The design and installation of the heating system has a major impact on the success of any tightening approach. Unless the heating system is designed and installed properly, a home with an otherwise tight envelope may still have unacceptably high air leakage.

The following guidelines will reduce the negative impact of the HVAC system on the air leakage.

• Try to get the heating plant and the ductwork inside the heated envelope. That way, any air that leaks will still provide heat to the home, and outside air cannot be pulled into the return side of the system. In homes with heated basements this is fairly easy. In homes built on crawlspaces or slabs, try locating the mechanical room centrally within the living space. Ducts can run in a dropped soffit in a hallway.
• If ductwork must be run outside the insulated shell, make sure it is well sealed. In addition to sealing the joints between lengths of duct, you also should seal the long seams in metal duct and any crimped edges on plenum boxes or register boots. Use a high-quality “mastic” specifically designed for ductwork. Metal duct often has a thin coating of oil remaining from the fabrication process. Wipe the oil off before applying the mastic. A rag with mineral spirits usually will do the trick. Foil tape can be used on access panels that need to be removed for maintenance.
• Design the system in such a way that the supply and return portions are in balance. This may mean using more returns than you normally do. If a room doesn't have a return, the door to that room should be undercut so that there is at least 3/4 in. between the door and the finish floor. Other options are a louvered door an open transom to the hallway or a short "bypass" duct that allows air to flow around the closed door.