A fire door with a hole in it is not a fire door, which is a problem when the room behind it needs ventilation. An intumescent air transfer grille solves it: an open grille in normal use that closes itself when heated, restoring the door's resistance at the moment it matters.
How they work
The grille contains intumescent material arranged so that air passes freely between the elements in normal conditions. When the temperature rises the material expands and the elements swell together, closing the opening completely. The grille is tested as part of a doorset assembly, so its rating is expressed in the same terms as the door it is fitted to.
Where they are used
Where a room behind a fire door needs transfer air: plant rooms, some cupboards housing equipment, riser cupboards, and rooms with mechanical ventilation that needs a make up air path. They are common in commercial fit outs and in blocks of flats where ventilation strategy and compartmentation meet, and they are the correct answer to a problem people otherwise solve by leaving the door open.
Smoke is the limit
A standard intumescent grille closes on heat, not on smoke. Where the door is required to restrict the passage of cold smoke as well as fire, an ordinary transfer grille undermines that, and a smoke rated product or a different ventilation solution is required. This distinction is frequently missed and it matters most on exactly the doors protecting escape routes, where smoke is the hazard.
Fitting intumescent vents for fire doors
Cutting an aperture in a certified fire door is an alteration to a tested assembly, so it must be within the door manufacturer's permitted alterations: position, maximum size and distance from edges are all specified. A grille cut into a door outside those limits invalidates the certification even though the grille itself is certified, and the whole door then has to be justified afresh or replaced.
Questions people ask about intumescent air transfer grilles
Can I cut a vent into a fire door?
Only within the door manufacturer's permitted alterations, which specify position, maximum aperture size and distance from the edges. Outside those limits the door's certification is invalidated.
Do intumescent grilles stop smoke?
A standard grille closes on heat, not on cold smoke. Where the door must restrict smoke as well, a smoke rated product or a different ventilation approach is needed.
Where are air transfer grilles normally used?
In doors to plant rooms, equipment cupboards, risers and rooms needing make up air, where ventilation is required through a wall or door that also has to resist fire.
Are intumescent door vents the same as an air transfer grille?
The same product under two names: a grille in a fire door or a wall that lets air through in normal use and closes under heat, so ventilation and compartmentation both survive. Two things matter in practice. It has to be certificated with the door or wall type it is fitted in, because it was tested as part of an assembly. And the aperture has to be cut to the manufacturer's detail, which is the part a site gets wrong: an oversized cut out or a grille fitted on one face only leaves a fire door with a hole in it.