All condensing unit coils accumulate dirt over time. Placing plywood over the unit will hasten the process and does absolutely nothing to keep the condenser coil clean. Plywood deteriorates physically, chemically, and biologically by sun, wind, water, and temperature. Wet plywood will attract dirt and other micro-organisms and leach adhesives, resins, chemicals, preservatives, dirt, fungus, bacteria, and mold all of which can stick to the coil and reduce the efficiency of your system. A dirty coil reduces heat transfer, wastes energy, stresses the equipment and does the exact opposite of what you are trying to achieve. There are vast differences in the four grades of commercially available plywood ranging from treated to marine plywood and two basic categories, interior and exterior. Each grade uses different wood, chemicals, adhesive glue and manufacturing processes. Even the term "waterproof plywood" doesn't necessarily mean that the plywood is waterproof but rather that the glue is waterproof. Even marine grade plywood will eventually rot if left exposed to weather if not properly sealed with varnish, paint, or other coatings. For the above mentioned reasons we do not recommend you place plywood over any unit. 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.