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Chimney: Is B-vent on top of masonry chimney possible?
Is it possible to just add a metal "B-vent" chimney directly on top of an existing masonry chimney -- without the masonry chimney below being changed or any metal liner being placed in the masonry chimney?
The reason that I am asking is that one of the many chimney companies and masons that I had look at my chimney submitted an estimate that included tearing the existing chimney down to the roof line, then just adding a triple wall stainless steel B-vent on top of the existing chimney from the roof line up. According to them, the B-vent would go directly on top of the existing 8"x8" clay flue with an adapter, and no new liner would be placed in the existing chimney. This, of course, would be assuming that the existing chimney and clay flue from the roof line down is all in good condition.
Since they gave their estimate, I have been doing tons of research and I cannot find anything -- no codes, no descriptions, etc. -- that would address this particular situation.
Yes, it would make sense to at least add a liner to the existing chimney if possible, but an issue has come up regarding that. The issue is that going into the chimney is an existing 7-inch metal flue from the gas-fired steam radiator heating unit (187,500 btu) and a 3-inch metal flue from the gas-fired hot water heater (36.000 btu). Apparently, since a 7-inch metal flue is already going into the 8"x8" clay chimney flue, the minimum new metal chimney liner size would have to be at least 7 inches. But, a 7-inch liner will not fit into an 8"x8" clay flue. So, a new metal liner or B-vent cannot be placed inside the existing masonry chimney without removing the existing 8"x8" clay flue all the way down.
So, again, my question is, "Is it possible to just add a metal "B-vent"
chimney directly on top of an existing masonry chimney -- without the
masonry chimney below being changed or any metal liner being placed in the
P.S. This is a side-by-side twin home, so this is actually two chimneys instead
of one -- one for each house. The height of the chimney is about 10 feet up
from the roof line.
Here are two photos: