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 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
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
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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