Commercial Grilles & Valves

Frequently asked questions

What is the difference between a grille, a diffuser and an air valve?

A grille is a faceplate with fixed or adjustable blades that covers a duct opening and directs air in a broad throw, typically wall mounted. A diffuser is designed to spread air in several directions at once, usually from a ceiling, so the supply air mixes with room air before it reaches anyone sitting under it. An air valve is a small circular terminal, normally 100mm to 160mm, with a screw-adjustable centre disc that sets the flow rate at that outlet. Grilles and diffusers are about distribution; valves are about balancing.

How do I size grilles and diffusers for a commercial system?

Size to the design airflow at that outlet, not to the duct diameter. Work from the airflow in cubic metres per hour, the face velocity the manufacturer quotes, and the throw needed to reach the occupied zone. Undersizing raises face velocity, which causes noise and draughts; oversizing drops the throw so air dumps straight down instead of mixing. On multi-room systems calculate every outlet individually, then balance the whole thing against the fan or air handling unit rather than assuming the branches share equally.

Should I use fixed or adjustable grilles?

Fixed grilles are cheaper, have nothing to work loose and suit extract duties or any position where the throw direction will never change. Adjustable grilles let you set blade angle on site, which matters on supply air where you are steering air away from desks, around obstructions or down from high level. If the layout of the space is likely to change, adjustable is worth the extra.

What should I consider when selecting air valves?

Four things. Duty: supply, extract or both, since the cone profile differs. Airflow capacity: match the rated volume to the design figure with the valve part open, not fully open, so there is adjustment left in both directions. Pressure drop: a valve throttled almost shut to hit its figure is a sign the branch is oversized. Material: steel or coated finishes for damp, dusty or commercial kitchen positions. Leave the valve accessible, because it will need re-setting after any change to the system.

How do I minimise noise from grilles and diffusers?

Most terminal noise is face velocity, not fan noise. Increase the free area by going up a size, choose a low pressure drop pattern, and keep the last section of duct straight for a few diameters before the terminal so air arrives evenly across the face. A bend or a flexible duct squashed against the back of a grille will generate turbulence no amount of blade adjustment will fix. Where the duct run itself is the source, an inline silencer upstream is the answer.

What is a grille box and when do I need one?

A grille box, or plenum box, is a sheet metal box that sits behind the grille with a spigot for the duct connection. It steadies airflow across the face of the grille, gives a clean fixing for the faceplate and lets you bring the duct in from the side or the back. You need one wherever a round duct meets a rectangular grille, and anywhere the connection would otherwise be visible through the blades.

What matters when installing weather louvres and roof cowls?

Free area, position and access. A weather louvre loses a substantial share of its gross area to the blades, so size on free area rather than the opening size or you will add static pressure the fan was never sized for. Keep intakes clear of exhaust discharges, flues and roof edges where turbulence and recirculation happen. Roof cowls need an upstand that keeps them clear of standing water. Leave both accessible for inspection, because mesh blocks with leaves and salt long before anything mechanical fails.

How often should grilles, diffusers and valves be inspected?

Every six to twelve months in offices and typical commercial spaces, quarterly in industrial or high-dust environments, and annually for external louvres and cowls. Look for debris on the face and in the plenum, valves that have drifted or been turned by an occupant, and any corrosion on external terminals. A terminal that has been adjusted by someone chasing a draught is one of the most common reasons a commissioned system drifts out of balance.

Do you supply grilles and valves for trade and project work?

Yes. We supply M&E contractors, installers, facilities teams and specifiers, holding stock across the standard sizes for next day delivery. Send a schedule or a drawing and we will price the lot, confirm sizes and finishes, and quote any made to order items alongside. Call 0161 399 3825 or email hello@efans.co.uk.