During the operating season rain will help keep your unit clean and will wash airborne dust off the unit. However over time airborne pollutants such as cottonwood or dandelion seeds, leaves, grass, bugs, dirt, and foliage will get drawn into the system and lodged deep between the coils. Low pressure rainwater will not remove all debris or clean bugs, sticky residues, corrosion, or foliage from the coils. Rainwater will also have no effect on debris that has fallen inside the unit. Different manufacturers use different materials to construct condensing unit coils and have different recommendations regarding cleaning. Always follow the manufacturers safety and cleaning instructions precisely or have maintenance performed by a professionally trained technician. Many complications can arise from improperly trying to clean a coil. The fins are very delicate and can become easily damaged if touched incorrectly. Trying to wash the coil from the outside may further lodge seeds and debris deep between the coils. A professional can safely clean the equipment from the inside which will remove more debris. There are many specially formulated coil cleaners on the market and some are extremely corrosive. Unless you are a trained HVAC professional never use a coil cleaner to clean your coil. These chemicals are extremely dangerous to use and are usually either acidic, alkaline, solvent, or detergent based. Misuse, wrong concentrations, poor rinsing, or repeated use can completely destroy a coil or drastically shorten the coil's life. During the off season regular preventative maintenance is the best way to ensure trouble-free operation and peak performance. When not using your equipment use a Brinmar cover. It will help keep your system running at its optimal efficiency levels, which keeps energy usage - and your energy bills - in check. But just as important using a cover gives you peace of mind.