Brochure
Engineering Data
76 Recommended Noise Criteria and Face Velocity Ranges are on page 75
Engineering Data
Keeping face velocity low
• Returns should be at 400-600 fpm maximum
• Filter Returns should be at 450 fpm maximum
*ACCA recommends 300 max for filter grilles and 500 max for non-
filter grilles.
• The rule of thumb is 2 cfm per square inch of filter size. See table
below.
• Low velocity reduces noise, especially on stamped face grilles
(672/673); fixed-bar grilles can handle more velocity without noise
(94A/96AFB/RH45/RHF45/RCB).
• A single point return cannot be oversized like a supply. The system
will not be affected adversely, only improved. *This does not apply
to multiple return locations where balancing is more critical to pull in
relevant amounts from each room.
• Static pressure is also reduced. Pressure works against & reduces
blower delivery volume (cfm)
• Noise is not expected from a return.
Location
• Returns should be put in stagnant air locations that need to be
reconditioned.
- High for cooling mode (hot air rises)
- Low for heating mode (cold air falls)
- Both modes, choose a primary season
• Returns should not be near a supply register’s throw range. If at all
possible place the return at an opposite corner of the room.
Room Air Movement
• Returns do NOT have much effect on a room’s air movement,
regardless of face velocity. They only grab air about a duct diameter
away from the face. Most of the room air movement is done by the
supplies.
Unlisted Sizes–Engineering Data
When a size is not listed there are a couple ways to do an engineered
estimate. Airflow principles permit you to utilize existing sizes to
determine sizes not shown.
Method 1: Use nearest nominal size table entry. If a 14x14 is not
given, but a 20x10 is, since the these two sizes have an approximate
equal core area (196 and 200) the table entry for a 20x10 can be used
to approximate what the 14x14 grille would perform to.
Method 2: A more exact method would be to do interpolation pro-
cess between two listed sizes. If 14x14 is not given, but 18x10 and
20x10 are, then this equation will get more exact 14x14 data. Y = Y1
+ [{(X - X1) * (Y2-Y1)} / (X2 - X1)] where:
Y = unknown CFM or throw that is being computed for 14x14
Y1 = CFM or throw of listed 18x10 (for ex 600 cfm)
Y2 = CFM or throw of listed 20x10 (for ex 640 cfm)
X = 196 in² (nominal area of 14x14)
X1 = 180 in² (nominal area of 18x10)
X2 = 200 in² (nominal area of 20x10)
Using equation above computes Y = 600 + [{(196 – 180) * (640 –
600)} / (200 – 180)] =
600 + [{16 * 40} / 20] = 600 + 32 = 632 cfm for Y
Method 3: Sizes beyond the table (smaller or larger) can have their
CFM or Throw determined by using listed sizes by the following:
CFM for larger sizes:
If looking for 24x6 or 24x12 cfm that is
not listed, using the listed 12x6 cfm and
doubling it or quadrupling it will give the
answer for the 24x6 and 24x12, respec-
tively.
CFM for smaller sizes:
If looking for a 6x6 cfm that is not listed, using the listed 12x6 cfm
and halving it will give the answer for a 6x6.
Throw:
Double the size and CFM, multiply the throw by 1.5
Quadruple the size and CFM, multiply the throw by 2
Half the size and CFM, multiply the throw by .67
One quarter the size and CFM, multiply the throw by .5
*Pressure loss, face velocity and noise criteria will all remain the
same relative to the listed size used to determine the larger or smaller
sizes not shown.
Sizing a Return
Air volume going back to the air handler (fan) must equal what is supplied from the air
handler. Therefore the total AFM capacity of the return grilles must equal or exceed the
total CFM capacity of all the supply diffusers.
Keeping face velocity low
• Returns should be at 400-600 fpm maximum
• Filter Returns should be at 450 fpm maximum
*ACCA recommends 300 max for filter grilles and 500 max for non-filter grilles.
• The rule of thumb is 2 cfm per square inch of filter size. See table.
• Low velocity reduces noise, especially on stamped face grilles (672/673); fixed-bar
grilles can handle more velocity without noise (94A/96AFB/RH45/RHF45/RCB).
• You really can’t oversize a single point return like you can with a supply. The system
will not be affected adversely, only improved. *This does not apply to multiple return
locations where balancing is more critical to pull in relevant amounts from each room.
• Static pressure is also reduced. Pressure works against & reduces blower delivery
volume (cfm)
• No one expects noise from a return.
Location
• Returns should be put in stagnant air locations that need to be reconditioned.
- High for cooling mode (hot air rises)
- Low for heating mode (cold air falls)
- Both modes, choose a primary season
• Returns should not be near a supply register’s throw range. If at all possible place the
return at an opposite corner of the room.
Room Air Movement
• Returns do NOT have much effect on a room’s air movement, regardless of face
velocity. They only grab air about a duct diameter away from the face.
Filter Size
Area
(in²)
Ton (cfm) Filter Size
Area
(in²)
Ton (cfm)
12 12 144 n/a 20 20 400 2 (800)
12 20 240 1 (400) 20 25 500 2.5 (1000)
12 24 288 1.5 (600) 20 30 600 3 (1200)
12 30 360 1.5 (600) 20 36 720 3 (1200)
14 14 196 1 (400) 24 24 576 3 (1200)
14 20 280 1.5 (600) 24 30 720 3 (1200)
14 24 336 1.5 (600) 24 36 864 4 (1600)
14 30 420 2 (800) 25 25 625 3 (1200)
16 20 320 1.5 (600) 30 30 900 4 (1600)
16 24 384 2 (800) 30 36 1080 5 (2000)
Unlisted Sizes_Engineering Data
When a size is not listed there are a couple ways to do an engineered estimate. Airflow
principles permit you to utilize existing sizes to determine sizes not shown.
Method 1: Use nearest nominal size table entry. If a 14x14 is not given, but a 20x10 is,
since the these two sizes have an approximate equal core area (196 and 200) the table
entry for a 20x10 can be used to approximate what the 14x14 grille would perform to.
Method 2: A more exact method would be to do interpolation process between two
listed sizes. If 14x14 is not given, but 18x10 and 20x10 are, then this equation will get
more exact 14x14 data.
Y = Y1 + [{(X - X1) * (Y2-Y1)} / (X2 - X1)] where:
Y = unknown CFM or throw that is being computed for 14x14
Y1 = CFM or throw of listed 18x10 (for ex 600 cfm)
Y2 = CFM or throw of listed 20x10 (for ex 640 cfm)
X = 196 in² (nominal area of 14x14)
X1 = 180 in² (nominal area of 18x10)
X2 = 200 in² (nominal area of 20x10)
Using equation above computes Y = 600 + [{(196 – 180) * (640 – 600)} / (200 – 180)] =
600 + [{16 * 40} / 20] = 600 + 32 = 632 cfm for Y
Method 3: Sizes beyond the table (smaller or larger) can have their CFM or Throw
determined by using listed sizes by the following:
CFM for larger sizes:
If looking for 24x6 or 24x12 cfm that is not listed, using the listed 12x6 cfm and doubling
it or quadrupling it will give the answer for the 24x6 and 24x12, respectively.
CFM for smaller sizes:
If looking for a 6x6 cfm that is not listed, using the listed 12x6 cfm and halving
it will give
the answer for a 6x6.
Throw:
Double the size and CFM, multiply the throw by 1.5
Quadruple the size and CFM, multiply the throw by 2
Half the size and CFM, multiply the throw by .67
One quarter the size and CFM, multiply the throw by .5
*Pressure loss, face velocity and noise criteria will all remain the same relative to the
listed size used to determine the larger or smaller sizes not shown.