MVHR and fire rated doors

[Douglas]

Hi,

We are renovating our current two storey property and taking the opportunity of building dormers in the loft and introducing a MVHR system. As all floors will  be accessed from a single stairwell, all of the rooms leading on to it need to have FD30S rated doors. Consequently I cannot create sufficient gaps under the doors to allow the flow of air for the MVHR system ( I understand the maximum for smoke rated door is 3mm whereas I need 8 -10mm).

I do not wish to use  smoke thresholds as the intention is to have 'unbroken'  flooring throughout the area and  wish to avoid cutting holes in the oak doors. Can anyone assist in suggesting either a suitable fire and smoke rated  vent product that can be inserted in the walls for each of the rooms , or  is there  an option to use another fire protection solution for the stairwell which would obviate the need for FD30S doors?     

Many thanks for your assistance