Determining Charge Method - Lennox Merit Series13HPD Units Installation Instructions Manual

3. Set the room thermostat to 75° to 80° and a call for
heat. This will create the necessary load for properly
charging the system in the cooling cycle.
4. Use a digital thermometer to record the outdoor
ambient temperature.
5. When the heating demand has been satisfied, switch
the thermostat to cooling mode with a set point of 68_F
(20_C). When pressures have stabilized, use a digital
thermometer to record the liquid line temperature.
6. The outdoor temperature will determine which
charging method to use. Proceed with the appropriate
charging procedure.

Determining Charge Method

Use figure 24 to determine the correct charging method.
Above 65ºF
(18ºC)?
APPROACH
WHEN TO CHARGE?
S
Warm weather best
S
Can charge in colder weather
CHARGE METHOD? Determine by:
S
Metering device type
S
Outdoor ambient temperature
REQUIREMENTS:
S
Sufficient heat load in structure
S
Indoor temperature between 70-80ºF (21−26ºC)
S
Manifold gauge set connected to unit
S
Thermometers:
− to measure outdoor ambient temperature
− to measure liquid line temperature
− to measure vapor line temperature
Figure 24. Determining Correct Charging Method
CHARGE USING THE WEIGH−IN METHOD −
OUTDOOR TEMPERATURE < 65_F (18_C)
If the system is void of refrigerant, or if the outdoor ambient
temperature is cool, first, locate and repair any leaks and
then weigh in the refrigerant charge into the unit.
CTXV or RFC
Below 65ºF
(18ºC)?
WEIGH-IN or
SUBCOOLING
1. Recover the refrigerant from the unit.
2. Conduct leak check; evacuate as previously outlined.
3. Weigh in the unit nameplate charge. If weighing
facilities are not available or if charging the unit during
warm weather, use one of the following procedures.
CHARGE USING THE SUBCOOLING METHOD −
OUTDOOR TEMPERATURE < 65°F (18°C)
When the outdoor ambient temperature is below 65°F
(18°C), use the subcooling method to charge the unit. It
may be necessary to restrict the air flow through the
outdoor coil to achieve pressures in the 200−250 psig
(1379−1724 kPa) range. These higher pressures are
necessary for checking the charge. Block equal sections of
air intake panels and move obstructions sideways until the
liquid pressure is in the 200−250 psig (1379−1724 kPa)
range as illustrated in figure 25.
Figure 25. Blocking Outdoor Coil
1. With the manifold gauge hose still on the liquid service
port and the unit's pressure stabilized, use a digital
thermometer to record the liquid line temperature.
2. At the same time, record the liquid line pressure reading.
3. Use a temperature/pressure chart for HCFC−22 to
determine the saturation temperature for the liquid line
pressure reading.
4. Subtract the liquid line temperature from the saturation
temperature (according to the chart) to determine
subcooling.
5. Compare the subcooling value results with those listed
in table 7. If subcooling is greater than shown, recover
some refrigerant. If subcooling is less than shown, add
some refrigerant.
Table 7. Subcooling Values
º
º
= º
Model −018 −024
°F
6
(°C)* (3.3)
NOTE − For best results, use the same electronic
thermometer to check both outdoor-ambient and liquid-line
temperatures. *F: +/−1.0°; C: +/−0.5°
Page 15
Blocking Outdoor Coil
*Outdoor coil should be blocked one
side at a time with cardboard or plastic
sheet until proper testing pressures
are reached.
cardboard or plastic sheet
*Four−sided unit shown.
Saturation Temperature ºF (ºC)
Liquid Line Temperature ºF (ºC)
Subcooling Value ºF (ºC)
−030 −036 −042
11 8 6 6
(6) (4.4) (3.3) (3.3)
−048 −060
4 9 (5)
(2.2)
13HPD SERIES