Poor Duct Layout
Poor duct layout can cause pressure differences within a home. Many homes have a supply duct for each room, but only a single return duct located in a central portion of the house.
When interior doors are closed it may be difficult for the air in these rooms to circulate back to the return duct. The pressure in the closed-off rooms increases and the pressure in rooms open to the return decreases. This uneven pressure can increase air leakage into one area of the house while increasing leakage out of another. Installing return ducts in rooms frequently isolated from the central return duct alleviates this problem.
Transfer ducts, which allow air to circulate from one room to another, are another option. At a minimum, all interior doors should be undercut at least one inch to allow for air to flow to the return duct. Be certain that carpeting does not block the undercut area.