If a forced-air HVAC system is pushing more air into a space or room than return pathways can match, some spaces in the home become pressurized and others depressurized. This imbalance can cause problems: thermal comfort, moisture, and even combustion safety.
Types of HVAC returns
In an ideal world, every supply would have a companion return duct. This approach is called a dedicated return for each supply. But most older and even many new homes have one or maybe two central returns located in common spaces because it is less expensive and difficult to locate and run just a supply to each space. With central returns, here are the ways to provide return pathways: door undercuts, jump ducts, transfer grilles, and the deKeiffer bypass.
This is certainly the easiest and cheapest way to attempt a return pathway but it is rarely sufficient. For most situations, door undercuts need to be greater than 1.5 inches to be sufficient and that amount of undercut is very often not acceptable for aesthetic or privacy reasons.
A jump duct is a large insulated flex duct that connects two ceiling grilles, one just inside the door to the supplied room and the other just on the other side of the door in the hallway ceiling. The sharp bend and variegated surface of the interior lining of the jump duct reduces sound transfer without compromising air movement. Since this approach penetrates the ceiling plane, the jump duct must be carefully air sealed if it breaks into an unconditioned space like the attic.
There are two types of transfer grilles, above-door and high-low offset. The above-door transfer grille simply has two grilles located above the door between the room and common space, one on each wall, with offset baffles to limit sound transmission.
The high-low offset is located in an interior wall framing cavity adjacent to the door between the room and common space. One grille is located high on the wall on one side (usually the common space), the other low on the wall (in the room). The offset reduces sound transmission. Note that this interior framing cavity must be clean and without wiring penetrations (or seal these penetrations as you would if it were an exterior wall framing cavity).
The de Keiffer bypass
An HVAC engineer from Boulder, CO, Rob deKeiffer of the Boulder Design Alliance, came up with an elegant return pathway: the deKeiffer Bypass. The door trim is spaced off of the wall on both sides enough to create around 23 square inches of free air space connecting the room and the common space. The framing cavity space above the door is sleeved with sheet metal to create a dedicated pathway connecting the two sides of the spaced-out door trim.
Armand Magnelli’s (Livable Housing, Inc.) perspective
Another approach is to install a product that combines a metal sleeve with an interior baffle and grilles on both sides of the wall, such as the Tamarak Return Air Pathway (RAP). Or one of Halton’s transfer grilles (these more commercial in nature). These products purportedly provide better noise reduction with good air flow and are relatively easy to install. The fit between standard stud spacing in interior partition walls. For an independent analysis of several approaches see the following review by the Building America Program.
An advantage of using the products from Tamarak is that they are sized for CFM capacity, an important variable in balancing the airflow between rooms. The issue with the products from Tamarak and Halton is that while they provide an attractive and effective packaged solution it may be difficult to convince local HVAC distributors to stock their lines of products forcing contractors to order them online. Some of the other solutions are fabricated from stock components available at most HVAC distributors.
The use of any of these solutions may also improve performance of whole house ventilation systems by enabling better circulation of fresh air from bedrooms at night, a crucial time to deliver fresh air to occupants.
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