Chris Bacon wrote:
Rick wrote:
What I would expect to find is 6 vents up one chinmey brest. each vent
should be sealed from each of the others, else you'd get smoke come
out upstaris when you light a fire downstairs. I would also expect to
find that the boiler has a metal flue liner.

I suspect that this is not what you will find in this case.

I would be very entertained to find out what the OP has
got here!

OK. Let me explain a little more.....

The house was once a large Victorian manor house that has been
converted into 4 houses. The chimney breast in question is the one that
used to service the old kitchen, and it's shared between us & next
door. It has 6 vents, all sealed from each other. Only 4 of these are
in use (one for our boiler, one for next door's, one for our woodburner
and one for next door's). The boilers are both in the same 'boiler
room' which is adjacent to our bedroom.

At one time the 2 boiler flues used to go into the same chimney vent
but a few years ago, before we moved in, someone spotted this and
decided it was a bad idea so they moved next door's into a vacant vent.

I'm pretty sure that a chimney liner was installed at this time - you
can see it through the airbrick.

I assume that the airbrick is still necessary though to avoid the build
up of moisture.