VENTIlATION OR AIR CyClING
(CONTINUED)
DETERMINE fRESH AIR DElIVERy RATE
1 . Measure the negative static pressure of the return system at the location where the outdoor air duct enters the return duct .
2 . See
Table 4
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 4 – 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: For the table above, 6” flex duct is laid loose with two, wide 90° bends and a fully opened damper . Rigid pipe values are based on 6” 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 5
.
2 . The values are listed for a 1 hour Cycle Period . For a 2 hour Cycle Period, set the Cycle Time to twice the values listed in
Table 5
.
3 . 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 .
TAblE 5 – 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
60
20
30
40
50
60
70
80
90
100
80
15
25
30
40
45
55
60
70
75
100
15
20
25
30
35
40
50
55
60
120
10
15
20
25
30
35
40
45
50
140
10
15
15
20
25
30
35
40
45
160
10
10
15
20
25
25
30
35
40
16
