ASHRAE Journal - October 2011 - 20

sooner and increases the number of hours when the economizer is able to provide all the necessary cooling. For zones with low cooling loads, when the supply airﬂow has been reduced to the minimum Room 204 setting of the VAV terminal, raising the supplyair temperature also decreases the use of reheat at the zone level. However, because the supply air is warmer, those zones that require cooling will need more air to satisfy the cooling load. This increases supply fan energy. Finally, in climates that experience humid weather, warmer supply air means less dehumidiﬁcation at the coil and higher humidity levels in the zones. If dehumidiﬁcation is a concern, use caution when implementing this strategy. Average SAT reset should be implemented so that it minimizes overall system energy use. This requires 6 a.m. 8 a.m. 10 a.m. 12 p.m. 2 p.m. 4 p.m. 6 p.m. considering the trade-off between compressor, reheat, and fan energy, while not ignoring space Figure 3: Identifying rogue zones. humidity levels. Although there are several different approaches to implementing this strategy, many of the papers and articles written on the 61 subject tend to agree on some general principles 60 for balancing these competing issues.1,2,3 First, when it is warm outside, keep the SAT 59 cold. This takes advantage of the signiﬁcant en58 ergy savings from unloading the fan. Then, begin Reset Based on to raise the SAT setpoint during mild weather Worst-Case Zone 57 when it can enhance the beneﬁt of the airside economizer and reduce reheat energy. 56 Although there are several possible control se55 quences, the example in Figure 4 demonstrates one way to attempt to balance these competing 45 50 55 60 65 70 75 impacts of SAT reset. Outdoor Dry-Bulb Temperature (°F) With this approach, the SAT setpoint is reset based on the changing outdoor dry-bulb tem- Figure 4: Example of supply-air temperature (SAT) reset control. perature. When the outdoor dry-bulb temperature is warm—higher than 65°F (18°C) in this example—no Finally, the amount of reset is limited, to 60°F (16°C) in this reset takes place and the SAT setpoint remains at its design example, which allows the system to satisfy cooling loads in value of 55°F (13°C). When it is warm outside, the outdoor air interior zones without needing to substantially oversize VAV provides little or no cooling beneﬁt for economizing, and the terminals and ductwork. cooling load in most zones is likely high enough that reheat A possible drawback of this approach is that if a zone has a is not required to prevent over-cooling. Keeping the air cold high cooling load, even when it is cool outside, it is possible allows the fan to turn down, taking advantage of the energy that the warmer supply air may not provide enough cooling savings from reducing airﬂow. In addition, the colder supply- and that zone may overheat. To prevent this from occurring, air temperature allows the system to provide sufﬁciently dry when SAT reset does takes place, the amount of reset should air to the zones, improving part-load dehumidiﬁcation. depend on the cooling need of the worst-case zone. (Zones When the outdoor temperature is cooler, the controls begin that are expected to have near-constant cooling loads should to reset the SAT setpoint upward. At mild or cold outdoor tem- be designed for the maximum reset SAT.) peratures, reset enhances the beneﬁt of the economizer, and if As depicted in the example in Figure 4, when it is 50°F there is any zone-level reheat, it is reduced or even avoided. (10°C) outside, the system will attempt to raise the SAT setAt these cooler temperatures, the supply fan has likely already point to 60°F (16°C). However, if there is a zone that is at unloaded signiﬁcantly, so the incremental energy use of hav- near-design cooling load, this air may not be cold enough. Using to deliver a little more air is lessened. ing the current temperature and VAV damper position for that
20 ASHRAE Journal
Supply-Air Temperature Setpoint (°F) Zone VAV Damper Position

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October 2011

ASHRAE Journal - October 2011

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