The box holds flow regardless of inlet pressure.

Duct pressure changes as other zones modulate.

A fixed damper position would deliver varying flow.

The box measures its own airflow instead.

It moves the damper to hold the commanded flow.

That is why it carries a flow sensor.

The box quietly becomes pressure dependent.

It reads low and opens further than needed.

Or reads high and starves the zone.

Nothing alarms and the room simply misbehaves.

Sensor cleaning belongs in maintenance.

Check it when a zone complains persistently.

It sets how much air is reheated for nothing.

Minimum must cover ventilation and air movement.

Every unit above that is wasted at low load.

It is set once and rarely revisited.

Occupancy changes make old settings wrong.

Review minimums when a floor is refitted.

In a reheat VAV system, by design.

Air is cooled centrally and warmed locally.

That is what allows many zones from one plant.

The waste is bounded by the minimum flow.

Other system types avoid it differently.

Judge the system, not the box.

Throttling against high inlet pressure.

Sound rises sharply with pressure drop.

Manufacturers publish sound against pressure.

Excess system pressure is the usual cause.

Attenuation after installation is expensive.

Select at the actual design pressure.

Considerably, and it is often ignored.

A box fed straight off a bend sees a distorted profile.

Its flow sensor reads that distortion.

Commissioned flows then do not match reality.

Straight inlet duct is specified for a reason.

Allow it in the ceiling coordination.

Hot water coils and electric elements.

Water suits buildings with a heating circuit.

Electric suits zones where pipework is impractical.

Electric reheat needs an airflow interlock.

Running cost differs considerably between them.

Match to the building's heat source.