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’s 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 backward. Centrifugal fans still supply some air even when running backward—typically about 50% 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.
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’s cycling rapidly, call in a qualified HVAC technician to check the settings on the fan controller, as it may need 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 the 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’s possible to safely do so, 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’re 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 let belts run until they break. Depending on the price of a belt, it may make sense to forestall breakage with periodic replacement. 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.
Switching from standard to cogged V-belts is an easy upgrade that can improve drivetrain efficiency by 2% to 8% (figure 5).
Figure 5: 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%. Cogged belts run on conventional smooth pulleys, but the notches on the inside of the belt reduce internal bending losses and improve gripping action.