What does ASHRAE 90.1 require for HVAC controls?

ASHRAE 90.1-2019 requires DDC (Direct Digital Control) for most HVAC systems in buildings over a certain size, mandatory demand-controlled ventilation using CO₂ sensors in densely occupied spaces, and automatic setback of temperature setpoints during unoccupied hours. It mandates economizer controls (air-side or water-side) with high-limit cutoffs, requires VAV fan systems to have variable-speed drives with static pressure reset, and specifies that supply air temperature must be reset based on zone demand. Section 6.4.3.4 also requires DDC-controlled terminal units in most VAV applications, and the 2019 version added requirements for automated fault detection and diagnostics (FDD) in many system types.