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DETERMINE FRESH AIR DELIVERY RATE
1. Measure the negative static pressure of the return system at the location where the fresh air intake duct enters the return duct or dehumidifier
inlet.
2. See Table 2 for estimated inlet airflow in CFM, based on duct type, length and available negative pressure. Use an airflow measuring device
for a more accurate airflow delivery rate.
3. Record the delivered CFM. ________
TABLE 2 – CFM Delivered
Duct Length
Negative Static Pressure (“w.c.) as Measured for Return Duct or Plenum
0.05 0.1 0.15 0.2 0.25 0.3
Flex Pipe Flex Pipe Flex Pipe Flex Pipe Flex Pipe Flex Pipe
10 ft.
60 65 85 90 105 110 120 125 135 140 150 160
20 ft.
55 60 80 85 100 105 115 120 130 135 140 150
30 ft.
50 55 75 80 95 100 110 115 125 130 130 140
NOTE:Forthetableabove,6”flexductislaidloosewithtwo,wide90°bendsandafullyopeneddamper.Rigidpipevaluesarebasedon6”
duct, two 90° elbows, and a fully open damper. In both cases, the air intake is through a metal vent hood with inlet screen. Airflow may need to
be adjusted up or down for variations in duct work.
DETERMINE CYCLE TIME
1. Use the Required CFM and Delivered CFM from the above steps to determine the Cycle Time from Table 3.
2. The values highlighted in gray cannot be set due to the maximum 60 minute Cycle Time. A second ventilation device (i.e., bigger duct or
second duct) will be required to meet ventilation needs.
VENTILATION (CONTINUED)
DETERMINE VENTILATION REQUIREMENTS
CALCULATING AIRFLOW REQUIREMENT
1. The MINIMUM ventilation requirement is calculated using ASHRAE
62.2-2010.
ASHRAE Airflow in CFM = [House Area in Sq. Ft. x 0.01] +
[(Number of Bedrooms +1) x 7.5]
NOTE: Use ‘Number of Bedrooms + 1’ or ‘Number of Occupants’,
whichever is larger.
2. Table 1 shows the calculated airflow values to the nearest 5 CFM.
3. Record the required CFM. ________
TABLE 1 – CFM Required
House Sq. Ft.
Number of Bedrooms
2 3 4 5 6
1000
35 40 50
1500
40 45 55 60 70
2000
45 50 60 65 75
2500
50 55 65 70 80
3000
55 60 70 75 85
3500
75 80 90
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