Fans are the heart of a building’s HVAC system. Depending on the type of system, several types of fans may be used. Here are some general fan maintenance practices as well as some special considerations for specific fan applications.

The first step to maintaining fan performance is to clean the evaporator and condenser coils and the filters. Dirty coils and filters will decrease airflow and the efficiency of your fan system. Along the same lines, make sure the air intake to the fan motor is clear. For fans that operate outside or at the building’s exterior, use screens to block debris from entering the fan and keep the screens clean. Also make sure nothing is blocking the fan blades and check for obvious signs of damage. Damaged fan blades can reduce performance.

Maintain fan motors. After cleaning the coils and filters, verify that the motor amperage is as expected. If it is not, this could indicate that the motor is failing or that the fan assembly needs maintenance. For belt-driven motors, be sure to verify belt tension and condition to ensure safe and efficient operation.

Inspect dampers. Periodically inspect the damper, linkage, and actuator for proper operation. Frozen or shut dampers can affect fan performance and increase energy consumption considerably.

Fans move air across the condenser and the evaporator coils in packaged, or rooftop unit, HVAC systems (Figure 1). Fans can account for about 45 percent of annual HVAC energy use because although fan power use is a small fraction of compressor power use, the fan operates for many more hours.

Supply fans. In a rooftop unit, the airflow across the evaporator is the supply air for the building. Supply air is moved through a rooftop unit by a belt-driven, forward-curved centrifugal fan like the one shown in Figure 2.

A supply fan’s two key maintenance requirements are bearing lubrication and blade cleaning. Fans in older rooftop units have sleeve bearings, which are simple 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. 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.

In addition to screening, filter the supply air. 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 because 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.

Condenser fans. Rooftop units have one condenser fan and motor for about every five tons of cooling capacity. Condenser fans are propeller fans, usually coupled directly to 1-horsepower induction motors. A condenser motor operates in much more difficult conditions than an evaporator fan motor: A condenser motor is exposed to rain and snow, and the stream of hot condenser air makes it more difficult to cool the motor. These conditions can lead to premature failure, so it is important to perform regular maintenance, including lubrication and blade cleaning.

Also monitor fan cycling. 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. If you observe rapid fan cycling, call in a qualified HVAC technician to check the settings on the fan controller—they may be in need of adjustment.