The previous owners of my little home installed a (then) high efficiency gas furnace, replacing an older one. The older one was installed when the house was built in 1973, and the vent pipe went up through the ceiling into the attic, elbowed west into the double chimney (one flu for the wood fireplace, a separate one for the gas furnace) into its dedicated flu. Instead of using this same flu, the new vent pipe, pvc and smaller, goes through a new hole in the ceiling, elbows south and out the south wall of the attic through a hole cut too large for the pipe. I've shot the open space full of expanding foam for a quick fix this winter (after removing a mud dauber nest) and will attend to the outside this summer -- but has anyone got any good ideas why on earth this reroute would be preferrable to running the furnace exhaust out the chimney as before? Thanks! best, M.