through the wet pads and into the house. A float valve, like the one in a toilet tank, refills the reservoir as water evaporates.
Evaporating one pound of water absorbs roughly 1,000 BTU of heat, and that heat comes out of the air passing through the pad. A gallon of water weighs a little over 8 pounds, so every gallon per hour the cooler evaporates removes something like 8,500 BTU per hour from the airstream. That is why a big roof cooler can do the work of a multi-ton air conditioner: it moves a lot of water.
A swamp cooler never recirculates indoor air. It brings in 100 percent outdoor air and relies on open windows to let warm indoor air escape. There is no refrigerant, compressor, or outdoor condenser, unlike the refrigerant cycle explained in how air conditioning works.
Wet-bulb temperature decides everything
A swamp cooler cannot cool air below the outdoor wet-bulb temperature, the reading of a thermometer with a wet wick in moving air. In very dry air the wet-bulb sits 30 or more degrees below the regular (dry-bulb) temperature. In humid air the two are only a few degrees apart, so there is almost no room to cool. Real pads never quite reach the wet-bulb; good residential coolers get about 80 to 90 percent of the way there.
What Temperature Will a Swamp Cooler Produce?
"Works in dry climates" is easier to judge in degrees. Each cell below is the approximate air temperature leaving a direct swamp cooler with 85 percent effective pads, calculated from standard psychrometric formulas at sea level. High-elevation cities like Denver and Albuquerque do slightly better.
| Outdoor temperature | 10% humidity | 20% humidity | 30% humidity | 40% humidity | 50% humidity |
|---|---|---|---|---|---|
| 90°F | 63°F | 67°F | 71°F | 74°F | 77°F |
| 95°F | 66°F | 70°F | 74°F | 78°F | 81°F |
| 100°F | 69°F | 74°F | 78°F | 82°F | 86°F |
| 105°F | 72°F | 77°F | 82°F | 86°F | 90°F |
| 110°F | 75°F | 81°F | 86°F | 90°F | 94°F |
The house settles a few degrees warmer than the supply air as it picks up heat from people, appliances, and sun. Supply air in the low 70s or below works well. The upper 70s is marginal and feels acceptable mainly because of the strong breeze. Near 80 or above is poor, and the added moisture makes it feel worse than the number.
A quick field rule: if your afternoon dew point is under about 50°F, a swamp cooler will do well. Between about 50 and 60°F it gets marginal. Above 60°F you need refrigerated air.
| Climate | Swamp cooler fit |
|---|---|
| High desert (Denver, Albuquerque, Salt Lake City, Reno, Boise) | Works well most of the summer |
| Low desert (Phoenix, Tucson, Las Vegas) | Works well in late spring and early summer; marginal to poor in monsoon humidity, so many homes pair it with AC |
| Inland West valleys (Central Valley, eastern Washington and Oregon) | Usually works; marginal on muggier days |
| Midwest, Northeast, Gulf Coast, Southeast | Poor; choose AC or a heat pump |
Types of Swamp Coolers
Roof-mounted whole-house coolers. The classic Southwest setup: a large down-draft cabinet on the roof blowing into a ceiling diffuser in a central hallway, or into ducts feeding several rooms. Side-draft units sit on the ground or a wall bracket, which makes them easier to service. Residential coolers range from about 3,000 to 25,000 CFM.
Window units. Mounted like a window air conditioner, they cool one room or a small zone and still need a separate exhaust opening.
Portable coolers. Units on casters with a tank you fill by hand or hose. Fine for a garage, patio, or single room in a dry climate. In humid weather or a closed room they mostly raise humidity; to cool one room in a humid area, look at portable air conditioners instead.
Direct, indirect, and two-stage
- Direct: air passes straight through the wet pad. Cheapest and most effective per watt, and it adds moisture. Almost every residential swamp cooler is direct.
- Indirect: water evaporates on one side of a heat exchanger and cools the supply air on the other side without adding moisture. It cools less but keeps indoor humidity down.
- Two-stage (indirect-direct): an indirect stage pre-cools the air, which lowers its wet-bulb temperature, so the direct pad that follows can deliver cooler, less humid air than a single pad. These cost more and are less common in homes.
What Size Swamp Cooler Do You Need?
Swamp coolers are sized by airflow in CFM, not BTU. The Department of Energy notes that manufacturers recommend about 20 to 40 air changes per hour, with hotter, drier climates at the high end.
CFM = square feet x ceiling height x air changes per hour / 60
| Home size (8-foot ceilings) | 20 changes/hour (milder) | 30 changes/hour (typical) | 40 changes/hour (hot desert) |
|---|---|---|---|
| 1,000 sq ft | about 2,700 CFM | about 4,000 CFM | about 5,300 CFM |
| 1,500 sq ft | about 4,000 CFM | about 6,000 CFM | about 8,000 CFM |
| 2,000 sq ft | about 5,300 CFM | about 8,000 CFM | about 10,700 CFM |
| 2,500 sq ft | about 6,700 CFM | about 10,000 CFM | about 13,300 CFM |
Many CFM ratings are measured with no duct attached, and a ducted roof unit delivers less, so compare models at the same static pressure if the spec sheet lists it.
Why You Have to Open Windows
Because the cooler pushes fresh air in, the house needs an exit. The Department of Energy's starting point is 1 to 2 square feet of open window or vent area per 1,000 CFM of cooler output, on the side of the house away from the cooler. A 6,000 CFM cooler needs roughly 6 to 12 square feet. Too little opening and humid air builds up indoors; too much and hot air comes in on its own. Open windows in the rooms you are using; air goes where the openings are.
How Much Water Does a Swamp Cooler Use?
A large whole-house cooler on a hot, dry afternoon commonly goes through several gallons per hour, and the bleed-off line adds more. A University of Arizona analysis estimated about 150 gallons a day for a cooler running through a Tucson summer. A central AC uses no water on site, though the power plant behind it does. If your city has drought restrictions, check them before you buy.
Swamp Cooler vs Air Conditioner
| Factor | Swamp cooler | Central air conditioner |
|---|---|---|
| How it cools | Evaporates water into 100% outdoor air | Refrigerant cycle moves heat outside |
| Upfront cost | Lower; often a fraction of a central AC install | Higher, especially if ducts are needed |
| Electricity | Roughly a quarter of central AC by DOE's estimate | Compressor plus fans; usually the largest summer electrical load |
| Water | Uses water continuously | None on site |
| Indoor humidity | Raises it | Lowers it |
| Climate fit | Dry climates only | Any climate |
| Lowest temperature | Limited by outdoor wet-bulb | Holds your setpoint |
| Maintenance | Frequent: pads, pump, pan, winterizing | Annual tune-up and filters |
Many Southwest homes keep both: the cooler for dry spring and early summer, the AC once monsoon humidity arrives. Never run them at the same time, since the cooler adds moisture the AC then has to remove and the open windows let conditioned air escape. In a home without ducts, a ductless mini split is the usual way to add refrigerated cooling for the humid weeks. For the broader picture, see our guide to the types of air conditioners. If your nights cool off well, a whole house fan is the other low-energy option for dry climates, and it uses no water.
Swamp Cooler Maintenance
Always unplug the cooler or switch off its disconnect before reaching into the cabinet: the blower and pump run on line voltage, and a belt-drive blower can start without warning. Roof units also mean ladder and roof work, so use fall protection or leave it to a technician.
| Task | When | What to do |
|---|---|---|
| Replace pads | At least once a season; more often with hard water | Aspen pads typically last one season; thicker rigid media lasts several if kept clean and allowed to dry |
| Clean the reservoir | Startup and every few weeks | Drain, scrub out sediment and scale, rinse, refill |
| Check the float valve | Startup and monthly | Set the level so the pan fills without overflowing |
| Clean pump and trough | Startup and monthly | Clear the pump screen and trough holes; dry streaks on a pad mean air is bypassing |
| Check bleed-off | Monthly | Confirm the bleed line drips or the purge pump cycles |
| Winterize | Fall, before freezing | Shut off and drain the water line and pan, close the ceiling damper, cover the cooler |
Mineral scale and bleed-off. When water evaporates, its minerals stay behind. In hard-water areas, which includes much of the Southwest, scale builds on the pads and pan, cuts airflow, and shortens pad life. A bleed-off line sends a small, steady stream of pump water to a drain so fresh water replaces it; a purge pump does the same in batches. It uses extra water, but without it you replace pads more often.
Winterizing. An undrained line can freeze and split, and a cooler left open to the house is a large hole in your ceiling that leaks heated air outside all winter. Close the damper or block the duct from inside, and cover the roof unit.
Health Notes: Keep the Water Clean
A neglected cooler is a pan of warm, stagnant water with debris in it. That grows algae, bacteria, and mold, produces the musty smell that gave swamp coolers their name, and can breed mosquitoes in portable units left full outdoors. Drain coolers you are not using, replace pads that smell musty or show dark growth instead of trying to clean them, and keep the bleed-off working. People with mold allergies or asthma should be especially strict about upkeep.
How Much Does a Swamp Cooler Cost?
Most published cost guides put a professionally installed roof-mounted whole-house swamp cooler at roughly $1,500 to $3,800, with averages around $2,500. Small portable units start under a couple of hundred dollars. What moves the price:
- Size (CFM): bigger cabinets cost more and are harder to lift onto a roof.
- Mounting: a roof unit needs a curb or stand and a flashed roof penetration.
- Ductwork: a single-diffuser drop is simplest. Multi-room duct runs cost more, and cooler ducts are larger than AC ducts because coolers move more air.
- Type: indirect and two-stage units cost more than direct coolers.
- Electrical and plumbing: a circuit and disconnect, a water line to the roof, and a drain for bleed-off.
What to ask an installer
- How did you calculate the CFM, and at what static pressure is that rating?
- What pad media does it use, and is it direct, indirect, or two-stage?
- Does the quote include bleed-off and a winter damper or cover?
- Where should I open windows, and how much, for this size unit?
Frequently Asked Questions
Do swamp coolers work in humid weather?
Poorly. A swamp cooler can only cool air toward its wet-bulb temperature, and in humid air the wet-bulb is close to the actual temperature. At 95 degrees and 50 percent humidity, a good cooler delivers air around 81 degrees and adds moisture to it. In the desert Southwest, coolers work well in May and June and lose much of their punch once monsoon moisture arrives.
Do you need to open a window when using a swamp cooler?
Yes. The cooler pushes outdoor air into the house, and that air needs a way out. The Department of Energy's starting point is 1 to 2 square feet of open window or vent for every 1,000 CFM of cooler output, on the side of the house away from the cooler. Too little opening traps humid air; too much lets hot air in.
How much water does a swamp cooler use?
A whole-house roof unit on a hot, dry afternoon commonly evaporates several gallons an hour, plus bleed-off water that flushes minerals from the pan. A University of Arizona analysis estimated about 150 gallons a day for a cooler running through a Tucson summer. Portable units use far less.
Is a swamp cooler cheaper to run than an air conditioner?
On electricity, by a wide margin. A swamp cooler runs only a blower and a small pump, with no compressor, and the Department of Energy puts its energy use at roughly a quarter of central air conditioning. You pay some of that back in water and maintenance, and the savings only exist in a climate dry enough for the cooler to work.
Can a swamp cooler make you sick?
A clean one is not a known health problem for most people, but a neglected one can be. Warm standing water grows bacteria, algae, and mold, and old pads blow a musty smell through the house. Clean the pan regularly, replace pads on schedule, never leave water sitting in an idle cooler, and drain it completely at the end of the season.
Where should you place an evaporative cooler?
Place it where it can pull in outdoor air and push it through the house to an open window on the far side. A portable unit sits beside an open window or door on the intake side, with an exhaust opening across the room, and never in a closed room. Keep the pads clear of walls and furniture, set the unit level near a water supply, and keep it away from electronics. Whole-house units go on the roof or beside the house, as described in the types section.