Plenum Boxes (Plastic Back Boxes)

Frequently asked questions

What is a plenum box used for?

Joining a round duct to a square grille, diffuser or louvre. Without one, a round duct feeding a square grille gives uneven airflow across the face and more noise. The box slows and spreads the air so the grille performs as designed, and gives a neat airtight connection.

What sizes do you stock?

The box comes in a range of combinations of square face size and round spigot size, with spigots from 100mm to 315mm. Choose the face to suit the grille or diffuser and the spigot to suit the duct. The size options are listed on the product.

Plenum box, back box or plenum chamber?

They all describe the same thing: a box fitted behind a grille or diffuser that provides the duct connection. Back box is the common trade term, plenum box or plenum chamber the technical one.

What does top entry mean?

The spigot is on the top of the box, so the duct comes down into it from the ceiling void above. That suits ceiling grilles and diffusers with the duct running in the void.

Do I need a plenum box behind every grille?

For square ceiling grilles and diffusers on a round duct, yes. Round air valves fit the duct directly and do not need one.

How do I install a plenum box?

Fix the box in the void above the opening, connect the duct to the spigot with a clip and foil tape, then fit the grille or diffuser to the underside. Seal the joints, especially on MVHR, and support the box independently so the duct is not carrying its weight.