Sometimes the solution to a poor design is simple. Tear it down. Our client's roof was leaking and the reverse board and batten wood siding was rotting. We replaced it with Hardi-Plank and left the vented fireplace with a short chimney. The owner installed a non-vented gas log set years ago and didn't want to lose the function of the "fireplace". Although shortening the flue will prevent them from building a "real fire" in the fireplace - they love this option and hope to spend this winter enjoying a leak free holiday season.