Duct Leakage Science

 

No Leaks If the duct system was perfectly designed and sealed, the same amount of air would enter the return grille and leave the supply registers. This is the ideal situation, and so it rarely happens.
Supply Leaks When supply ducts leak, air escapes before it gets into the building. However, the blower still tries to pull the full amount of air out of the building. This creates negative pressure that pulls outside air into the building.
Return Leaks The return side of the system is under suction, so leaks here pull air into the ducts. The blower then forces more air into the building than it’s taking out (supply flow exceeds return flow), creating a positive pressure. Until recently, duct installers routinely skipped the taping of return ducts and plenums. It was considered a waste of time, since it was the supply ducts that “delivered the goods.” Perhaps this is one reason researchers find the most significant leaks on the return side.
Closed Doors Some experts claim closed doors can have a greater impact on pressure balance than duct leaks. Most houses have one or two centrally located returns with supply registers dispersed throughout the house. Closing a door blocks the air flow from the supply register to the return. This increases the air pressure in the room with the supply register. The room with the return has a negative pressure.