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An amazing number of rooftop units 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 US$100 per year in energy costs (9.0 EER, 2,000 hours of operation, at 8 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.
Leaking cabinets waste expensive air. A leak of 200 cubic feet per minute costs about $100 per year in energy.
Source: E Source
Most rooftop units are covered with access panels 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 should keep 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:
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 a rooftop unit when it is first installed (or first incorporated into a comprehensive service program) makes this type of measurement much easier.
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