I'm a certified gas tech and the thought gives me chills! Sounds more like the makings of a Ripleys Believe It Or Not story to me. Besides all the obvious reasons, the water vapor in flue products are corrosive (related to sulfuric acid) when they condense and there's a LOT of it. I'm better on NG than LP but with NG there's 9 gals. of water, as vapor, goes up the stack per every 100 c.f. of gas burnt. You'd have acid rain everywhere that's below the dew point. You also don't need it if you're talking a little below 15F. Our last 5er that had no underbelly or tank insulation, at all, was good for a 15F night. At 10F we had a little water line freeze but was thawed by 10AM. Use your furnace and let the underbelly covering do it's thing. Sorry for the rant but I have a good friend who's badly scared and lucky to be here from misusing a homemade LP heater. Craig
