A cold-air return is a vent that sucks cold air into a furnace so that it can be heated and brought back to a room through a furnace register. These vents are seen in many types of furnace systems, although there are alternatives, such as drawing air from the outside or from the area around the furnace. Depending on the design, each room in a house may have a return, or they may be strategically located at various points around the house. A heating and cooling specialist can make recommendations about the number of returns and their optimal placement.
In addition to being used in heating systems, cold-air returns can also be used in cooling systems. In both cases, the benefit is that air drawn from a room is naturally closer to the desired temperature, which means that less energy is required to heat or cool it to the necessary level. Furnaces can also combine a cold-air and main return, a vent on the furnace that brings in air from the area immediately surrounding the furnace.
Typically, this vent is located near the floor; because heat rises, the coldest air will be close to the floor. Locating a vent up high would leave pockets of cool air near the floor, which would be undesirable when the goal is to heat a room. Registers that emit hot air can be located at various heights, depending on the design of the system.
Often, the interface of a register and that of a cold-air return look identical. People can tell which is which by waiting for the furnace to turn on, and holding a piece of tissue in front of the vents. If the tissue is pushed out, the vent is a register, and if it is pulled in, it is a return. Both returns and registers typically have filters that prevent the passage of dust and dirt, and these filters need to the periodically changed to avoid clogging the vent.
People can control the amount of air moving through the furnace by opening or closing the vents. With the installation of a new furnace system that takes advantage of existing ductwork, people may be told that they need additional or fewer returns, depending on how the system works. In the case of unnecessary returns, they can be capped or simply closed, depending on personal taste.