For all their benefits and cost savings, swamp coolers still only work in the right climate, and that unfortunately doesn't include areas like Philadelphia.
It needs to be not only hot but also dry, because as the dry bulb temperature approaches the wet bulb temperature, the difference between the two gets smaller, and the cooling effect of the evaporating water follows suit. A wet bulb temperature above 70 degrees Fahrenheit means the swamp cooler won't be able to adjust the temperature enough to keep it in the comfort zone. (This varies based on humidity, personal preference and activity, but it generally falls in the low 70s.) [source: Canadian Centre for Occupational Health and Safety].
If the air gets too saturated with water, the water condenses. 100 percent humidity outside the house means it might rain, and while a swamp cooler won't cause a downpour in your home, it won't have any cooling effect either -- that's the hot, sticky feeling you might associate with swamps. Your perspiration just doesn't evaporate into the saturated air. The Environmental Protection Agency recommends between 30 and 60 percent humidity in a house to keep down mold and mildew, and the humid air may keep your wooden furniture from drying out. Unfortunately, it can also cause metal to rust, even in the swamp cooler itself [source: Environmental Protection Agency]. The problem of excess humidity can be solved in evaporative coolers by using a heat exchanger to heat air inside the house while the humid air vents to the outside, but these systems are considerably less efficient than direct evaporative coolers.
Swamp coolers also require maintenance. The pads needs to be cleaned or changed regularly to avoid that swampy smell and associated problems with air quality. Minerals in the water can also build up as water evaporates, requiring a bleed-off of mineral-rich wastewater over time. The coolers also require a steady supply of water -- 3.5 to 10.5 gallons (13.25 to 39.75 liters) an hour, according to the National Association of Home Builders. That can be a tall order in the hot, dry climates where swamp coolers function best.
For lots more information on swamp coolers and