Btuh = 1.08cfm x dt is the formula but it's not much good w/o accurate measurements and knowing the net btus of the furnace. I just took apart every inch of my heating system and taped, stapled, screwed, caulked and reclamped all of it and what an improvement. I can feel warm air several feet away now. My heated basement is actually a return air plenum - the return air passing thru the basement to get to the furnace. My typical access doors had shrunk up gaskets so cold outside air was being sucked in. An rv furnace isn't nearly big enough to heat all that! I also sealed all the plumbing/electric penetrations between the basement and under belly. Sucking OSA into the under belly is also counter productive to keeping the tanks warm. Seal everything! Craig
