When I was looking at a stove to buy a pellet stove (and where to locate it in my house), I knew the EVL would be close to 15, so I asked the store owner if I should go with 4" pipe. He said, "you don't want to move to 4" unless you have to because of a loss of efficiency." I noticed that he only sold 3" pipe on the shelves of his store, so I have been wondering if what he said was true or convenient for his business.