Grille Box (Plenum Box) – 350mm x 350mm x 150mm

It bridges the gap between round ducting and a square or rectangular air terminal. The box mounts behind a grille, louvre or valve, presenting a square face that matches the terminal’s neck and a circular spigot that connects to spiral or flexible ducting. It also settles the airflow so the terminal delivers air evenly across its face.

The first two numbers are the face dimensions, which should match the neck size of the grille or louvre you are fitting; the third is the diameter of the round duct connection. So a 300mm x 300mm x 150mm box suits a 300mm x 300mm neck grille fed by 150mm ducting.

High-quality, impact-resistant plastic — sturdy enough for daily service in both residential and commercial installations, while staying light and easy to mount. The spigot fits standard duct sizes, so connecting spiral duct or flex is straightforward: push on, then secure and seal the joint as you would elsewhere in the run.

Any of our square-neck terminals with a matching neck size — eggcrate grilles, single and double deflection grilles, weather louvres and similar. Match the box face to the grille neck size (not the larger overall face) and the spigot to your duct diameter. Stock sizes run from 100mm x 100mm faces up to 550mm x 550mm with spigots from 100mm to 500mm.

Yes — the plastic grille boxes are stock items with our fastest lead time, shown on each product page, and orders over £400 excluding VAT qualify for free delivery. If you need a size or configuration that is not listed, including steel construction, contact us for a made-to-order quote.