118
Superheat and Subcooling
When determining superheat, use
to convert the low side pressure gauge (suction line) to the
appropriate temperature. Subtract the converted temperature from the suction line surface temperature.
There should be approximately a
20°F
difference. Superheat monitors what state the refrigerant is when it
leaves the evaporator coil. High superheat indicates the refrigerant has picked up more heat than
designed. Low superheat indicates the refrigerant has not picked up enough heat and can cause flooding
in the compressor. If superheat is incorrect, verify subcool first before making changes to the system.
When determining subcool, convert the high side pressure gauge (condensing coil liquid line) to the
appropriate temperature. Subtract the converted temperature from the liquid line surface temperature.
There should be approximately a
10-20°F
difference. Subcooling monitors what state the refrigerant is
when it leaves the condensing coil. High subcooling means the condenser is flooded. Low subcooling
means the condenser is starving.
• If the superheat is high and the subcool is low under normal operating conditions, the system may be
low on charge. Determine the cause of low refrigerant and repair as necessary. Refer to
System Low On Refrigerant” on page 107
• If the superheat is low and the subcool is high under normal operating conditions, the system may be
“Recovering Refrigerant from the System” on page 104
.
• If the superheat is high and the subcool is high under normal operating conditions, there could be a
blockage in the coil, or line set.
NOTE: Ambient temperatures may affect subcooling.
Table 16 - R410A Pressure Temperature
Temperature (°F)
Refrigerant Pressure
Temperature (°F)
Refrigerant Pressure
-45
7.7
55
156.6
-40
10.8
60
170.7
-35
14.1
65
185.8
-30
17.8
70
201.8
-25
21.9
75
218.7
-20
26.3
80
236.5
-15
31.2
85
255.4
-10
36.5
90
275.4
-5
42.2
95
296.4
0
48.2
100
318.6
5
55
105
341.9
10
62.3
110
366.4
15
70.2
115
392.3
20
78.7
120
419.4
25
87.8
125
447.9
30
97.5
130
447.9
35
107.9
135
509.4
40
118.9
140
542.5
45
130.7
145
577.3
50
143.3
150
613.9
