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23 BALANCING THE AIRFLOWS
Determining the CFM:
After balancing the airflows, calculate the CFM flow rate.
Example
This example shows how to determine the airflow for a 6 in diameter duct. If the duct velocity pressure reads
0.025 in w.g. on the digital manometer, use the chart that came with the pitot tube to determine a duct
velocity of 640 ft/min. for a duct velocity pressure of 0.025 in w.g.
CFM Calculation
CFM = feet per minute x cross section area of duct
= 640 x 0.196
= 125
Units with Balancing Collars:
Install these units with the dampers fully open and damper down the duct with the higher airflow equal to the
lower airflow. All other units require dampers for balancing airflows installed into the “Fresh Air to Building”
and “Stale Air from Building” ductwork.
Hard/Rigid Ducting
• Push and turn with slotted screwdriver. Damper
automatically locks when pressure released.
Insulated flexible ducting
• When connecting ductwork to the collar, take note where screws
are located. Screws should be located no further than 1/2 in from
outside edge of collar, so as not to impede operation of the
damper.
• Installations where the HRV is ducted directly to the return of a furnace may require additional dampening on
the fresh air to building duct. This is due to the high return static pressures found in some furnace
installations.
Cross section area of some common duct sizes:
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