Do evaporative coolers add humidity?

A compressor air conditioner moves heat from indoors to outdoors and removes water from the air as a by-product. An evaporative cooler does something entirely different: it converts sensible heat into latent heat, in place, by evaporating water.

Evaporating a pound of water at room temperature absorbs roughly 1,050 BTU, and that energy comes out of the air passing through the wet pad. The air leaves the machine cooler and wetter than it arrived. Nothing is taken out of the room and nothing is sent outside; the heat is still there, carried now by water vapor instead of temperature.

Manufacturers publish tank capacity rather than moisture output, so the conversion is worth doing once. A US gallon of water weighs 8.34 pounds, and an evaporative cooler evaporates essentially all of what it uses.

Those are the published tank sizes of real machines — the Hessaire MC37M holds 10.3 gallons — and the cooling column is our arithmetic at 1,050 BTU per pound evaporated. Emptying that tank into a closed house puts 86 pounds of water into the air inside it. For scale, a family of four generates a few pounds a day from cooking, washing and breathing, which is enough to cause condensation problems in winter.

How fast a given machine gets through a tank, and what that means for refilling and plumbing, is in evaporative cooler water usage.

Evaporation is driven by the difference between the water’s surface and the air’s capacity to hold more vapor. As indoor relative humidity rises, that difference shrinks, evaporation slows, and the temperature drop the machine can deliver shrinks with it.

So a swamp cooler in a sealed room is a self-limiting device. It cools well for the first while, then progressively less, and eventually it is an expensive fan blowing damp air. The point where that happens depends on the outdoor air it started with, which is the subject of whether evaporative coolers work in humidity.

Three consequences worth planning around:

• Leave a window or door open on the far side of the room. This is not optional ventilation advice, it is the operating instruction. The moist air has to be able to leave.
• Do not run one in a bathroom, laundry or kitchen during use. Those rooms already have a moisture source and usually have an exhaust fan working against you.
• Watch for condensation on cold surfaces — windows, metal fixtures, tile. Water forming on a surface is the clearest sign the room has taken on more vapor than it can carry.

A cheap hygrometer settles the argument in an afternoon. Measure before you start and again after an hour of running:

• Relative humidity below about 50% and falling temperature: working as designed.
• RH climbing through 60% with the temperature flat: the room is saturating and the machine has stopped earning its electricity. Open more window.
• RH above 70% indoors: stop. This is the range where mold and dust mites become a realistic concern rather than a theoretical one, and where a dehumidifier is doing the opposite job to your cooler in the same room.

There is also a comfort argument buried in those numbers: humid air at the same temperature feels hotter, because it slows the evaporation of sweat, which is your own cooling system. That mechanism is worked through in why humidity makes heat worse.

In genuinely dry climates — the desert Southwest in summer, where afternoon relative humidity can sit in the teens — adding moisture improves comfort rather than harming it. Dry air at 15% RH is hard on skin, eyes and sinuses, and a machine that cools and humidifies at once is solving two problems with one motor.

That is the climate case, and it is why this category makes sense in Phoenix and makes no sense in Houston. The regional detail, with dew-point data, is in evaporative coolers by US climate.