Inside a variable air volume system

A variable air volume (VAV) system sends conditioned air from a central air handler to zones through VAV boxes, each with a damper that opens or closes to meet that zone's need. When the zones need less air, a variable speed drive slows the supply fan instead of choking it with dampers. Because fan power varies roughly with the cube of airflow, running at half flow can take only about an eighth of the power.

Controls

Labels

Animation speed

Airflow (percent of design)

Fan power at full flow

Step through it

Mixing and filteringOutdoor air enters through dampers, mixes with return air from the building and passes through filters that catch dust.
CoilsA chilled water coil cools and dries the air; a hot water coil warms it when needed. Most VAV systems deliver air at a steady cool temperature.
Supply fan and driveThe supply fan pushes air into the ductwork. A variable speed drive adjusts fan speed to hold a target duct pressure.
VAV boxesEach zone's box opens or closes its damper to deliver just enough cool air to hold that zone's thermostat setting.
The cube law payoffAt 50 percent airflow an ideal fan needs only 12.5 percent of full power. Buildings spend most hours at part load, so savings add up fast.

Fan affinity laws

For a given fan and duct system, airflow is proportional to fan speed, pressure to speed squared and power to speed cubed. So power at part load = full power x (flow fraction) cubed. At 60 percent flow that is 0.216, about a fifth of full power.

This is the ideal case. Real VAV systems hold a minimum duct static pressure so the far boxes still get air, and motors and drives lose some efficiency at low speed, so real savings follow something closer to a 2 to 2.5 power. Treat the readout as an upper bound.

Try it: set full flow power to 30 kW and slide airflow from 100 to 50 percent. Fan power falls from 30 kW to under 4 kW, while the duct pressure readout drops to a quarter.

Why not just close dampers

Older constant volume systems ran the fan at full speed all day and throttled or reheated the air. Slowing the fan instead removes energy at the source, which is why variable speed drives on fans are one of the most common efficiency upgrades in commercial buildings.

Sources

Chapter 6. Fans and Ventilation (fan affinity laws)NC State Extension
How Low Can You Go? Low Pressure Drop Laboratory Design (LBNL-49366)U.S. DOE Office of Scientific and Technical Information
Large Commercial Buildings: Re-tuning for Efficiency, Terminal Units in Air Distribution SystemPacific Northwest National Laboratory

How to cite this model

Avid Solutions International. (2026, November 11). Inside a variable air volume system [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/vav-air-handler

Last updated November 11, 2026

Keep exploring

Buildings & HVACHow a heat pump moves heat uphill Buildings & HVACHow a cooling tower rejects heat Buildings & HVACWhy elevators use a counterweight Buildings & HVACHow an energy recovery ventilator works