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10A. HOW TO SIZE A FRESH AIR DAMPER
10. SIZING OF FRESH AIR DAMPER AND HRV/ERV
Select the size of the Fresh Air Damper (FAD) based on the continuous ventilation CFM requirement
multiplied by 3 and adjusted for the actual fresh air ductwork installation parameters to allow the
FAVC system operate 10 minutes on every 30 minutes.
There are two methods available to Size the Fresh Air Duct: Estimated and Design.
Estimated Method for FAD Sizing:
Use Tables 6A and 6B to select fresh air damper air fl ow rate in CFM.
Table 6A shows estimated FAD air fl ow rates based on smooth wall fresh air pipe ductwork at various
lengths. Table 6B shows estimated FAD air fl ow rates based on Flex Duct fresh air pipe ductwork at
various lengths. Short Run Length section is for short simple installations with no elbows or reducing
fi ttings installed with a total of 10 foot equivalent duct length or less. Medium Run Length section is for
8-19 foot duct runs with 1 or less elbows or reducing fi ttings installed with a total of 30 foot equivalent
duct length or less. Long Run Length section is for 15-28 foot duct runs with 2 or less elbows or reducing
fi ttings installed with a total of 30 foot equivalent duct length or less.
P/N 780100700 07/18 Rev F
Table 6A: FAD Damper Size Air Flow in CFM for Smooth Wall Pipe
Table 6B: FAD Damper Size Air Flow in CFM based on Flex Wall Duct
Design Method for FAD Sizing:
The design method uses the combination of equivalent duct length
based on actual ducting design and static pressure measurement at inlet side of fresh air ducting with
air handler running in cooling and/or heating modes. Figure 15, on page 15, shows a typical FAD
installation in a basement confi guration. Tables 2A and 2B are used to select the continuous ventilation
CFM requirement according to size of home (in square foot), number of bedrooms and applicable
ASHRAE62.2 code year. Tables 7A and 7B are used to calculate equivalent feet for vent piping and
reducer/increaser fi ttings. Table 8 is used to verify your fresh air damper air fl ow selection based on
actual negative return air static pressure (inches WC) measurement taken at the intake of fresh air
ductwork.
