The connection between the existing ductwork and new fittings / ducting should be air sealed using aluminum tape or paint-on mastic. Any air leaks in the existing ductwork should also be sealed where possible. The primary reason for air sealing the ductwork is to reduce the possibility of generating negative air pressure in the vicinity of the furnace/ hot water tank, which can cause down drafting of hot water tank B-vents. Well sealed ductwork also ensures that the designed air volumes are delivered to different locations in the home thereby improving thermal comfort.