"Supply Registers and Return Grilles"
The Comfort Zone by Maury Tiernan, Geary Pacific Corp.
Let's talk about the supply outlet registers first. 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
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 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.
Well, that was probably more than you ever wanted to know about the function of
the supply register and return grille. But following a few basics will insure
your customer feels like they are in ... The Comfort Zone.