Perhaps the most common rooftop unit malady is improper damper operation. A study of 13 rooftop units on small commercial buildings found that not a single one had properly operating outside-air dampers. This problem can have major energy consequences in regions that otherwise could take advantage of economizer operation; it can also have potentially serious indoor air quality impacts in all climates.

Outside-air dampers on a packaged rooftop unit are shown in Figure 1. These dampers must endure a continual flow of dirty air that fouls the pivot points and actuator mechanism (the coarse prefilter only keeps leaves and birds out of the unit). If operating properly, the dampers will prevent the compressor from running 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).

During damper servicing, moveable surfaces should be cleaned and lubricated. The cleaning can be done with a power washer or with a bucket of soapy water and a brush. As long as a service technician is already on the roof, this cleaning and testing should take about 15 minutes, costing US$10 (at $40 per hour for labor). If this prevents one of the 5-ton compressors in a 10-ton unit from running for just 100 hours, it will save 465 kilowatt-hours ($37 at 8¢ per kilowatt-hour).

After cleaning and lubrication, a damper should be tested. First, it should be run 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 the economizer setpoint should be checked. 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.

One of the easiest ways to make a service technician groan is to mention enthalpy controllers for outside-air economizers, which attempt to account for both the temperature and humidity of outside air. These devices are notoriously unreliable because of the difficulty of measuring humidity with inexpensive sensors. Until a reliable, accurate, and cheap humidity sensor appears, rooftop unit economizers should be controlled by drybulb temperature sensors. (However, a possible exception to this rule is a building with a good-quality central humidity sensor that sends control commands to rooftop units through an energy management system.)