Maintaining Air-Handling Equipment

Proper maintenance of air-handling equipment can prevent energy waste and help to ensure the comfort of building occupants. Fans alone—the main energy-user of air-handling equipment—use about 7 percent of the energy consumed by buildings that use conditioned air. They can be either in self-contained equipment—for example, with air handlers used in chilled-water systems—or integrated into packaged equipment like rooftop air conditioners. Other air-handling components include filters and dampers, which can also waste energy if not maintained.

In this topic, we cover maintenance measures that apply to air-handling equipment, including changing filters and maintaining fans and outside-air dampers. Other measures that apply specifically to rooftop units (RTUs), such as cleaning the condenser and evaporator coils, verifying refrigerant charge levels, and maintaining the RTU cabinet integrity, are covered in the topic Maintaining Packaged Rooftop Units.

Changing filters
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Filters play two important roles: They help maintain indoor-air quality and they protect the downstream components of an air-handling system (the evaporator coil and fan) from the accumulation of dirt that can cripple performance. A dirty filter will slow down standard fans; although this reduces the fan’s energy usage, it also decreases the capacity and efficiency of the rest of the system. Figure 1 shows a typical RTU filter arrangement.

Figure 1: Rooftop unit filter rack
Filter changes are the most frequently required maintenance task for rooftop units. The unit pictured also has a nearby folder with a schedule and log for documenting filter changes—a practice that we recommend.

Filter-changing intervals can be based on the pressure drop across the filter or (more commonly) by calendar scheduling or visual inspection. Scheduled intervals should be between one and six months, depending on the pollutant loading from indoor and outdoor air. More-frequent changes may be required during the economizer season, because outdoor air is usually dirtier than indoor air.

Measuring pressure drop is the most reliable way to rate filter loading. However, this approach requires some effort—most rooftop units do not have built-in pressure taps. Taps can be made by drilling into the cabinet wall and installing quarter-inch tubing with removable caps. A technician then can use a hand-held pressure meter or manometer to check filter status. (To get accurate readings, cabinet access panels must be shut tightly, with all screws replaced.) In facilities with predictable and regular filter loading, pressure measurements can be used to establish the proper filter-change interval; thereafter, filter changes can simply be scheduled.

Pressure-measurement taps are a bargain, given that they cost less than a single change of a high-quality filter. Complete air-filter pressure kits that include a dial gauge cost between US$50 and $100. Hardware for installing taps that can accommodate a portable gauge costs less than $10, and this task can be performed by a service technician in a matter of minutes. Automated pressure tracking is a further step in filter management. It makes the most sense for very large rooftop units (20 tons or greater) and for large, custom-built air handlers (for which a single filter change may cost $100 or more). There are two ways to track pressure automatically:

  • Install transducers that feed pressure-drop data to a building automation system. This data then can be used by building operators to check filter status, or it can trigger an alarm when pressure drop exceeds the design limit. Unfortunately, this strategy yields a long payback period when applied to small RTUs, because the cost of pressure transducers starts at about $260 (for a model with ± 3 percent accuracy).
  • Use a differential pressure switch that flips from “off” to “on” at a certain pressure difference across the filter. This is a less-expensive option, but these switches require time and care to calibrate, and one experienced service technician maintains that they cause more problems than they solve.
Maintaining fans
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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.
Maintaining outside-air dampers
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Dampers are notorious for falling into disrepair. A 2002 study of 123 RTUs equipped with economizers performed by Architectural Energy Corp. found that dampers on 24 percent of them would not move at all and another 10 percent displayed poor operation. This problem can have major energy consequences in regions that otherwise could take advantage of economizer operation; it can also have potentially serious impacts on indoor-air quality in all climates.

Outside-air dampers on a packaged RTU are shown in Figure 4. These dampers must endure a continual flow of dirty air that fouls the pivot points and actuator mechanism (the coarse prefilter only keeps larger things, such as leaves and birds, out of the unit). If operating properly, the dampers can alleviate the need to run the compressor when outside air temperature is below about 60° Fahrenheit (F). Unless they are kept clean and well-lubricated, however, they can stick in place and rob the unit of free cooling potential (if closed) or overload the cooling coil with too much hot outside air (if open).

Figure 4: Outside-air dampers
These dampers have a dirty job: directing the flow of particulate-laden urban air. Unless they are cleaned and lubricated regularly, they cannot perform well. On the right is a pre-filter that keeps out large objects.

During damper servicing, clean and lubricate moveable surfaces. Though the cleaning can be done with a power washer, the high pressure can bend the fins and cause problems. A safer method is to scrub the surfaces with a brush, using foam cleaner and a hose. As long as a service technician is already on the roof, this cleaning and testing should take about 15 minutes.

After cleaning and lubrication, test the damper. First, run it through its full range of motion. Tools can generate electrical control signals to drive the actuator, or the economizer setpoint can be manipulated at the control panel. Then check the economizer setpoint. Although many economizers are set at about 60°F, the setpoint can be as high as the return-air temperature (about 74°F) to provide beneficial ventilation. However, in high-humidity climates (or where outside air is very polluted), it may not make sense to maximize outside-air flow at low drybulb temperatures.

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