52
Before Charging
Unit being charged must be at or near full
load conditions before adjusting the charge.
Units equipped with hot gas reheat must be
charged with the hot gas reheat valves closed
while the unit is in cooling mode to get the
proper charge. After charging, unit should be
operated in reheat (dehumidification) mode
to check for correct operation.
Units equipped with heat pump options
should be charged in heating mode to get the
proper charge. After charging, unit should be
operated in cooling mode to check for correct
charge. Charge may need to be adjusted for
cooling mode. If adjustments are made in the
cooling mode heating mode must be rerun to
verify proper operation.
After adding or removing charge the system
must be allowed to stabilize, typically 10-15
minutes,
before
making
any
other
adjustments.
The type of unit and options determine the
ranges for liquid sub-cooling and evaporator
superheat. Refer to the tables below when
determining the proper sub-cooling.
Checking Liquid Sub-Cooling
Measure the temperature of the liquid line as
it leaves the condenser.
Read the gauge pressure at the liquid line
close to the point where the temperature was
taken. Use liquid line pressure as it will vary
from discharge pressure due to condenser
pressure drop.
Convert the pressure obtained to a saturated
temperature using the appropriate refrigerant
temperature-pressure chart.
Subtract the measured liquid line temperature
from the saturated temperature to determine
the liquid sub-cooling.
Compare calculated sub-cooling to the table
below for the appropriate unit type and
options.
Table 15 - Acceptable Refrigeration Circuit Values
Cooling
Mode
Liquid Sub-
Cooling
Values
Cooling Only Unit
4
8-15°F
Cooling Only Unit with Hot
Gas Reheat
1,4
5-15°F
Heat PumpUnit
2,4
2-4°F
Heat Pump Unit with Hot
Gas Reheat
3,4
2-6°F
Cooling Only Unit with
LAC
4
8-15°F
Cooling Only Unit with Hot
Gas Reheat & LAC
4
8-15°F
Notes:
1.
Must be charged with the hot gas valve closed.
After charging, unit should be operated in reheat
(dehumidification) mode to check for correct
operation.
2.
The sub-cooling value in this table is for the unit
running in cooling mode of operation. After
charging, unit should be operated in heating mode
to check for correct operation.
3.
The sub-cooling value in this table is for the unit
running in cooling mode of operation and the hot
gas valve closed. After charging, unit should be
operated in reheat (dehumidification) mode to
check for correct operation and then in heating
mode to check for correct operation.
4.
Sub-cooling must be increased by 1°F per 10 feet
of vertical liquid line rise for R-410A (AHU
above CU). For example, a cooling only unit with
hot gas reheat and a vertical liquid drop can
charge to a sub-cooling value of 5-15°F, but a
cooling only unit with hot gas reheat and a vertical
liquid rise of 30 ft must charge to a sub-cooling
value of at least 8-15°F.
