Key points
- A direct cooler passes air through wet pads 8 to 12 inches thick, dropping its temperature by about 15 to 40°F (roughly 8 to 22°C) while adding humidity.
- Suited to climates with a 1% summer design wet-bulb temperature below 70°F (about 21°C); performance falls as humidity rises.
- It can use about a quarter of the energy of central AC, but it uses water and needs relief openings: 2 ft² of free area per 1,000 CFM.
- Empty the pan if the unit will sit idle for more than a few days; standing water breeds mosquitoes and bacteria.
Evaporative (desert) coolers are common in rest houses, workshops, warehouses and some homes. The idea is simple: evaporating water draws heat out of the air. But they are not a universal replacement for ordinary AC; success depends on dry air.
1) How it works
- Direct: a housing with a large blower and sponge-like media 8 to 12 inches thick kept wet; air passes through, water evaporates, and the air temperature drops by 15 to 40°F (roughly 8 to 22°C)1, but the air entering the house becomes more humid.
- Indirect: an air-to-air heat exchanger with two separate air paths, cooling supply air without adding humidity because the moist air is exhausted separately1.
- Two-stage (indirect then direct): combines both, with an evaporative cooling efficiency of 95% or more according to the source1.
2) When it helps and when it doesn't
It suits areas where the 1% summer design wet-bulb temperature is below 70°F (about 21°C), and can work up to 74°F (about 23°C) with reduced performance1. The source contrasts a hot, dry city where it works well with a cooler but humid one where it does not1.
3) Compared with ordinary (compressor) AC
| Item | Evaporative cooler | Compressor AC |
|---|---|---|
| Electricity | Can use about a quarter of the energy of central AC1 | Higher; see the AC guide and efficiency label |
| Cost | Can cost less than half as much to install and run as central AC1 | Higher |
| Water | Uses water; for a 1,500 ft² (about 140 m²) home, annual use in the source's examples ranged from 2,980 to 12,457 gallons depending on the city1 | Uses no water (it produces condensate) |
| Humidity | Direct type adds indoor humidity | Removes humidity |
| Windows | Needs relief openings | Works with the space closed |
4) Relief openings: a requirement, not an option
An evaporative cooler pushes outside air in, so it needs a way out. The source states: for every 1,000 CFM of air delivery, 2 ft² (about 0.19 m²) of unrestricted relief area is required1. Without it, pressure and humidity rise and cooling weakens.
5) Maintenance
- Start of season: clean or replace the pads, check belt tension, and clean the cabinet1.
- During the season: empty the pan if the unit will sit idle for more than a few days, because standing water breeds mosquitoes and bacteria1.
- End of season: shut off the water line, drain and clean the pan, and cover the unit1.
- Bleed-off valve: reduces mineral build-up but increases water use by 10% to 50%1.
Practical advice: hard, salty water speeds scale on the pads; watch them and replace them when they stiffen. Water and power connections for a roof unit must be rated for outdoor use; see outdoor electrics.
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1Evaporative Cooling Systems (Building America Solution Center) — Pacific Northwest National Laboratory / U.S. DOE