We recently purchased a home built in 1940. The existing wood fireplace is in the center of the home and chimney goes through the center of the house and terminates out the roof at its peak. We are looking to install a Quadra-Fire gas insert since the chimney has no existing liner and we have have dust allergies from wood fireplaces.

The fireplace calls for (2) two 3" flex liners (one for intake air; the other exhaust/vent gas) to then terminate at the chimney cap. Because of the existing exposed chimney's very poor condition above the roof line, I will be knocking it down below the roof decking when I re-roof in a few weeks. So I am look at a slightly unique approach to terminating the intake and exhaust pipes through the roof.

What I would like to do is:
-from the fireplace through the chimney use (2) two 3" diam. aluminum flex chimney liners
-where the chimney is torn down to, I will install a custom fabricated sheet metal cap/plate (I have sheet metal brake) and seal to the brick to prevent air leakage in or out of the masonry chimney cavity.
-at that metal cap/plate, I will adapt from the 3" flex liners (intake and exhaust) to individual b-vent and go with b-vent through the roof decking, shingles, etc.

Almost all direct vent gas insert manufacturers only show transitioning from the 3" flex liners to a co-axial to co-linear connector. I assume to reduce the number of roof penetrations and since most installations utilize an existing chimney above the roof line. As you can see, I would like to keep both 3" pipes separate and terminate two 3" b-vents above the roof. The vent/exhaust would be a foot higher than the intake to prevent it from bringing vent gas in for combustion.

What are your thoughts?