I know the flow rate of gas (products of combustion + air) entering a natural draught (no fans) flue. If the flue is removing all the gas: velocity of flue gas = flow rate (corrected for thermal expansion) / flue csa As the flue gas moves up the flue what happens to its velocity? In particular, I need to know the exit velocity at the flue terminal. Obviously, this will depend upon the flue height, but what is the relationship between height and velocity? I know the gas will lose heat as it moves up in contact with the flue wall, slowing it down. Are there any other factors involved? Does the constant upthrust from the pressure difference between the top and bottom of the flue mean the gas continues to accelerate throughout its ascent?