Clean out the cooler in the spring. Remove all dust, dirt and debris. Oil moving parts. Inspect the belt for wear and cracks and replace if necessary. Fill the cooler reservoir with water and test the pump, hoses and fan to make sure they are all working. Make sure all the hoses are moving water to the pads and are properly aligned to drop water onto the pads. Install new pads and make sure the water is wetting the whole pad evenly. Dry areas on the pads will allow hot air through.
Use good pads. Some pads are made with thin strips of wood inside plastic mesh; these are very inexpensive. The wood strips can slide down and bunch up. This type of pad is usually very thin also. Empty spaces that develop in the pads allows hot air to flow through without being cooled by evaporation, making it hotter in your house. For best cooling, use the thickest pad your cooler will accommodate. Also, use one made of material that will not move or shift around. This will ensure even and more efficient cooling.
Water and Water Level
Check the water level in the bottom of the cooler. Ensure it is high enough to allow the pump to pump only water with no air entering the pump. Running the pump without water can burn it out. If it is too high (leaking) or too low, adjust it by adjusting the float -- the float is attached to the valve which lets more water in. Loosen it (unscrew it on most coolers) where it connects to the valve and adjust it higher or lower to control the flow of water into the cooler. You may have to experiment to find the correct adjustment. Ensure there is no debris or floating matter in the water which could block the pump. It is best to have some screening around the pump to filter the water. These are available at some retailers.
Mineral Build Up
Look for mineral buildup on the pads. Most water contains minerals. As the water evaporates in your pads, it leaves mineral deposits behind. These mineral deposits can become so thick that they prevent the pads from getting evenly wet and thus prevent proper evaporation and cooling. If you see substantial buildup on the pads, replace them. This is the main reason why it is necessary to install new pads at the beginning of the warm season when you are doing your start-up maintenance.
Humidity and Cooling
Check the humidity first if the cooler is not cooling well. Because evaporative coolers cool the hot outdoor air via evaporation, anything that prevents evaporation will make them less efficient. High humidity will prevent good evaporation, making the cooler seem like it is not working well. If the humidity is low and the cooler is not cooling well, check to see if there is adequate water flowing into the cooler reservoir, that the pump is working well, that all hoses are transporting water to all the pads, that all the pads are evenly wet, and that there is no mineral buildup on the pads. Also ensure that the fan is blowing an adequate volume of air. Correct any of these faults to restore proper cooling.
Do shut down maintenance every autumn. Empty out all water from the cooler. Empty all water from the water hose that carries water to the cooler, because if the water in it freezes it will split the hose, even if it is made of copper. It is best to cover the cooler with a cooler cover, tarp or plastic sheeting; this will keep out dust and insects and help prevent the loss of warm air from the house during the winter. Also, be sure to install the metal plate that goes into your duct work; this will prevent warm air from escaping out the cooler vents during the winter. If there is no plate, try to purchase a replacement or cover your duct vent with cardboard or plastic sheeting and tape if possible.