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Checking Liquid Sub-cooling:
1. Measure the temperature of the liquid line
as it leaves the condenser coil.
2. Read the gauge pressure reading of the
liquid line close to the point where the
temperature was taken. You must use liquid
line pressure as it will vary from discharge
pressure due to condenser coil pressure drop.
3. Convert the pressure obtained in Step 2 to
a saturated temperature using the appropriate
refrigerant temperature-pressure chart.
4. Subtract the measured liquid line
temperature in Step 1 from the saturated
temperature in Step 3 to determine the liquid
sub-cooling.
5. Compare calculated sub-cooling to Table
2 for the appropriate unit type and options.
Checking Evaporator Superheat:
1. Measure the temperature of the suction
line close to the evaporator.
2. Read gauge pressure at the suction line
close to the evaporator.
3. Convert the pressure obtained in Step 2 to
a saturated temperature using the appropriate
refrigerant temperature-pressure chart.
4. Subtract the saturated temperature in
Step 3 from the measured suction line
temperature in Step 1 to determine the
evaporator superheat.
5. Compare calculated superheat to Table 2
for the appropriate unit type and options.
Table 2 - Acceptable Refrigerant Circuit Values
* Sub-cooling must be increased by 1°F per
10 feet of vertical liquid line rise for R-410A
** Superheat will increase with long suction line
runs.
The system is undercharged if:
1. The superheat is too high and
2. The sub-cooling is too low
Correct an undercharged system by adding
refrigerant to the system to reduce superheat and
raise sub-cooling.
If the sub-cooling is correct and the superheat is
too high, the TXV may need adjustment to correct
the superheat.
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