In a central return duct system, or whole-house return, return grilles are installed in central locations on each floor, usually close to the air handler. In order to keep supply air from pressurizing closed rooms, transfer grilles or jump ducts are installed to allow supply air to flow back to the central return grille.
A transfer grille is a grille that allows air to move from one space to another to alleviate pressure differences. For example, a pressure grille installed above a bedroom door enables air to move between the bedroom and the hallway, regardless of whether the door is open or closed. The standard practice for homes with central returns is to undercut doors to allow for air flow, but undercutting alone doesn't provide enough room for air to move; the Best Practice is to use transfer grilles or jump ducts, so air can easily reach the central return grille.
Although they're often located above interior door frames, transfer grilles can also be installed in a full wall cavity in order to reduce noise transmission. When installed in a full wall cavity, the Best Practice is to install grilles using the high/low method, where one grille is low on one side of a wall, and the corresponding grille is high on the opposite side of the wall. The standard practice is to use the wall cavity as the duct between the two grilles, but this creates noise problems that can bother homeowners. Instead, the HVAC trade contractor should install ductwork connecting the high and low grilles, significantly reducing sound transfer.
Jump ducts are short ducts that run in the attic from the ceiling of a closed room to a ceiling in the main body of the home to provide an alternative pathway for air to flow when a door is closed. They're routed through the attic in order to minimize noise transmission in a home. Jump ducts are often less than 6 feet long and connect one room to another. Each end of a jump duct is then connected to a standard return air register vent.
Transfer grilles and jump ducts work to equalize the pressure in rooms and allow for conditioned air from the supply ducts to flow evenly throughout the home. Consider these points about central return duct systems:
Over-the-door transfer grilles are normally smaller vents installed over bedroom doors. Most floor plans will place bedrooms off of a main hallway; when the bedroom doors are shut, a grille allows air to flow into the hallway and back to the central return duct in the main living area.
Transfer grilles can be installed with baffles inside. These baffles work to minimize sound traveling from the hallway, allowing for a more soundproof room.
Jump ducts are a great way to reduce ductwork. They're often installed in the ceiling and provide pressure balance between bedrooms and main living areas.
Good duct design provides a home with proper air distribution, economical heating and cooling system operation, and economical duct installation. Central return duct systems are a good way to provide a comfortable indoor environment for homeowners.
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