High efficiency condensing gas furnaces differ from the standard efficiency models in several ways. The primary difference is the manner in which they are vented. Since the heat lost through the flue of a high efficiency condensing furnace is a lot less than the heat lost through the flue on a standard efficiency furnace, the steam is much cooler on the high efficiency condensing furnaces. So much cooler that the steam condenses into liquid water. This is a big problem for venting into masonry or b-vent flues. The solution is to vent the flue gases through water-tight, sealed pvc piping. This requires a dedicated pvc flue and does not allow for a “common vent” option. The pvc exhaust and intake (when necessary) are usually sidewall vented. The high efficiency gas furnaces also require a condensate drain to remove the water produced by the furnace. A qualified person should repair per standard building practices in this John’s Creek home.