Plenum Boxes

A plenum box is a chamber fitted between ductwork and a diffuser or grille. It equalises pressure, reduces noise, and delivers a stable, even airflow across the outlet for better comfort and balancing.

Common types include supply plenum, return air plenum, linear slot diffuser box, swirl diffuser plenum, and starter/transition plenums. Choose side-entry for low ceiling depth or retrofit, and top-entry for straight vertical drops and minimal pressure loss.

Match the neck size to the diffuser/grille and the spigot size to the duct. Consider airflow (L/s or CFM), available static pressure, diffuser type/throw pattern, and required leakage class. Oversizing can reduce noise but may increase space requirements.

Plenums are typically made from galvanised steel, aluminium, pre‑insulated phenolic/PIR panels, or EPP. Insulated plenums improve R‑value and help prevent condensation; higher thickness and low‑conductivity cores reduce heat gain/loss.

Many supply plenums include an in‑spigot or opposed‑blade damper to fine‑tune airflow to each outlet. Balancing dampers simplify commissioning and help maintain design airflow across multiple branches.

Requirements follow local ductwork and fire standards. In Australia, AS 4254 applies to HVAC ductwork components; internationally, DW/144 (UK) and SMACNA (North America) guide construction and leakage. Fire performance often references BS 476 or EN 13501 classifications. Always check project specifications.

Seal all joints and spigots, support the plenum independently from the ceiling grid, and maintain a continuous vapour barrier on insulated boxes to avoid sweating. Provide access for cleaning and periodic inspection of dampers and fixings.