AIR HANDLING UNIT SHUTDOWNS DURING SCHEDULED UNOCCUPIED HOURS: U.S. COMMERCIAL BUILDING STOCK PREVALENCE AND ENERGY IMPACT

Abstract

Commercial buildings account for 18% of U.S. energy consumption, with 44% used for heating, ventilation, and air conditioning (HVAC). ASHRAE-90.1 requires HVAC systems to shut down fans and outdoor air ventilation during unoccupied times, only allowing fans to cycle on, without outdoor air, to maintain thermostat setpoints. However, it is minimally understood how often existing building operations align with energy code requirements and the energy implications of not doing so. This study used building automation system data from 843 buildings containing 5,706 AHUs to determine three unoccupied air handling unit (AHU) shutdown control schemes ranging in efficiency and then estimated their prevalence in the U.S. commercial building stock. ComStock was then used to analyze the energy savings potential of implementing the most energy efficient unoccupied shutdown control scheme in non-participating buildings across the U.S commercial building stock. Results show only 23% of AHUs align completely with the ASHRAE 90.1 requirement. ComStock modeling results show 4% annual stock energy savings by switching all non-participating buildings to the most efficient scheme, with 19% annual energy savings demonstrated for the median building switching from the least efficient scheme to the most efficient. Findings also show 114.5 TBtu electricity and 75.8TBtu natural gas savings when converting to the most efficient scheme. These findings help stakeholders understand the prevalence of buildings not aligning with the ASHRAE 90.1 requirements for unoccupied AHU shutdowns and the energy savings potential of utilizing the most efficient scheme.