What is supply air temperature reset and how is it implemented?

Supply air temperature (SAT) reset raises the AHU discharge air temperature setpoint when all zones indicate they are over-cooled (dampers near minimum position), reducing compressor/coil energy and enabling more hours of economizer operation. Per ASHRAE 90.1-2019, SAT reset is mandatory for systems with DDC at the zone level, and per Guideline 36, the reset should be trim-and-respond: the SAT setpoint increases by 0.2°F every 5 minutes (to a maximum of typically 60–65°F) as long as no zone is near its maximum cooling demand. The reset range is typically the design SAT (53–55°F) up to a maximum of 60–65°F, with the upper limit set by the zone requiring the most dehumidification. Siemens Apogee and JCI Metasys both implement SAT reset using rolling-average zone demand polling across all connected VAV controllers.