In an old house with an unlined 7.5 x11.5 chimney, I need to know what is the proper metal flue liner size to use. There is an old gas furnace with a 4 " vent and old water heater with a 3 " vent. They both use the flue. Some contractors are saying that there must be two liners, one for each device. Another, says that one larger flue is enough. Does any one know what the normal code is?