This project developed and demonstrated approaches to synergistically improve ventilation and indoor environmental quality during replacements of packaged heating, ventilation, and air conditioning (HVAC)systemsin California schools. The research team 1) characterized HVAC systems, carbon dioxide concentration, and indoor thermal conditions in 104 classrooms that had replaced packaged HVAC systems serving a single classroom (single-zone) between 2013 and 2016; 2) evaluated HVAC system retrofits with regard to energy efficiency and indoor environmental quality performance at two field sites; and 3) completed building energy and indoor environmental quality modeling of HVAC equipment and filter selection forfour climate and outdoor air conditions representative of California’s regional variation and two different classroom vintages.Inspections of 104 classrooms with HVAC equipment installed between 2013-2016 showed that only 15 percent of classrooms’ estimated median daily ventilation rates met the 7.1 liters per second per person Title 24 code requirement,and 9 percent had carbon dioxide levels above2,000 parts per million for significant portions of the school day, which implies a ventilation rate of less than half of that required. Where under-ventilation occurred, it tended to affect several observed classrooms within a given schoolandnot occur as an isolated case. Periodic testing of ventilation systems andcontinuous real-time carbon dioxide monitoring could help to detect ventilation problems.Field testing and modeling of HVAC technologies determined that variable speed motors for indoor blowers, two-speed compressors, economizers, demand control ventilation technology, and air filters with a minimum efficiency reportingvalue of 13 constitute an HVAC package suitable for all of California’s climates. The combination of technologies can save between 28 and 57 percent of HVAC electricity use, depending on climate. Filters with a minimum efficiency reporting value of13can reduce indoor particulate matter exposuresby 40 percent or more compared to filters with avalue of 8.