3
A
SSUME
4-T
ON
EQUIPMENT
WITH
10"
ROUND
SMALLEST
ZONE
1600 CFM
(
EQUIPMENT
)
– 400 CFM
(
SMALLEST
ZONE
– N
ORMAL
CFM )
= 1200 CFM N
EEDED
SO,
FROM
THE
R
OUND
D
AMPER
MAX CFM,
A
14"
ROUND
DAMPER
SHOULD
BE
USED
FOR
BYASS.
EXAMPLE
2 -2.5
10"
3 -3.5
12"
4 -4.5
14"
5
14 -16"
UNIT SIZE
ROUND DAMPER
(TONS)
SIZE (INCHES)*
BYPASS DAMPER “RULE OF THUMB”
BELOW CHART ASSUMES 400 CFM PER TON *USE
NEXT SIZE UP ROUND BYPASS DAMPER FOR ANY
SYSTEM WITH A SINGLE ZONE LESS THAN 200 CFM.
A PROPERLY SIZED BYPASS DAMPER AUTOMATICALLY MAINTAINS CONSTANT PRESSURE INSIDE THE SUPPLY AIR
DUCT WHEN ZONES OPEN AND CLOSE. WHEN THE CORRECT SIZE BYPASS DAMPER IS INSTALLED AND ADJUSTED
ACCURATELY, IT WILL BE FULLY CLOSED WHEN ALL ZONES ARE CALLING (NO AIR BYPASSING) AND WILL OPEN
PROPORTIONATELY AS ZONE DAMPERS CLOSE.
RECTANGULAR CFM
*
SURFACE AREA IN SQ. FT. =
(“HEIGHT” X “WIDTH”) / 144
NORMAL CFM =
(SURFACE AREA IN SQ. FT.*) X 600 FPM
MAX CFM =
(SURFACE AREA IN SQ. FT.*) X 900 FPM
6”
100
200
7”
150
250
8”
200
300
9”
300
450
10”
400
600
12”
600
900
14”
900
1400
16”
1400
2000
DAMPER NORMAL MAX
SIZE
CFM
CFM
ROUND CFM CHART
BYPASS - PRESSURE RELIEF
BYPASS SIZING
SELECT A DAMPER SIZE WITH A MAX CFM GREATER THAN THE CFM-NEEDED TO BYPASS. USE THE FORMULA
BELOW TO CALCULATE CFM NEEDED TO BYPASS AND THE ROUND DAMPER CFM CHART TO CHECK MAX CFM.
TYPICALLY 400
CFM
PER TON
USE A BYPASS DAMPER THAT
HAS LARGER “MAX CFM”
800
2
1200
3
1600
4
2000
5
EQUIPMENT AIRFLOW
CAPACITY IN CFM (HIGH SPEED)
CFM OF
SMALLEST ZONE
CFM-NEEDED
TO BYPASS
USE “NORMAL CFM”
BYPASS
DUCT
SMARTZONE DUCT SYSTEM
A ZONING SYSTEM CAN TYPICALLY USE THE SAME DUCT SIZING AS A TRADITIONAL SINGLE-THERMOSTAT SYSTEM IF THE PRESSURE RELIEF
IS INSTALLED CORRECTLY AND THE SYSTEM IS 4 OR LESS ZONES. TO MINIMIZE BYPASS AIR FLOW, INCREASE THE DUCT CAPACITY BY ONE
SIZE FOR EACH ZONE LESS THAN 25% OF THE TOTAL SYSTEM AIR FLOW CAPACITY FOR SYSTEMS WITH MORE THAN 4 ZONES, INCREASING
THE DUCT & DAMPER SIZES OF THE SMALLER ZONES (OR ALL THE ZONES) WILL MINIMIZE THE AMOUNT OF PRESSURE RELIEF NEEDED
WHEN ONLY THE SMALLEST ZONE IS OPEN.
NOTE: CONNECT DAMPERS DIRECTLY TO THE PLENUM WHEN POSSIBLE AND BRANCH OFF SMALLER DUCTS GOING TO DIFFERENT AREAS
WITHIN THE ZONES. USING THIS TRUNK/BRANCH DUCT DESIGN WILL MINIMIZE COST AND REDUCE AIR NOISE.
DESIGN & PRE-INSTALL
