Grilles & Diffusers

Frequently asked questions

Grille or diffuser: what is the difference?

A grille is a face plate over a duct opening. It lets air through and, on deflection types, points it in a set direction. A diffuser spreads the air out across the ceiling in several directions at once, so supply air mixes into the room without a cold draught on the people underneath. As a rule, use diffusers for supply air in occupied spaces such as offices and shops, and grilles for extract, return air, wall openings and doors.

Which ventilation grille do I need?

Pressed steel grilles are the general purpose choice for walls and doors, for extract, return air and simple transfer between rooms. Single deflection grilles have one set of adjustable blades so you can direct supply air up, down or sideways. Double deflection grilles add a second set at right angles, so you can steer the air both vertically and horizontally, which is useful on long throws from a high wall. Egg crate grilles have a square cell core with a large free area, which keeps resistance and noise low on return and extract air, and they are the usual choice in suspended ceilings.

Will a ceiling diffuser fit a 600 x 600 suspended ceiling tile?

Yes, if you choose the right size. The 450mm neck 4-way diffuser has a 595mm x 595mm overall face, and the 550mm neck egg crate grille has a 595mm x 595mm outer flange, so both drop straight into a standard 600 x 600 lay-in grid in place of a tile. Smaller necks have smaller faces and need a cut-out in the tile or plasterboard. Each product lists the neck size, which is the duct connection, and the overall face size.

What size grille or diffuser do I need?

Match the neck size to the duct or opening, then check the airflow. A grille that is too small for the air volume pushes the face velocity up, which is the main cause of whistling and draughts. As a rough guide, keep face velocity below about 2 m/s on supply in occupied rooms and below about 3 m/s on extract and return. If you are working from an airflow figure, send it to us with the room use and we will size it.

Supply or extract air valve?

Air valves are the round adjustable discs used on domestic ventilation and MVHR. Supply valves have a profile that spreads the incoming air across the ceiling, and extract valves have a profile that draws air in evenly. They are not interchangeable, because using an extract valve on a supply run throws the air straight down. We stock metal supply and extract valves in 100mm, 125mm, 150mm and 200mm. Set the opening during commissioning and lock it so the system stays balanced.

Do I need a damper behind the grille?

An opposed blade damper lets you trim the airflow at each grille during commissioning, so every room gets its design rate without adjusting the fan. It is worth fitting wherever several grilles or diffusers share one duct system. Our egg crate grilles are available with or without a damper. On a single grille on its own fan, a damper is usually unnecessary.

Can I use a grille on a fire door?

Not a standard one. A grille in a fire door or fire rated wall needs to be an intumescent fire rated transfer grille that seals in a fire and holds the rating of the door. Our pressed steel grilles are suitable for transfer air in non fire rated doors and partitions only. If the door is a fire door, ask us and we will point you to the right product.

Can grilles be used outside?

Our pressed steel and deflection grilles are for internal use. For an external wall opening use a louvred wall grille or a weather louvre, which is designed to keep rain out. For small domestic ducts, the Domus plastic louvred wall grilles in 100mm, 125mm and 150mm include a flyscreen. For larger openings see our weather louvres.

Do you supply grilles and diffusers for trade and projects?

Yes. We supply M&E contractors, ceiling and partition installers, facilities teams and housing providers. Send a grille schedule and we will price the lot, including dampers and non-standard sizes made to order. Call 0161 399 3825 or email hello@efans.co.uk.