Here a Home Warranty company was replacing a water heater. At the home owner's request and extra money, they upgraded from a 40 Gallon to a 75 gallon, 75,000 btu water heater. But then what to do with the different flue sizes. Reduce to fit! Notice the 4"x3" reducer. You could feel the spillage as soon as you walk into the room. Look how big that draft diverter is.