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A "return wall" is a term used in the construction industry and refers to a wall through which fresh air or room air is drawn into a forced air circulation system. It provides oxygen to ensure complete combustion of the fuel as well as circulates air throughout the building. This allows the air to warm or cool with each pass.
Return walls play a passive but critical role in the design of a building. In a building’s forced air heating system that burns oil, natural gas, coal or other carbon fuels, oxygen is required for combustion. With modern construction technologies, buildings are often sealed from outside fresh air. If the fuel is completely burned, as will occur when there is an excess of oxygen, the carbon byproduct is carbon dioxide. In incomplete combustion, carbon monoxide is produced, and it is deadly to humans and pets, so it is important that fresh air be supplied to the furnace whenever it is in use.
In residences, a passive system is employed to supply fresh air to the furnace. There are no fans that push fresh air into the system. Instead, fresh air is pulled into the furnace through the interior spaces between the studs of short lengths of particular walls. These are the return walls.
The return wall differs from the neighboring wall section in that it is open at either the top or the bottom, where the framing is, not on the outside portion, where it seen in the room. There it connects with a distribution system, such as ductwork from the floors above it or below it, or both. Some ductwork will connect with vents in the attic or basement to bring in outside air and oxygen. Other ductwork will connect with grates in the return walls to circulate room air. The circulation is necessary to avoid a sense of “staleness” and to allow the air to be exposed to the heating or cooling elements in the system and thus raise or lower the room temperature.
From a homeowner’s point of view, it is important to know where the return walls are. They can be found if one looks for the grates at either the top or bottom of room walls. A return wall should be left intact. Small holes such as those caused by hanging pictures on the wall exterior surface are not harmful. The return wall, however, is not the place for one to build a nook or hang a between-stud structure such as a bathroom cabinet.
It is important for the return wall channels to be maintained so that the system has the mechanical capacity to suck in the air. One can imagine the sucking capacity needed to draw fluid through a straw with a small diameter compared to a straw with a large diameter. If the straw is split down the side, it can’t be used at all. If a return wall is removed, an alternate path for the circulating air must be found.