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!