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Design Method:
Step 1:
Select the continuous ventilation CFM fi gure according to size of home (in square
feet), number of bedrooms and applicable ASHRAE62.2 code year using Tables 2A
and 2B on page 6.
Step 2:
Measure static pressure at the return intake (in Inches WC).
Step 3:
Calculate equivalent feet of duct between fresh air inlet and the central fan.
i.
ii.
iii.
Step 4:
Use Table 8 to select fresh air damper size based on measuring static pressure at the
return intake, multiple the continuous ventilation CFM value by 3 for fresh air damper
CFM and calculated total equivalent duct length. Find the static pressure column, match
the FAD CFM rate equal to or greater than 3 times the continuous ventilation rate
required. The correct damper and intake hood size is listed to the left on Table 8.
When in doubt, use the next larger damper.
It is recommended that the damper be as close to the return air plenum and the FAVC as possible
while maintaining the minimum separate distance of 20 inches, and that the inlet duct connection
to the return air plenum upstream of the system fi lter and downstream of any duct-mounted sensors.
P/N 780100700 07/18 Rev F
Determine the total equivalent feet for each type of fi tting used in the system
from Tables 7A and 7B.
Calculate the total feet for the straight lengths of pipe.
Add the equivalent feet of the fi ttings to the total amount of straight feet pipe.
This fi gure becomes the total equivalent feet of duct length.
Table 7A: Equivalent Feet for Vent Pipe Fittings
Table 7B: Equivalent Feet for a Reducer/Increaser Pipe Fitting
Page 32 of 36
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