5" flue, double wall from basement through the middle of the house all the way to the roof (2 story).
Furnace has a 4" going into the 5" along with a 3" from the gas water heater.
I have NOT measured the flue temp.
Those who dance, appear insane to those who do not hear the music.
Those who believe, appear ignorant to those who do not know God.
I would measure the flue temp, the furnace is induced draft correct, since the main flue is 5" I am not sure if the standing pilot in the water heater will produce enought heat to keep it from condensating. In a cold climate area I had to insulate the doulble wall flue to stop this problem . Mainly in the basement.