Have you ever feel cooler when you stand by the ocean or a river? The reason is simple and normal because as hot air blows across the water, it causes some of the water to evaporate and take away the heat. The air is cooled when the evaporation process.
The evaporative air conditioners use this theory to work. When hot air enters the unit which is installed on your roof, that is where the outdoor hot air is filtered and cooled as it passes through specially designed moistened pads. A fan then blows this comfortable cooled air into every corner of your home. Hot air in the house is forced to be pushed out through the open windows and doors providing your entire home with a complete change of air at least every two minutes. So in other words, you can enjoy the fresh outdoor air all the time instead of recycled indoor air.
Like all the appliances you have at home, in order to make it work properly and efficiently, and serve you a long time, the routine check and service are necessary. Because the evaporative cooler unit is always staying outside, that means the dust and dirt can go inside of the unit and blow into your home via the ducts.