*
These charts are an example of a generic
superheat charts for a typical fixed orifice, split res-
idential system. These charts should not be used
for charging. A typical manufacturer’s recommend-
ed subcooling is 12°F (7°C). These are only exam-
ples of what the manufacturer’s may recommend.
Heed all manufacturer’s indications, instructions
and warnings above those in this manual.
The indoor wet bulb measurement can be
accomplished by a Fieldpiece ARH4 or ATWB1 and
should be taken as close to the evaporator coil inlet
as possible. The outdoor dry bulb reading can be
taken with an ARH4, ATB1, ATA1 or any other
Fieldpiece air thermocouple and should be taken as
close to the condenser air inlet as possible.
Measuring actual superheat and
subcooling
Superheat is the difference between the actual
temperature of the refrigerant (gas) as it leaves the
evaporator and the boiling point temperature of the
refrigerant in the evaporator coil. 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 evap-
orator boils off near the end of the evaporator coil.
To make sure liquid doesn’t enter the compressor
under the worst case condition (low load), the AC
manufacturers publish charts indicating 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 work-
ing 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 and the
actual 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
TXV) the subcooling will continue to rise. If the
subcooling rises to high, liquid may be backed into
the compressor causing damage and catastrophic
failure.
On TXV systems, subcooling is the best indica-
tion of the state of charge in the refrigerant system
since these systems are designed to maintain con-
stant superheat.
Properly charging a system ensures maximum
efficiency and longer equipment life.
The hose must have a schraeder valve depress-
er on one end to release the refrigerant from the
suction or liquid line. This is the same type of hose
available with most pressure gauge sets. We sug-
gest EPA sanctioned “no leak” hoses.
Exercise caution whenever working with any
electricity and high pressure liquid or gas. Follow all
instructions provided with equipment being serviced
or installed.
Target superheat and subcooling
Heed all equipment manufacturer’s specifica-
tions, warnings and suggestions above anything
found in this manual.
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 temper-
ature), indoor wet bulb, and the manufacturers tar-
get superheat chart or subcooling chart.
You can use the ARH4 Fieldpiece accessory
head for both indoor wet bulb and outdoor dry bulb.
Or you can use any Fieldpiece meter that has a
temperature function along with a ATWB1 wet bulb
thermocouple. Below is a diagram of the ASX14 on
a split-system residential A/C unit.
Condenser
Evaporator
Compressor
Throttle Valve (TXV, Cap tube, Fixed Oriffice)
Outside
Indoor
Liquid
Vapor and Liquid
Vapor
Liquid
Vapor
Vapor and Liquid
Liquid
Vapor
Outside
Indoor
AUTO-
OFF
T/C
Cal
STABLE
Pressure
Temp
SH or SC
ON
LO BATT
Air
Conditioner
Superheat
and Subcooling
ENGLISH
METRIC
R22
R410A
Set
ATM
ASX14
SH
SC
AUTO-
OFF
T/C
Cal
STABLE
Pressure
Temp
SH or SC
ON
LO BATT
Air
Conditioner
Superheat
and Subcooling
ENGLISH
METRIC
R22
R410A
Set
ATM
ASX14
SH
SC
Checking
Subcooling
Using the
ASX14
Checking
Superheat
Using the
ASX14
Re
frig
e
ra
nt
Flo
w
Re
fri
g
e
ra
nt
Flo
w
Generic Target Superheat Charts*
