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These four temperature differentials are the critical
measurements used to determine all refrigerant related problems. Often a
manifold gauge set is not even necessary.
Superheat is a temperature differential.
Subcooling is a temperature differential.
Evaporator entering air versus leaving air temperature
is a differential.
Condenser entering air versus leaving air temperature
is a differential.
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Critical Temperature Differentials
Air temperature drop over the evaporator should not
exceed 20 degrees F.
Air temperature rise over the condenser should not
exceed 30 degrees F.
The low side superheat should be between 20 and 30
degrees.
The condenser subcooling should not exceed 15
degrees.
An air temperature drop over the evaporator greater
than 20 degrees indicates low evaporator airflow.
An air temperature rise over the condenser greater
than 30 degrees indicates low condenser airflow.
A low side superheat less than 20 degrees indicates
too much liquid refrigerant is in the low side.
A low side superheat greater than 30 degrees
indicates too little refrigerant is in the low side.
A condenser subcooling exceeding 15 degrees indicates
too much liquid refrigerant is in the high side.
- Comparing these readings will lead to an understanding of what is
wrong with the system. For example, assuming adequate airflow over both
the evaporator and condenser the following is true.
High superheat with high condenser subcooling
indicates a restriction. Too much liquid is in the high side and too
little in the low side.
Low superheat with high subcooling indicates an
overcharge. Too much liquid on both sides.
High superheat with low condenser subcooling indicates
an undercharge. Not enough liquid on either side.
Low side superheat and condenser subcooling simply
tell us where the refrigerant is located.
Too much refrigerant on the high side and too little
on the low side indicates a restriction.
Too much on both sides indicates an overcharge and not
enough on either side indicates an undercharge.