Hole size to cut for my Grille, Diffuser or Louvre?
The same as the nominal.
Unless stated otherwise and in the vast majority of cases, the size you give us will be taken to be the nominal, i.e. the size of the aperture into which you will be fitting the grille, diffuser or louvre.
From this size HVC shall deduct a tolerance, permitting the neck of the unit to fit into the hole comfortably. This tolerance is normally between 9 – 15mm, however, does vary by product type, therefore please download the specific product PDF for further details.
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There are notable exceptions to the above:
Series CD ceiling diffusers are normally specified by their neck size, not their nominal. However, you should still cut any holes into which CDs are to be fitted to the nominal. This also applies to Series SFCD and LFCD circular diffusers.
The nominal size of dampers will normally refer to the ductwork size they are to suit, not to any aperture size.
