Depending on the location of the supply register, an air distribution system could be described as a perimeter system, a ceiling supply system, or an inside wall supply system. Some general comments about the strengths and weaknesses of these systems are made below.

Perimeter Systems:
Perimeter systems blanket portions of the exterior walls with supply air. This is accomplished by using floor, baseboard, or low sidewall outlets that are designed to discharge the supply straight up the wall. If the outlets are sized correctly, the discharge pattern will extend up to the ceiling. (Never use outlets that blow air into the interior of the room.) It is also possible to use ceiling outlets that discharge air straight down the wall, but this arrangement is more suited for heating rooms that cannot be served by a below-the-floor duct system. (Discharging cold air straight down a wall will cause the air to stratify along the floor. A horizontal, parallel to the ceiling, discharge is preferred for cooling.)

Traditionally, perimeter systems have been recommended for buildings that are located in cold climates because they provide more comfort at the floor level than the two other types of systems. However, ceiling or inside wall systems can be used in a cold climate if the building has a thermally efficient envelope and a heated room (like the first floor of a 2 story or basement) below the room the perimeter system will be installed.

Ceiling Supply Systems:
Ceiling supply outlets should discharge air parallel to the ceiling. If ceiling outlets are sized correctly the discharge pattern will extend to the walls. (Never use outlets that blow the air down into the interior of the room.) Ceiling systems provide optimum performance during the cooling mode, so they are commonly used in buildings that are located in warm climates. (Cold floor problems could be experienced during the heating season when ceiling outlets are installed in a building that has an exposed floor.)

High Inside Wall Supply Systems:
High sidewall supply outlets should discharge air parallel to the ceiling toward the outside wall. If the outlets are sized correctly, the discharge pattern will extend to the opposite wall and high velocity air will not drop into the occupied zone. (An excessive drop during the cooling season is a common problem that is associated with sidewall outlets.) Sidewall outlets perform best during the cooling mode, so they are more suitable for buildings that are located in warm climates. (Cold floor problems could be experienced during the heating season when high sidewall outlets are installed in a building that has an exposed floor.)

Now for the return air grille locations. Return duct systems are commonly characterized by the number of the return openings. Return inlet locations, duct run geometry, and duct material are secondary features that can be used to describe a return air system.

Number of Return Inlets:
Return duct systems can be classified as a single central return system, a multiple return system , or as a system that has a return in every room. Regardless of which type of return duct system is used, there must be a low-resistance return air path between every room and the building HVAC unit. A system that features a properly sized return in very room automatically satisfies that requirement. If a single return system, or a multiple return system is used, there must be a low resistance path between every isolated room and the closest return air opening. This can be established by using jump ducts, wall transfer grilles, or door grilles. (Observe caution as to not increase noise, or air velocity issues.)

Return Inlet Location:
Return air duct systems can be characterized by the location of the return air openings. If all of the return openings are installed in the ceiling or located high on the walls, the system is called a "high return system". If located in the floor or low on the sidewall then it is referred to as a "low sidewall system". Since the return air location (high or low) has negligible effect on the air motion within the room, the return openings should be placed at positions that are compatible with the HVAC unit and duct runs. Return openings do not need to be located on the opposite side of the room from the supply, and the return does not "draw" the air across the room. The return opening is simply a path for the air from the room to return to the HVAC unit.

The air motion within the occupied zone depends on the performance of the supply register. If the register is sized correctly, the jet of conditioned air will join with a large amount of room air as it develops into a secondary air pattern. The supply air mixing with the air in the room causes a secondary air pattern that is 10 to 20 times greater than the register supply air cfm. Even more mixing will take place as the secondary air exchanges its momentum with the room air. Selecting the correct supply air register is important to insure the mixing action takes place outside the occupied zone, which means it must occur near the walls or ceiling. Ultimately, all of the air in the occupied zone will be induced into motion and there will be no drafts or stratified air in the occupied zone. Stratification does not cause discomfort if it occurs outside of the occupied zone, such as near the ceiling.

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