Supply air is moved through rooftop units by a belt-driven, forward-curved centrifugal fan like the one shown in Figure 1. A supply fan's two key maintenance requirements are bearing lubrication and blade cleaning. Fans in older rooftop units have sleeve bearings, which are simply oiled metal-to-metal running surfaces. These should be lightly oiled two or three times per year with the recommended lubricant. A label near the bearings should indicate the lubrication interval, lubricant type, and perhaps a log of past service.

Newer fans are equipped with self-lubricating bearings (sealed-cassette ball-bearing cartridges preloaded with grease). There is no way to regrease these bearings, so when they finally fail—typically after several years of service—the bearing cassette must be replaced. Warning signs of impending failure are excessive noise, vibration, or heat emanating from the bearing.

Conventional greased ball bearings are found occasionally in rooftop units. The most common problem with these bearings is overgreasing—the service technician connects a grease gun to the fill fitting and pumps in grease until it flows out of the bearing seals. Overgreasing can be as damaging as undergreasing for a variety of reasons. The proper procedure is to open the drain plug and inject grease through the fill fitting until clean grease comes out of the drain. If it is possible to do so safely, regrease the bearings while the motor is running to help ensure a complete grease exchange. Take care not to get grease or oil on the pulley wheels or belt, because that will cause slip-stick action that will jar the system.

The impeller blades on a forward-curved fan are scoops that can fill up with dirt. Good filtration helps keep dirt out of the fan, but an annual visual inspection still makes sense. If impeller blades are coated with dirt, fan efficiency will suffer. Cleaning the blades on a small fan takes an hour or more, as the technician must remove the impeller from the fan housing. Cleaning larger fans, especially those with multiple wheels on a single shaft, can be a major project.

Many HVAC technicians have at least one story of finding a fan motor running in the wrong direction. Centrifugal fans still supply some air even when running backwards (typically about 50 percent of rated flow), so the problem may not be readily apparent. The most common cause of reverse fan operation is switched wire leads on the motor. Clear labels on the fan housing, pulleys, motor, and wires can help prevent this problem.

Measuring the fan speed can be an important part of diagnosing air-side problems and is a key part of checking the total airflow. When a rooftop unit is down for service (or first installed) affix a small reflective sticker to the fan shaft to allow speed measurement with a hand-held stroboscopic tachometer.