Will an evaporative cooler work in your climate?
Evaporative coolers (also called swamp coolers, desert coolers or evaporative air coolers) can only cool air toward the wet-bulb temperature. Enter outdoor conditions to see how much cooling the air allows. For a fair assessment, use a typical hot afternoon or your local summer design condition, not a mild morning.
What the numbers mean
- Wet-bulb temperature
- The lowest temperature air can reach by evaporating water into it. A perfect (100% effective) evaporative cooler would deliver air at this temperature.
- Wet-bulb depression
- Dry bulb minus wet bulb. This is the maximum possible temperature drop, so the bigger it is, the better evaporative cooling works. A real cooler achieves a fraction of it, set by its effectiveness.
- Dew point
- The temperature at which the air's moisture would condense. It changes slowly through a day, so a high dew point is an early warning that humid conditions are limiting your cooler.
How the verdict is decided
We use guidance from the US Department of Energy's Building America Solution Center: evaporative coolers work best where the summer design wet-bulb temperature is below about 21 °C (70 °F), and can still provide cooling up to about 23 °C (74 °F) with reduced performance. Above that, refrigerated air conditioning or a heat pump, which also removes moisture, is usually the better choice.[1]
| Wet bulb | Verdict |
|---|---|
| ≤ 21.1 °C (70 °F) | Well suited |
| 21.1–23.3 °C (70–74 °F) | Works with reduced performance; consider indirect or two-stage |
| > 23.3 °C (74 °F) | Poorly suited at these conditions |
Worked example
The DOE source compares two US design days. At 42 °C (108 °F) dry bulb with a 21 °C (70 °F) wet bulb, typical of a desert city, the wet-bulb depression is 21 °C (38 °F) and evaporative cooling works very well. On a humid coastal design day of 33 °C (92 °F) with a 26 °C (79 °F) wet bulb, the depression is only 7 °C (13 °F).[1] Try both in the calculator.
Limitations, with no guarantees
- One reading describes one moment. Humidity often rises in rainy or monsoon seasons and near coasts, so check your most humid hot weather too.
- Psychrometric properties are calculated with ASHRAE equations for moist air, accurate to well under 0.1 °C for these inputs (see methodology). The weather data you enter is usually far less certain than that.
- Comfort also depends on airflow, ventilation, insulation and personal tolerance. A "well suited" climate does not guarantee a comfortable house if the cooler is undersized or the air has nowhere to exit (see climate & ventilation).
Next steps
Estimate outlet air temperature · Size the airflow · Estimate water use · Compare running cost with AC
Sources
- U.S. Department of Energy, Building America Solution Center (PNNL), "Evaporative Cooling Systems": wet-bulb suitability thresholds, design-day examples, definition of wet-bulb depression.
- ASHRAE Handbook—Fundamentals (2017), Chapter 1 "Psychrometrics": moist-air equations used by the calculator.