284809-YIM-B-0707
Unitary Products Group
45
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 22.
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 operation is
available and, if so, which components to energize.
For gas heating, the UCB monitors the "W1" call but
does not handle the operation of the gas furnace. An
ignition control board controls the gas heater operation.
For electric heat units, the UCB passes the call to the
electric heater.
In both cases, when the "W1" call is sensed, the indoor air
blower is energized following a specified heating delay.
If at any time a call for both heating and cooling are
present, the heating operation will be performed. If oper-
ating, the cooling system is halted as with a completion
of a call for cooling. Heating always takes priority.
COOLING SEQUENCE OF OPERATION
CONTINUOUS BLOWER
By setting the room thermostat fan switch to "ON," the
supply air blower will operate continuously.
INTERMITTENT BLOWER
With the room thermostat fan switch set to "AUTO" and
the system switch set to either the "AUTO" or "HEAT"
settings, the blower is energized whenever a cooling or
heating operation is requested. The blower is ener-
gized after any specified delay associated with the
operation.
When energized, the indoor blower has a minimum run
time of 30 seconds. Additionally, the indoor blower has
a delay of 10 seconds between operations.
OPTIONAL VAV BLOWER OPERATION
For units with VFD, the speed of the indoor blower
motor is controlled by duct static pressure. The duct
static set point is the pressure that the drive will main-
tain when operating the unit in VAV mode. If the duct
static pressure reaches or exceeds the high-limit set-
point, then the supply fan motor will be shutdown.
FIGURE 22 - 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 damage.
PRESSURE DROP ACROSS A DRY INDOOR COIL VS SUPPLY AIR CFM
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
4000
5000
6000
7000
8000
9000
10000
11000
12000
NOMINAL AIR FLOW (CFM)
PRESSURE DROP (IWG)
