I have a question about how I should write this up. We don’t find a lot of gas furnaces and water heaters in the south. I ran across a gas furnace in a crawl space today. The vent pipe fed into a masonry chimney. In the garage a gas water vent also vented into the side of the chimney above the furnace vent. I believe the two are tied together but can’t confirm because it is behind the masonry. Out of the top of the chimney is a single metal flue.