I need to make provision for a supply of air for an open hearth. This
should be ducted through a concrete floor which will have underfloor
heating installed after the air duct is in place. The idea is to duct the
air to directly under the hearth where the fireplace design allows the air
to flow around the ashpan and enter the hearth via grating. I need to
estimate the cross section of the duct and also to source appropriate
ducting and an external vent which I would prefer to have a fine mesh
screen to prevent scorpions entering via the duct.

Has anyone sourced this sort of thing in the past and can they provide a
pointer to suitable standards for construction? The architect is being
totally bloody useless at present and the only sensible advice I can get
relates to construction of the chimney (25cm diameter and must exceed the
height of the roof ridge by 0.5 metres minimum, cannot be located within 8
metres of trees or other buildings.)