Unlike the evaporator coil, the condenser coil is exposed to unfiltered outdoor air—so it suffers much greater degradation due to dirt. The performance penalty of a dirty condenser makes this task one of the most cost-effective energy-efficiency practices available for rooftop unit maintenance. A dirty coil reduces the cooling ability of the air blowing across the condenser coils. If the condensing temperature is raised from 95° Fahrenheit (F) to 105°F as a result, then cooling capacity will be cut by 7 percent and increase power consumption by 10 percent, with a net compressor efficiency reduction of 16 percent. Such performance degradation in a 10-ton unit operating at 9.0 EER for 2,000 hours per year at 8 cents per kilowatt-hour wastes about US$250 per year in operating costs (Figure 1). It takes a technician about an hour to clean the condenser, which typically costs about $50. This is money well spent—in this example, cleaning the unit would have a payback of just over two months, with a net annual savings of $200.

The best tool for this maintenace job is a power washer that feeds cleaning solution into a high-pressure water flow. Some companies specialize in performing this type of cleaning for a competitive price. They typically use tank trucks and custom self-contained equipment. Spray-on cleaning solutions that are intended to be used with a brush and a hose will not do a good enough job of cleaning the coils, even though they may brighten the outer surface. A serious condenser cleaning involves before-and-after measurements of the temperature difference across the coil to verify the effectiveness of the cleaning. These measurements should be included in a report to the owner or supervisor.

Power washing, if done improperly (for instance, using the wrong spray angle or excessive pressure), can damage coils by bending the fins—or even breaking them off if the coil is old. The task isn't very complicated, but it should be done only by people trained in the proper use of the necessary equipment.