I have two gas furnaces side by side.- for main floor and basement. The flue pipes for each are 4inches each going into 6 inch double wall flue pipes that goes to a power vent and then to side wall. The power vent pie going to out side is 4 inch. This installation has been working without problem for over 5 years.
My question is - is it required to change 4 inch power vent to 6 inch and will that meet code ? Or is it required to have separate 4 inch power vent for each furnace ?