175244-BIM-A-0706
46
Unitary Products Group
AIR BALANCE
Start the supply air blower motor. Adjust the resis-
tances in both the supply and the return air duct sys-
tems to balance the air distribution throughout the
conditioned space. The job specifications may require
that this balancing be done by someone other than the
equipment installer.
To check the supply air CFM after the initial balancing
has been completed:
1.
Remove the two 5/16" dot plugs from the blower
motor and the filter access panels shown in Fig-
ures 13 and 14.
2.
Insert at least 8" of 1/4 inch tubing into each of
these holes for sufficient penetration into the air
flow on both sides of the indoor coil.
NOTE:
The tubes must be inserted and held in a posi-
tion perpendicular to the air flow so that veloc-
ity pressure will not affect the static pressure
readings.
3.
Using an inclined manometer, determine the pres-
sure drop across a 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
assure a dry coil, the compressors should be deac-
tivated 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 Pressure Drop vs. Supply Air
CFM Figure 21.
After readings have been obtained, remove the tubes
and reinstall the two 5/16" dot plugs that were removed
in Step 1.
NOTE:
DE-ENERGIZE THE COMPRESSORS
BEFORE TAKING ANY TEST MEASURE-
MENTS TO ASSURE A DRY INDOOR COIL.
OPERATION
SEQUENCE OF OPERATIONS OVERVIEW
For these units, the thermostat makes a circuit
between "R" and "Y1" for the first stage of cooling.
The call is passed to the unit control board (UCB),
which then determines whether the requested opera-
FIGURE 20 - BELT ADJUSTMENT
FIGURE 21 - PRESSURE DROP ACROSS A DRY
INDOOR COIL VS SUPPLY AIR CFM
FOR ALL UNIT TONNAGES
Failure to properly adjust the total system air
quantity can result in extensive blower dam-
age.
PRESSURE DROP ACROSS A DRY COIL vs SUPPLY AIR CFM
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
NOMINAL AIR FLOW (SCFM)
PRESSURE DROP (IWG)
