Measuring actual superheat and subcooling, Target superheat and subcooling, Generic target superheat & subcooling charts – Mastercool 90413 4-way Valve Digital Manifold User Manual

Page 6: Fig. c

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upward “drift” of the vacuum gauge reading may indicate a leaking system.

MEASURING ACTUAL SUPERHEAT AND SUBCOOLING

(SEE FIG. C)

Superheat is the difference between the actual temperature (sensor temperature) of
the refrigerant (gas) as it leaves the evaporator and the boiling point temperature of the
refrigerant in the evaporator coil

(saturated temperature). After boiling, the refrigerant

continues to warm up. The number of degrees it “warmed up” after boiling is called the
superheat. Under worst-case conditions (low load for ﬁxed oriﬁce systems), the refrigerant
in the evaporator boils off near the end of the evaporator coil. To make sure liquid doesn’t
enter the compressor under the worst case condition, the AC manufacturers publish charts.
The charts indicate what the superheat should be at a given indoor wet bulb measurement
and outdoor air temperature. Measuring superheat is your best indication on a ﬁxed
oriﬁce system of the proper refrigerant charge and operating conditions. If everything else
is working properly and the actual superheat is too high, add refrigerant. If it’s too low,
remove refrigerant.
Subcooling is the difference between the boiling point of the refrigerant in the condenser
(saturated temperature) and the actual temperature (sensor temperature) of the
refrigerant as it leaves the condenser. The degrees that the refrigerant “cools down” below
the boiling point is the sub-cooling. Under worst case scenario low load for thermostatic
expansion valve systems (TXV) the subcooling will continue to rise. If the subcooling rises
too high, liquid may be backed into the compressor causing damage and failure.

FIG. C

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LOW

VAC

REF

HIGH

LOW

VAC

REF

HIGH

On TXV systems, the subcooling is the best indication of the state of the charge in the
refrigerant system since these systems are designed to maintain constant superheat.
Properly charging a system ensures maximum efﬁciency and longer equipment life.

Use caution whenever working with electricity and high-pressure
liquid or gas. Always wear safety glasses.

Follow all instructions provided with equipment being serviced or installed.

TARGET SUPERHEAT AND SUBCOOLING

Follow all equipment manufacturer’s speciﬁcations, warnings and suggestions.
To determine the target superheat (ﬁxed oriﬁce system) or subcooling (charts vary
dramatically from one system to another), you will typically need three things. Outdoor dry
bulb (outdoor air temperature), indoor wet bulb, and the manufacturers target superheat
chart or subcooling chart.
You can use Mastercool part number 52232 Humidity Temperature Meter for both indoor
wet bulb and outdoor dry bulb. (Refer to diagram.)

GENERIC TARGET SUPERHEAT & SUBCOOLING CHARTS*

*The required superheat chart is an example of a generic superheat chart of a typical ﬁxed
oriﬁce, split residential system. The required subcooling chart is an example of a typical
chart for a TXV, split residential system. These charts should not be used for charging.

This manual is related to the following products:

99972 4-way Valve Digital Manifold 99961 4-way Valve Digital Manifold 99903_99903-2 4-way Valve Digital Manifold