Air stops moving at low primary flow.

The zone temperature can be correct and the space still feel stagnant.

Distribution suffers at very low volumes.

The fan maintains room air movement.

It also recovers plenum heat.

That heat is already paid for.

Where the fan sits and when it runs.

Series puts it in the main path, running continuously.

The room gets constant volume at varying temperature.

Parallel puts it in a side branch, running only at low flow.

That uses less energy and gives varying volume.

Comfort against energy, essentially.

Series where constant air movement matters.

Open offices and densely occupied areas.

Parallel where occupancy is intermittent.

Perimeter zones with variable load suit it.

Energy priorities push toward parallel.

Comfort complaints push toward series.

It reduces the reheat actually needed.

Ceiling plenum air is warmed by lights and structure.

Mixing it in raises supply temperature for free.

Less energy goes into the reheat coil.

The effect is largest in interior zones.

It is a genuine part of the business case.

Noise and maintenance, above an occupied ceiling.

There is now a motor and a filter up there.

Filters need changing on a schedule.

Series boxes run continuously, so noise is constant.

Select against sound data, not capacity alone.

Avoid siting them over quiet rooms.

More than a standard terminal.

The fan and filter need regular attention.

An access panel beneath is essential.

Without one, filters simply never get changed.

A blocked filter degrades the whole box.

Coordinate panels with the ceiling layout.

Often, but less of it.

Plenum air raises the mixed temperature.

Perimeter zones with fabric loss still need a coil.

Interior zones may need very little.

Size the coil after allowing for plenum recovery.

Oversized coils reheat air unnecessarily.