343635-BIM-B-0408
10
Johnson Controls Unitary Products
CHECKING SUPPLY AIR CFM
To check the supply air CFM after the initial balancing has been
completed:
1.
Remove the two ¼ inch dot plugs in the duct panel.
2.
Insert at least 8 inches of ¼ inch tubing into each of these
holes for sufficient penetration into the airflow on both
sides of the indoor coil.
3.
Using an inclined manometer, determine the pressure drop
across the dry evaporator coil. Since the moisture on an
evaporator coil may vary greatly, measuring the pressure
drop across a wet coil under field conditions would be
inaccurate. To ensure a dry coil, the compressors should
be deactivated while the test is being run.
4.
Knowing the pressure drop across a dry coil, the actual
CFM through the unit can be determined from the curve in
Coil Delta P vs. Supply Air CFM figure.
After readings have been obtained, remove the tubes and
reinstall the two ¼ inch plugs removed in Step 1.
NOTE:
De-energize the compressors before taking any test
measurements to ensure a dry indoor coil.
FIGURE 4: COIL DELTA P VS AIRFLOW
Failure to properly adjust the total system air quantity
can result in extensive system damage.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
400
600
800
1000
1200
1400
1600
1800
2000
2.0 Ton
DPM Coil Delta P vs’ Airflow
3.0 Ton
2.5 Ton
3.5 - 5.0 Ton
ID Coil Delta P
Airflow (CFM)
