An air-supply method that changes how much air a zone receives as the load changes, instead of holding one fixed flow all day. The damper, the controls, and the fan all have to move together, or the room pressure drifts.
Constant Flow and Variable Flow
| Approach | What the room gets |
| Constant volume | One airflow whenever the system is on. Simple, and often louder and costlier to condition. |
| Variable volume | Airflow rises and falls with the load. The fan and the reheat have less work when the room is quiet. |
Why a Fume Hood Lists It
A laboratory hood that can run on a variable-volume exhaust uses less conditioned air when the sash is down. The product page is telling the facilities engineer that the hood can join that kind of system, and telling the safety officer that the airflow monitor still alarms if the reading drifts. The option is not a decoration. It changes the duct, the controls, and the energy bill.
