My house was built circa 1901 and has two chimney breasts. Presumably,
each would have originally had an open fire on the ground level and a
further one on the first floor (one of which on the ground floor is
still in operation). In the attic, these two breasts meet from either
side of the house before exiting through the roof.


The fireplace which is still in use exits through one of the four
chimney pots.


Having read lots of suggestions, I had decided to place one air
brick (well, technically not an airbrick but a bunch of holes drilled
through the brick with a vent in front) in the bricked up chimney
breast on the ground floor. A second air brick would be placed directly

above it in the attic.


....and now my problem:


....how can I tell whether there is simply one cavity in the chimney
breast
between the ground floor, first floor and attic meaning placement of
the vents is not critical; or two separate cavities meaning that the
attic air brick may simply ventilate the first floor flue but not the
ground floor if not placed correctly?


I have trawled the Internet for this information but have not been able

to answer my own question...are all chimneys representative of the
Santa type - whereby a fat drunk in a red suit can simply drop down a
single cavity to deliver presents!