16 17
DETERMINE VENTILATION REQUIREMENTS
2. Determine the ventilation controller fresh air delivery rate.
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
Measure the negative static pressure of the return system and consult table above for approximate inlet airflow.
These values are not absolute. An airflow measuring device (Nailor-Hart, etc.) will give the Ventilation Controller’s
airflow exactly.
For the table above, the flex duct is laid loose with 2 wide 90° bends, and the damper is full open. For the rigid pipe
the values are based on two 90° elbows, and the damper is open. For both cases the air intake is through a metal
vent hood with a bird screen. Adjust airflow up or down for variations, including elbows or bends, or if the length of
duct you’re using isn’t listed, etc.
Measure the delivered airflow or determine from above chart and record __________
SETTING VENTILATION TIME
3. Determine the ventilation controller knob setting.
TABLE 3 – Cycle Time Setting (minutes) for Airflow Delivered vs. Airflow Required for 1 Hour Cycle
CFM
Delivered
CFM Required
20 30 40 50 60 70 80 90 100 110 120
60 20 30 40 50 60 70 80 90 100 110 120
80 15 25 30 40 45 55 60 70 75 85 90
100 15 20 25 30 35 40 50 55 60 65 70
120 10 15 20 25 30 35 40 45 50 55 60
140 10 15 15 20 25 30 35 40 45 50 55
160 10 10 15 20 25 25 30 35 40 45 50
Use your entries from Steps 1 and 2 in the table above. From this you can determine the correct knob setting. For
example, if 80 CFM is required, and the Vent delivers 120 CFM set the knob to 40 minutes.
If more outdoor air is required, increase delivered CFM by reducing bends in duct, increasing duct size, adding a
second duct, or adding a powered fan.
