I got a house with a 7in vent running 21ft up alongside. Inside the one room basement is a Carrier 75k and a small water heater vented with a 4in sloping alittle connecting out to the 7in. I bought 25ft of 5in B vent at the store. The inspector called and said to go back to the store and get the sizing chart. He thinks a 3in outlet and a 4in outlet are OK with the 5in but isn't sure. I'm going to run straight up through the house with the firestops and the 1in clearance. Is 5in ok?