O & M Advisor
Your guide to optimizing equipment performance & efficiency

Rooftop units have one condenser fan and motor for about every 5 tons of cooling capacity. These propeller fans are usually coupled directly to one-horsepower induction motors. The condenser motor operates in much more difficult conditions than the evaporator fan motor: vertical mounting places a thrust load on its lower bearing, the stream of hot condenser air makes it more difficult to cool the motor, and the motor is exposed to rain and snow (Figure 1). Replacement costs for a typical condenser fan motor on a commercial rooftop unit vary by region and equipment, but are usually about half equipment and half labor.

Figure 1: Anatomy of a rooftop unit A rooftop unit packs complete refrigeration, heating, and air-handling capability into a single box. This figure shows the key parts of a rooftop unit with electric cooling and gas heating. Rooftop heat pumps use compressor-driven heating rather than the gas furnace section shown on this unit. Source: Platts

Condenser fan motors on most new rooftop units come with sealed-cassette ball bearings that need no lubrication. Excessive noise, vibration, or heat at the bearing indicate that the cassette should be replaced.

Rapid on-off cycling of a condenser fan (three minutes or less) leads to poor control of the refrigeration system and can wear out the fan motor prematurely. This problem is often caused by a too-narrow deadband on the condenser fan’s head-pressure controller. A healthy differential (20 to 50 pounds per square inch) between cut-in and cut-out pressure setpoints prevents rapid condenser fan cycling.

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