Is my cooler working?

How evaporative cooling works

There is no refrigerant, no compressor, and no clever engineering hidden anywhere in the box. A swamp cooler pulls outside air through a wet pad and lets water evaporate. That is the entire mechanism, and it is enough to drop the air temperature by 30 °F in the right climate.

Swamp cooler and evaporative cooler are the same machine

Most people say swamp cooler, and it is not a compliment. The nickname is usually traced to two things: the musty smell aspen pads develop when water is left standing in the sump, and the damp air a unit produces in a house with nowhere for that air to go. Both are failure modes — a neglected pad, a sealed house — rather than descriptions of how the machine works. A clean cooler in an open house does neither.

Evaporative cooler is what the manufacturers, ASHRAE, and the mechanical codes call it, and it is the term used everywhere on this site. It names the mechanism instead of the failure, and it carries a distinction the nickname cannot: a direct evaporative cooler sends the cooled, moistened air into the house, while an indirect one routes it through a heat exchanger so the house gets the cooling without the added water. That difference decides what happens to your indoor humidity. Nearly every residential unit — and everything on this site — is direct.

The vocabulary matters more than it looks like it should. "Swamp cooler" packages an assumption of failure into the name, and most coolers people call broken turn out to be running exactly as designed in air that cannot support them. Naming the mechanism keeps the question where it belongs: not is this thing any good, but how much room does today's air have left in it.

Where the heat goes

Evaporating water takes energy — about 1,060 BTU per pound of water at these temperatures. That energy has to come from somewhere, and the only thing available is the air passing through the pad. So the air gives up heat to evaporate the water, and the air gets colder.

Nothing is destroyed and nothing leaves. The heat moves out of the air and into the water vapor that the air is now carrying. Engineers call this adiabatic saturation: the total heat content of the airstream is unchanged, but its form has shifted from sensible heat, the kind a thermometer reads, to latent heat, the kind carried invisibly by water vapor.

That trade is why the process is nearly free. A vapor-compression air conditioner has to run a compressor to move heat across a temperature difference. An evaporative cooler only has to move air across a wet surface — a blower and a small pump. The physics does the rest.

The wet-bulb temperature is a floor, not a target

Air can only absorb so much water. As the air picks up moisture it approaches saturation, and once saturated it can take no more — evaporation stops, and so does the cooling. The temperature the air reaches at that point is the wet-bulb temperature, and nothing the machine does can get the air past it.

The name is literal. Wrap a wet cloth around a thermometer bulb, blow air across it, and the reading settles at the wet-bulb temperature of that air. A swamp cooler is that experiment at household scale.

This is the number that matters, and almost nobody knows theirs. The dry-bulb temperature on your weather app — the "it's 95 out" number — tells you very little about how much cooling is available. At 95 °F and 15 % humidity the wet-bulb is 61 °F. At 95 °F and 60 % humidity it is 81 °F. Same weather report, entirely different machine.

Psychrometric chart showing the cooling process Outdoor air at 95 degrees Fahrenheit and 15 percent relative humidity cools along a constant wet-bulb line to 68 degrees at typical effectiveness and 64.7 degrees at best case, approaching but never reaching the saturation curve at the 61.4 degree wet-bulb temperature. 60 70 80 90 100 110 Dry-bulb temperature °F Moisture in the air 20 % 40 % 60 % Saturation — 100 % humidity Outdoor air 95 °F · 15 % RH Cooler output air 64.7 – 68.1 °F Wet-bulb floor 61.4 °F
The cooling process at 5,000 ft. Air enters at the right and slides down-left along a line of constant wet-bulb — losing temperature, gaining moisture, at constant total heat. The saturation curve is the wall it is running into. A real cooler stops short of it; the gap is what saturation effectiveness measures.

Wet-bulb depression is the resource you are spending

The distance between the dry-bulb temperature and the wet-bulb temperature is the wet-bulb depression, and it is the entire budget for cooling. Everything a direct evaporative cooler can ever deliver is some fraction of that gap.

Dry air has a large depression, because it is far from saturated and can absorb a lot of water. Humid air has a small one. This is why these machines are commonplace in Phoenix and absent in Atlanta — not tradition, not building codes, just the amount of room the air has left for water:

Phoenix, June
105 °F, 12 % RH — wet-bulb near 68 °F, a depression of about 37 °F. A good cooler delivers air in the low 70s.
Albuquerque, July
95 °F, 15 % RH — wet-bulb near 61 °F, a depression of about 34 °F. Cooler output air in the mid 60s.
Houston, July
95 °F, 65 % RH — wet-bulb near 84 °F, a depression of about 11 °F. Even a perfect machine gets you to 85 °F, and it is humid air at that.

So do swamp coolers actually work?

Yes, emphatically, in the right air — and no, not really, in the wrong air. The disagreement people have about these machines is almost never a disagreement about the machines. It is two people in different climates describing different weather.

The argument fits on one grid. Each cell is the wet-bulb depression — the total cooling available before the machine's effectiveness is even considered:

Wet-bulb depression by temperature and humidity A grid of outdoor temperature from 80 to 110 °F against relative humidity from 10 to 70 percent. Available cooling is greatest at the top right — hot and dry, over 40 °F of depression — and collapses toward the bottom, where humid air at any temperature leaves under 10 °F to work with. 80° 85° 90° 95° 100° 105° 110° 10% 20% 30% 40% 50% 60% 70% 27 30 32 34 37 39 41 24 25 27 29 31 33 35 20 21 23 24 26 27 29 17 18 19 20 21 22 23 13 14 15 16 17 18 18 10 11 12 12 13 14 14 8 8 8 9 9 10 10
  • Over 30 °F — excellent
  • 22–30 °F — good
  • 15–22 °F — workable
  • 8–15 °F — marginal
  • Under 8 °F — pointless
Wet-bulb depression in °F at sea level — the cooling available before effectiveness is applied. Temperature runs across, humidity down.

Notice what the grid does and does not depend on. Moving right along any row — getting hotter — increases the available cooling, which is the opposite of most people's intuition. Moving down any column — getting more humid — destroys it. Humidity is the variable that matters; temperature barely is.

That is why a 110 °F afternoon in Phoenix at 15 % humidity is comfortable territory for an evaporative cooler, and an 85 °F afternoon in Atlanta at 70 % humidity is not. The Phoenix day is hotter and the machine works far better.

So the useful question is never "do swamp coolers work." It is "does the air where I live have room left in it." In the arid West the answer is yes for most of the season, and no for the monsoon weeks in mid-summer.

Saturation effectiveness: what you actually buy

No cooler captures the depression in full, because the air does not spend long enough against the wet surface to saturate completely. The fraction it does capture is called saturation effectiveness, usually written ε. An ε of 0.85 means the machine delivered 85 % of the cooling that was theoretically on the table.

Three things set it: what the pads are made of, how thick they are, and how fast the air moves through them.

0.85 – 0.90
Rigid media — the pleated cardboard-looking blocks, 8 to 12 inches thick, clean and evenly wet. The best a homeowner unit does.
0.70 – 0.80
Rigid media at 4 inches, or fresh aspen pads fully wetted. Perfectly respectable.
0.55 – 0.70
Aspen pads mid-season, partly scaled. Still cooling, but leaving real capacity on the table. Worth replacing.

Thicker pads and slower air both raise ε, because both give the air more contact time. They also cost airflow, which is why manufacturers settle around a face velocity of 200 to 250 feet per minute — past that, effectiveness falls off and water starts blowing through the pad instead of evaporating in it.

Relief air is not optional

This is the part that catches almost everyone, and it is not really about the cooler at all.

An evaporative cooler is a once-through machine. It does not recirculate house air the way central air conditioning does. It takes outside air, cools it, and pushes it in — continuously. Mass in must equal mass out. If that air cannot get out of the house, no more can come in, airflow collapses, and the moisture the cooler has been adding stays put and accumulates.

A sealed house therefore defeats an evaporative cooler completely, and does it in a way that feels exactly like a broken cooler: weak airflow, clammy rooms, no cooling. Open 1.5 to 2 square feet of window per 1,000 CFM, on the far side of the house, and the same machine works.

What it costs to run

The energy advantage is real and large, but it deserves stated assumptions rather than a bare multiplier, because the two machines are not doing the same job.

A 4,500 CFM residential evaporative cooler runs a blower motor and a small circulating pump — on the order of 500 to 800 watts in total, depending on motor size and speed. A 3-ton central air conditioner running its compressor and air handler draws roughly 3,000 to 3,500 watts. On those figures the cooler uses something like a fifth to a quarter of the electricity for a comparable amount of air movement.

Two qualifications. First, they are not equivalent services: the air conditioner recirculates, dehumidifies, and works in any weather, while the cooler supplies outside air, adds humidity, and depends on the climate. Second, evaporative coolers often run more hours per day, which narrows the gap in practice.

And there is water. At 95 °F and 15 % humidity, a 4,500 CFM cooler operating at ε = 0.80 evaporates roughly 12 gallons per hour — that follows directly from the airflow and the moisture the air picks up. Bleed-off to control mineral buildup adds more. In much of the arid West that trade is worth making, but it is a trade, and it should be made knowingly.

The air conditioner uses no water at the house, but it does use water at the power plant. A combined-cycle natural gas plant with a recirculating cooling tower consumes a median of 205 gallons per megawatt-hour generated — about a fifth of a gallon for every kilowatt-hour. Counting that on both sides, the total water behind an hour of cooling is roughly 12.1 gallons for the evaporative cooler and roughly 0.7 gallons for the 3-ton air conditioner.

Generation barely moves the cooler's figure, and it does not close the gap. Even taking the top of the published range for plant water use and the top of the air conditioner's power draw, the cooler still uses more than ten times as much water. What changes is where the water is spent: at your house, from your local supply, in the driest part of the country.

The real downsides

A page arguing that these machines are misjudged owes you the list of things that are genuinely wrong with them:

None of these are reasons the machine is not cooling today. That is what the check on the front page is for.