Code Reference
EnergyASHRAE 90.1-2022

Section 6.5.3/Air System Design and Control

ASHRAE 90.1 Section 6.5.3 covers VAV controls, fan power limitations, and simultaneous heating and cooling restrictions.

What this section requires

Multi-zone HVAC systems with a total fan system power exceeding specified thresholds must use variable air volume (VAV) fan control. Fan power is limited by Section 6.5.3.1 based on the system configuration and airflow. Zone-level controls must modulate airflow to match the load rather than using constant volume with reheat, which wastes energy by simultaneously cooling and reheating air. Reheat is limited: single-duct VAV systems must reduce airflow to the minimum before energizing reheat. Dual-duct and multizone systems must comply with the simultaneous heating and cooling limitations of Section 6.5.2.

Why this section exists

Air distribution accounts for a significant portion of HVAC energy use. A constant-volume system that cools all air to 55 degrees and then reheats it at each zone wastes enormous energy. VAV systems save 30-50% of fan energy by reducing airflow when zones are at partial load. Fan power limits prevent designers from oversizing fans, which wastes energy even on VAV systems. These provisions address the largest single category of HVAC energy waste in commercial buildings.

What plan reviewers look for

Plan reviewers check the mechanical drawings for VAV terminal units at each zone. They verify that the air handling unit has a variable frequency drive (VFD) on the supply fan. They check the control sequence for the VAV box minimum airflow setpoints and the reheat sequence (airflow must reduce before reheat activates). They also verify that the total fan brake horsepower does not exceed the fan power limitation.

Common violations

Constant volume system where VAV required
A multi-zone system uses constant volume fan-powered terminals instead of VAV terminals, resulting in full fan power at all times regardless of load.
Reheat before minimum airflow
The VAV control sequence activates reheat at the zone thermostat setpoint without first reducing airflow to the minimum. This causes simultaneous cooling (cold supply air) and heating (reheat coil) which wastes energy.
Compliance tip
Show VAV terminal units on the mechanical plan with minimum and maximum airflow settings. Include VFDs on the AHU fan schedule. Document the control sequence showing the airflow reduction before reheat sequence. Calculate total fan power and verify it meets the Section 6.5.3.1 limitation.

Related ASHRAE 90.1 requirements

Section 6.5.1 covers economizer requirements. Section 6.5.2 covers simultaneous heating and cooling limitations. Section 6.4.1 covers equipment efficiency. Section 6.5.3.1 provides the fan power limitation formulas.

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Related sections

6.4.1HVAC Equipment EfficiencyASHRAE 90.1-20226.5.1Economizer RequirementsASHRAE 90.1-20229.4.1Lighting ControlsASHRAE 90.1-2022