Swamp cooler or AC: the dew point decides, not the thermometer

Updated August 28, 2026 · Claude AC · 2 sources cited

In Phoenix in early June, an evaporative cooler is not a nostalgic compromise — it is frequently the better machine, at a quarter of the running cost. In Phoenix in late July, the same box turns a house into a warm, damp room that nobody can sleep in. Both statements are true in the same year, in the same house, and the variable that flips between them is not the number on the thermometer. It is the dew point.

The physics, in one paragraph

An evaporative cooler draws outdoor air through soaked pads. Water evaporates, and the energy for that phase change is taken out of the air as sensible heat, so the air leaves colder and considerably wetter. The floor of the process is the wet-bulb temperature: no direct evaporative cooler can supply air below it, and a well-maintained residential unit typically closes 70 % to 85 % of the gap between dry-bulb and wet-bulb. The Department of Energy describes the practical result as a 15 °F to 40 °F drop — a band that wide precisely because everything depends on how dry the incoming air already is. Our companion piece on how evaporative cooling works takes the psychrometrics further.

What that means at your front door

The table below shows approximate supply-air temperatures for a unit closing about 80 % of the wet-bulb depression. The values are indicative, rounded, and meant to show the shape of the curve rather than to be read to the degree.

Read down the dew-point column and the rule writes itself. Below roughly 55 °F, evaporative cooling is genuinely excellent and a refrigerated system is an expensive way to do the same job. Between 55 °F and 65 °F it still works, but the house feels merely « not hot » rather than cool. Above 65 °F the machine is fighting itself, and above 70 °F it is adding moisture to air that is already carrying too much of it — the exact reason a swamp cooler is a non-starter in Florida or coastal Texas, whatever the electricity bill argument says.

Where the map says yes

Our state pages classify climates as dry, mixed or humid alongside cooling degree days, and the dry column is where evaporative belongs: Arizona at roughly 2,800 CDD, Nevada near 1,900, New Mexico around 1,300, plus Utah, Colorado, Idaho and eastern Washington. The seasonal caveat matters as much as the map. Arizona and New Mexico get a genuine monsoon from roughly early July into September, when dew points climb into the 60s and low 70s and evaporative performance collapses for weeks at a time. Nevada and the Colorado Front Range see far less of that, which is why single-system evaporative houses are common in Las Vegas and Denver and increasingly rare in Tucson.

Running cost: the honest gap

An evaporative cooler runs a blower and a small circulating pump. There is no compressor, so the draw is measured in hundreds of watts rather than kilowatts, and the DOE puts whole-house evaporative energy use at roughly a quarter of central air conditioning, for about half the installed cost. On a 15-cent kilowatt-hour that is the difference between a few cents an hour and the figures in our guide to what running an AC actually costs.

Two costs go the other way. The first is water: a whole-house unit typically consumes something in the order of 3 to 15 gallons an hour depending on size and dryness, plus whatever a bleed-off line or purge pump dumps to prevent mineral build-up. Over a long desert season that is thousands of gallons, and in a metered, tiered-tariff city it is a real line on a real bill. The second is maintenance, which is not optional: pads scale up in hard water, pumps clog, and the DOE recommends replacing pads at least twice during a cooling season.

The three installation facts people learn too late

• You must open windows. An evaporative cooler is a one-pass machine: it pushes outdoor air in, and that air has to leave. A common design rule is one to two square feet of open exhaust area per 1,000 cfm, positioned in the rooms you actually want cooled. Close the house up as you would with refrigerated air and the unit simply humidifies it.
• Sizing is airflow, not tonnage. The usual rule of thumb takes the floor area multiplied by the ceiling height and divides by two to get the required cfm — a 1,600 sq ft house with 8 ft ceilings lands near 6,400 cfm. Undersized units are the most common complaint and the least often diagnosed.
• Winter matters. An unsealed roof-mounted unit is a chimney: drain it, cover it and close the damper, or the heating bill pays for the summer savings.

Two-stage and dual systems

Two-stage — or indirect/direct — coolers pre-cool the incoming air through a heat exchanger before it ever touches the wet pads. That first stage drops temperature without adding moisture, so the second stage starts from a lower wet bulb and the house ends up cooler and less humid than a single-stage unit can manage. It costs more and uses less water per degree delivered, and it is the sensible upgrade in Albuquerque or Denver where dew points are moderate rather than desert-low.

The other common Southwest arrangement is both machines: evaporative from May into June and again in September, refrigerated during the monsoon weeks. It works well on one condition — the evaporative dampers must be genuinely closed when the compressor is running, or you spend the afternoon paying to cool the roof. If you are specifying that refrigerated half, our BTU sizing guide covers the load calculation; a common mistake is sizing the AC as though it were the only system and ending up with an oversized, short-cycling unit for the six weeks a year it runs.

Frequently asked questions

At what humidity does a swamp cooler stop working?

Relative humidity alone is a poor guide, because 30 % at 110 °F and 30 % at 80 °F are completely different amounts of water. Use the dew point: below about 55 °F performance is excellent, 55–65 °F is workable, and above 65 °F the unit adds discomfort faster than it removes heat. Most weather apps show dew point, and forecasts for it are reliable a few days out.

Can a portable evaporative cooler cool a closed room?

No. A recirculating portable unit in a sealed room raises humidity until the air can absorb no more, at which point cooling stops and the room feels worse than before. Portables earn their keep on covered patios, in garages and workshops, and anywhere a window can stay wide open — not as a substitute for a fixed system.

Does a swamp cooler make mould more likely?

Indoors, rarely — the air is constantly exchanged rather than trapped, so moisture does not accumulate the way it does in a humid closed house. The mould risk lives inside the unit itself: standing water in the sump, pads left damp through the shoulder season, and a neglected bleed line. Drain and dry it when you shut down.

Can I convert my swamp cooler to refrigerated air?

Not the machine — but often the ductwork, with modifications. Evaporative systems move far more air at much lower static pressure, so existing ducts are usually large but poorly sealed and sometimes badly balanced for a compressor-based system. Any serious contractor will price duct modification separately; if a quote does not mention it at all, ask why, and compare it against the checklist in our guide to reading an HVAC bid.

Is evaporative cooling cheaper over ten years?

In a genuinely dry climate with reasonable water rates, yes, usually by a wide margin — lower purchase price, lower install cost, a quarter of the electricity, and no refrigerant circuit to leak. The equation flips wherever the monsoon or a maritime influence pushes summer dew points up, because the second system you end up buying wipes out the saving.

Read next

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• Running your AC on a generator: what starts, and what only stalls
• Wildfire smoke and your AC: what keeps it out, and what pulls it in
• Replacing only the outdoor unit: when it is fine, when it is a trap

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