Correct size flue for Furnace and water Heater?
I have a Gas furnace and a Gas water heater that each have a 4" flue pipe from them. These two are then connected into a single 8" pipe that goes up through my chimney. I was told that 8" is too large and it has probably been there for years and no one bothered chnaging it when they installed the new furnace.
Can anyone confirm what size flue I should use to cater for two 4" pipes?