Carrier 48NLT - Cooling System Start-Up

Start-Up Cooling Section and Make
Adjustments
A CAUTION
Complete the required procedures given in the Pre-Start-
Up section before starting the unit.
Do not jumper any safety devices when operating
the unit.
Do not operate the compressor when the outdoor tem¬
perature is below 55 F (unless accessory low-temperature
kit is installed).
Do not rapid-cycle the compressor. Allow 5 minutes
between “on” cycles to prevent compressor damage.
CHECKING COOLING CONTROL OPERATION - Start
and check the unit for proper cooling control operation as
follows:
1. Place room thermostat SYSTEM switch in OFF posi¬
tion. Observe that blower motor starts when FAN switch
is placed in ON position and shuts down when FAN switch
is placed in AUTO, position.
2. Place SYSTEM switch in COOL position and FAN switch
in AUTO, position. Set cooling control below room tem¬
perature. Observe that compressor, condenser fan and
evaporator blower motors start. Observe that cooling
cycle shuts down when control setting is satisfied. The
blower motor has an off delay of approximately one minute
on shutdown.
3. When using an auto.-changeover room thermostat, place
both SYSTEM and FAN switches in AUTO, positions.
Observe that unit operates in heating mode when tem¬
perature control is set to “call for heating” (above room
temperature) and operates in cooling mode when tem¬
perature control is set to “call for cooling” (below room
temperature).
CHECKING AND ADJUSTING REFRIGERANT CHARGE
— The refrigerant system is fully charged with R-22 refrig¬
erant, tested and factory-sealed.
NOTE: Adjustment of the refrigerant charge is not required
unless the unit is suspected of not having the proper R-22
charge. For all applications, the correct R-22 charge for the
best performance is the charge that results in a suction gas
superheat of 5 F at the compressor inlet when the unit is
operating at the ARI rating conditions of 95 F dry-bulb (db)
outdoor and 80 F db/67 F wet-bulb (wb) indoor.
A superheat charging label is attached to the outside of
the compressor access door. The label includes a “Super¬
heat Charging Table” and a “Required Suction-Tube Tem¬
perature (F)” chart.
An accurate superheat thermocouple- or thermistor-type
thermometer, a sling psychrometer and a gage manifold are
required when using the superheat charging method for eval¬
uating the unit charge. Do not use mercury or small dial-
type thermometers because they are not adequate for this
type of measurement.
A CAUTION
When evaluating the refrigerant charge, an indicated
adjustment to the specified factory charge must always
be very minimal. If a substantial adjustment is indi¬
cated, an abnormal condition exists somewhere in the
cooling system, such as insufficient airflow across
either coil or both coils.
Proceed as follows:
1. R emove caps from low- and high-pressure service
fittings.
2. Using hoses with valve core depressors, attach low- and
high-pressure gage hoses to low- and high-pressure ser¬
vice fittings, respectively.
3. Start unit in cooling mode and let unit run until system
pressures stabilize.
4. Measure and record the following.
a. Outdoor ambient-air temperature (F db).
b. Evaporator inlet-air temperature (F wb).
c. Suction-tube temperature (F) at low-side service
fitting.
d. Suction (low-side) pressure (psig).
5. Using “Superheat Charging Table,” compare outdoor-
air temperature (F db) with evaporator inlet-air temper¬
ature (F wb) to determine desired system operating su¬
perheat temperature. See Table 6.
6. Using “Required Suction-Tube (F)” table, compare de¬
sired superheat temperature with suction (low-side)
operating pressure (psig) to determine proper suction-
tube temperature. See Table 7.
7. Compare actual suction-tube temperature with proper
suction-tube temperature. Using a tolerance of ± 3 F,
add refrigerant if actual temperature is more than 3 F
higher than proper suction-tube temperature, or remove
refrigerant if actual temperature is more than 3 F lower
than required suction-tube temperature.
NOTE: If the problem causing the inaccurate readings is a
refrigerant leak, refer to Start-Up, Check for Refrigerant
Leaks section.
INDOOR AIRFLOW AND AIRFLOW ADJUSTMENTS
A CAUTION
For cooling operation, the recommended airflow is 350
to 450 cfm per each 12,000 Btuh of rated cooling ca¬
pacity. For heating operation, the airflow must pro¬
duce a temperature rise that falls within the range stamped
on the unit rating plate.
Direct-drive blower motors are factory connected to de¬
liver the proper heating and cooling airflows at normal ex¬
ternal static pressures (medium speed cooling, low speed
heating for 230-v units) and high speed cooling (units with
460-v have medium speed cooling and high speed heating).
For 208-V operation on 208/230-v rated direct drive units,
interchange motor leads to high speed for cooling and
medium speed for heating operation.
Table 5 shows the temperature rise at various airflow rates.
Tables 8 and 9 show both heating and cooling airflows at
various external static pressures. Refer to these tables to
determine the airflow for the system being installed.
NOTE: Be sure that all supply- and return-air grilles are
open, free from obstructions and adjusted properly.
A WARNING
Disconnect electrical power to the unit before changing
blower speed. (Be sure to turn off gas supply before
disconnecting electrical power.) Electrical shock can cause
personal injury or death.
A CAUTION
Do not change the blower-motor lead connections on
460-v units from the factory setting. Damage to unit
may result.
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