Maintaining Packaged Rooftop Units

Packaged air-conditioner and heat-pump systems, also called rooftop units (RTUs), fall into disrepair over time, leading to energy waste and compromised comfort for a building’s occupants. It’s crucial to maintain these RTUs on a regular basis to ensure that they cool buildings and occupants as intended without wasting energy. Regular maintenance will also help to prevent or quickly catch malfunctions before they have a chance to further degrade the equipment or waste prodigious amounts of energy. An economizer stuck in the open position in a climate such as that of Tampa, Florida, for example, can cause the HVAC system to use 50 percent more energy than a system without an economizer.

Because a large part of the cost of servicing an RTU is incurred just getting service personnel and equipment on site, it doesn’t make sense to use a piecemeal approach that only addresses one or two maintenance measures at a time. Rather, use an annual service or maintenance program or contract to maintain the entire unit.

These contracts typically include a host of measures. Several of these, including cleaning both the condenser and evaporator coils, verifying refrigerant charge levels, and maintaining the RTU cabinet integrity are covered here. Other measures, such as changing filters and maintaining fans and outside air dampers, are covered in our Maintaining Air-Handling Equipment topic, because they also apply to equipment other than RTUs. Based on a short survey of service providers, the cost for an annual maintenance contract that includes all of these tasks is between approximately US$1,000 and $1,200 for a 10-ton unit. Filter changes and inspections are frequently performed quarterly, with the remaining tasks performed annually.

Cleaning Condenser Coils
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In a packaged rooftop air conditioner, the condenser coil is exposed to unfiltered outdoor air, so it will accumulate dirt buildup at a much higher rate. The performance penalty of a dirty condenser makes cleaning it one of the most important practices available for RTU maintenance. As dirt accumulates, the coil’s ability to exchange heat—and thus its cooling capacity—will decrease, and its power consumption will increase. A dirty coil will also impede airflow, contributing to a capacity reduction. A 1999 study by the Proctor Engineering Group estimates that unmaintained condenser coils will gradually degrade unit efficiency by 5 percent over 17 years and by up to 30 percent over 20 years. Harsh environments or high equipment loading (that is, running equipment at or near capacity for long periods of time) can shorten these time periods by a factor of three.

The best tool for this maintenance job is a power washer that feeds cleaning solution into a high-pressure water spray. Some companies specialize in performing this type of cleaning for a competitive price. They typically use tank trucks and custom self-contained equipment. Spray-on cleaning solutions that are intended to be used with a brush and a hose will not do a good enough job of cleaning the coils, even though they may brighten the outer surface. A serious condenser cleaning involves before-and-after measurements of the temperature difference across the coil to verify the effectiveness of the cleaning. These measurements should be included in a report to the owner or supervisor.

Take care, however: Power washing, if done improperly (for instance, using the wrong spray angle or excessive pressure), can damage coils by bending the fins or even breaking them off if the coil is old. This task isn’t complicated, but it should be done only by people trained in the proper use of the necessary equipment.

Cleaning Evaporator Coils
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Dirt on the evaporator coil causes two problems: It reduces system airflow, and it directly degrades the coil’s heat-transfer efficiency (which significantly cuts cooling capacity). With good filtration, a rooftop unit’s evaporator coil will stay fairly clean.

Some technicians claim that specifying annual evaporator coil cleaning is an unnecessary expense, but others maintain that enough dirt gets around or through the filters to justify the expense. Those in favor of annual evaporator coil cleaning contend that as long as the equipment required to clean the condenser coil is on the roof anyway—as it should be for the unit’s annual cleaning—it’s worthwhile to clean the evaporator coil as well.

The evaporator coil should at least be inspected once a year to make sure that the filters have been doing their job. Shining a light through the coil is one way to inspect it (although wavy “enhanced” fin designs can make this type of check difficult). Another approach is to measure supply-fan amperage and filter/coil pressure drop (with fresh filters). If the amps are lower and the pressure drop is higher than last year’s measurement (also with fresh filters), then the evaporator coil is dirty and needs to be cleaned.

Fixing Refrigerant Leaks
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Fixing refrigerant leaks in RTUs is a tedious but important task. Refrigerant charge that’s too low can compromise efficiency and capacity. The following tips can help prevent leaks and speed their repair:

  • Use rubber-gasketed brass caps for all service-port connections and keep them wrench-tight (rather than just hand-tight). Have plenty of spares on hand—like the valve caps on your car tires, they have a way of wandering off. Replace these gaskets (or the complete caps) periodically, because they can become dry and cracked.
  • Enforce a standard of cleanliness for all refrigerant lines. Keep the copper pipes wiped down with a clean rag. Refrigerant leaks also leak oil, and it is easier to find oil on clean pipes.
  • Look for leaks on flanged and screw fittings first, rather than on soldered joints.
  • Minimize the number of times that refrigerant gauges must be connected to the service ports—each time this happens, it bleeds a bit of refrigerant from the system.
  • Repair a leaking coil only once; when you do, mark it with paint or chalk. The next time it leaks, replace the coil completely.

Repairing refrigerant leaks requires several steps, including refrigerant recovery, nitrogen charging, inspection with a leak detector, brazing or otherwise fixing the leak, vacuum drying, and recharging.

Maintaining Cabinet Integrity
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It’s not unusual for RTUs to spill much of their expensive chilled air onto the roof. The reason: poorly designed or poorly maintained cabinet hardware (Figure 1). Annual checkups should include a survey of air leaks, followed by corrective action such as replacing screws or latches and patching or replacing gaskets. Cabinet integrity is particularly important on the supply-air side, where high pressure created by the fan can force considerable air out of a small crack. Losing 200 cubic feet per minute (cfm) from a 10-ton rooftop unit reduces cooling and airflow capacity by about 5 percent and wastes more than $100 per year in energy costs (assuming an energy-efficiency ratio of 11.2 and 2,000 hours of operation at 10 cents per kilowatt-hour). Another potential source of air leakage is through the condensate drain pipe that leads out from the pan under the evaporator coil. A narrow pipe section or U-bend water trap can reduce or eliminate this type of leak.

FIGURE 1: Leaky rooftop cabinet
Leaking cabinets waste expensive air. A leak of 200 cubic feet per minute can cost about $100 per year in energy.

Most RTUs are covered with access panels that are held in place by small sheet-metal screws. Using hand tools to remove and reinstall these small screws can be exasperating—which explains why many panels have only one or two screws left in place after a few service calls. But loose panels mean leaky units, which in turn means valuable chilled air is spilling onto the roof. A cordless drill with the correct nut driver makes panel access quick and easy and is conducive to screw replacement. (Make sure that the drill has a clutch to prevent overtightening or stripping the screws.) Technicians will also find their work is made easier by keeping a bag of screws on hand to replace missing ones, including oversized screws for stripped holes.

The final step in checking the air side of the system is to measure the airflow and make sure it is within the expected range (350 to 400 cfm per ton of cooling capacity). This is a difficult task; nonuniform flow in the unit and ducts makes single-point air-velocity measurements nearly worthless in assessing total flow rate. The following three-step process is suggested:

  1. Measure total static pressure drop across the fan.
  2. Measure fan shaft rotation with a tachometer.
  3. Look up the flow from the manufacturer’s fan curve based on fan speed and pressure.

The hardest part of this process is likely to be obtaining the fan curve. You can call the manufacturer’s technical support service with the model number and field measurements and have them read back the corresponding flow rate. Collecting and archiving complete performance documentation on an RTU when it is first installed (or first incorporated into a comprehensive service program) makes this type of measurement much easier.

If the airflow is not within the expected range and the cabinet is properly sealed, then conduct a system investigation to find out why. Ducts could be leaking or obstructed, dampers may not be set or performing properly, or the fan may be malfunctioning.

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