Combustion Air ~ it is a Building Regulation requirement that in
the case of open fireplaces provision must be made for the
introduction of combustion air in sufficient quantity to ensure the
efficient operation of the open fire. Traditionally such air is taken
from the volume of the room in which the open fire is situated, this
can create air movements resulting in draughts. An alternative
method is to construct an ash pit below the hearth level fret and
introduce the air necessary for combustion via the ash by means of
a duct.Ref. BS 1251: Specification for open-fireplace components.Open Fireplaces and Flues