Plenum Boxes (Plastic Back Boxes)

A plenum box — also known as a back box or plenum chamber — is a plastic enclosure fitted behind a ceiling grille, diffuser or air valve to connect it to the round duct. The plenum equalises air pressure across the face of the grille, reduces noise and turbulence, and provides a neat, airtight transition between the square or circular grille and the flexible or rigid duct behind it. Our plastic back boxes are top-entry with a side spigot, supplied with a bird mesh over the spigot and designed to fit standard ceiling grilles and diffusers used in MVHR, extract and mechanical ventilation systems. In stock with next-day delivery available.

Ceiling Diffusers / Air Valves / Egg Crate Grilles / Flexible & Rigid Ducting

What is a plenum box used for?

A plenum box sits behind a ceiling grille, diffuser or air valve and provides the connection point for the round duct. Its job is to turn the air from a concentrated duct flow into an even, low-velocity flow across the face of the grille. This does three things: it improves airflow distribution so the grille works as designed, it reduces noise by slowing the air before it exits the grille, and it creates a clean airtight seal between the duct and the ceiling opening. Without a plenum, connecting a round duct directly to a square grille creates turbulence, noise and uneven airflow.

What sizes of plenum box do you stock?

Our plastic plenum boxes are top-entry with a side spigot connection and are available in sizes to match standard UK ceiling grilles and diffusers. The most common sizes we supply are 150×150mm and 200×200mm face with 100mm, 125mm or 150mm duct spigots — matching the grille sizes most commonly used on MVHR and mechanical extract systems. For larger diffusers or non-standard grille sizes, contact us on 0161 818 2050 and we can source the correct plenum.

What's the difference between a plenum box, back box and plenum chamber?

All three terms describe the same thing — an enclosure fitted behind a grille or diffuser that provides the duct connection and pressure equalisation. "Plenum box" is the most common technical term in UK ventilation; "plenum chamber" is the same thing used more in commercial HVAC specifications; "back box" is the common trade/installer term for the same product. You'll see all three used interchangeably on product listings and installation drawings.

Do I need a plenum box behind every grille?

For most ceiling-mounted grilles, diffusers and air valves, yes — a plenum box is the correct way to connect round duct to a square or round face grille. Without one, you lose airflow distribution, create turbulence noise, and make it harder to get an airtight connection. Exceptions are where the grille itself has an integral duct spigot (common on air valves) or where the grille is wall-mounted and connects directly to rigid ducting behind the wall. For ceiling installations with flexible duct, a plenum is effectively essential.

Can plenum boxes be used with MVHR systems?

Yes. Plenum boxes are standard on MVHR supply and extract terminals. They give the low-velocity, low-noise air delivery that MVHR systems depend on, and allow flexible duct to be routed through the ceiling void and connected neatly to ceiling grilles or diffusers at each room. For MVHR installations, the plenum also helps with commissioning by providing a predictable pressure drop across the terminal.

What is a top entry plenum box?

Top entry refers to the spigot orientation. On a top-entry plenum, the duct spigot enters through the top face of the box, meaning the duct runs vertically down from the ceiling void to the plenum. This is the most common orientation for ceiling-mounted grilles in residential and light commercial applications. Side-entry plenums have the spigot on the side face and are used where horizontal duct runs approach the grille position — more common in commercial ceiling void installations with long horizontal duct runs.

How do I install a plenum box?

The plenum box is fixed in the ceiling void above the grille opening. The duct is connected to the spigot using a duct clip and foil tape to give an airtight seal. The grille or diffuser is then fitted to the underside of the plenum through the ceiling opening, usually with integral clips or screws. All joints should be taped or sealed to maintain the airtight integrity of the ventilation system — especially on MVHR where leakage reduces heat recovery efficiency. The plenum should be supported independently of the flexible duct to avoid loading the spigot connection.

Can I use a plenum box with a fire damper?

Yes, but the fire damper is installed in the duct upstream of the plenum, not inside it. Where the duct passes through a fire compartment, the fire damper is fitted at the compartment line, the duct continues past the damper, and the plenum and grille are fitted beyond it. The plenum itself is a plastic enclosure and does not provide fire resistance. For full fire compartmentation under Building Regulations Part B, the fire damper specification must match the compartment rating — typically 60 or 120 minutes.