Fans are integral to a building’s HVAC system. It’s especially important to perform regular maintenance on fans that are exposed to the elements, such as the condenser fan in an RTU, because they are more prone to premature failure than fans that are in protected locations, such as in an air-handler inside a building (Figure 2).
Figure 2: Anatomy of a rooftop unit
Rooftop units include both a supply (evaporator) fan and a condenser fan. In this model, the former is belt-driven and the latter is a direct-drive fan. Though both fans are housed within the unit, the condenser fan is blowing straight up, so it is directly exposed to the elements.
Here are some general fan-maintenance practices.
Visually inspect the fan. Make sure nothing is blocking the fan blades and check for obvious signs of damage. Damaged fan blades can reduce performance and ruin a motor.
Check fan motors. 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. Also verify that the fan motor is running in the correct direction—many HVAC technicians have at least one story of finding a fan that’s running backwards. 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. Finally, 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 a fan that is cycling rapidly, call in a qualified HVAC technician to check the settings on the fan controller—they may be in need of adjustment.
Lubricate bearings. Sleeve bearings, which are simple oiled metal-to-metal running surfaces found in older fans, 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 occasionally found in fans. 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. But 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.
Clean fan blades. If impeller blades are coated with dirt, fan efficiency will suffer. Impeller blades on forward-curved fans are especially prone to filling up with dirt because they are shaped like scoops. Good filtration helps keep dirt out of the fan, but an annual visual inspection still makes sense. 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.
Adjust belts. Improperly adjusted belts rob the drivetrain of power, create noise, and require replacement sooner than well-adjusted belts. Loose belts slip on the pulley wheels, causing torque loss and rapid wear. Belts that are too tight put an excessive load on the motor and fan shaft bearings, causing early failure of the bearings or belts. Proper belt tension can be achieved with a deflection strain gauge, but most technicians are familiar enough with the proper tension to adjust it simply by pressing on the belt with a finger. Either method works well if performed consistently. In addition, belts should be aligned with a straightedge to prevent lateral wear.
Some technicians advocate belt changes once or twice a year, whereas others simply let belts run until they break. Depending on the price of a belt, it may make sense to forestall breakage with periodic replacement. According to Nationwide Heating and Cooling, a Denver, Colorado, service representative for Carrier Corp., belts can cost from $20 to $60 and a service call to replace a broken belt can run from $150 to $200. Experts recommend keeping one extra belt (an old one will do, if it’s in good shape) inside the cabinet to use as an emergency spare.
An easy upgrade that can improve drivetrain efficiency by 2 to 8 percent is to switch from standard to cogged V-belts (Figure 3).
Figure 3: Cogged V-belts
Specifying cogged V-belts instead of standard V-belts is an easy way to improve supply-fan efficiency by 2 to 8 percent. Cogged belts run on conventional smooth pulleys, but the notches on the inside of the belt reduce internal bending losses and improve gripping action.