Superheat and Subcooling
The two measurements that tell you how a refrigerant circuit is really being fed and drained.
What it is
Superheat
The number of degrees a refrigerant vapor has been heated above its saturation temperature at that pressure.
Subcooling
The number of degrees a refrigerant liquid has been cooled below its saturation temperature at that pressure.
Superheat = measured suction line temperature - saturation temperature at suction pressureMeasure temperature and pressure at the same point in the line.
Subcooling = saturation temperature at liquid pressure - measured liquid line temperatureWhy it matters
Pressures alone do not tell you whether the evaporator is fully fed or whether the condenser is holding liquid. Superheat and subcooling do. They are the difference between guessing at a charge and measuring it.
How to read them
| Reading | Typical direction | What it usually suggests |
|---|---|---|
| High superheat | Low | Undercharge, restriction, or a metering device that is underfeeding |
| Low superheat | High | Overfeeding, an oversized or stuck open metering device, or low indoor load |
| High subcooling | High | Overcharge or a liquid line restriction backing liquid up in the condenser |
| Low subcooling | Low | Undercharge or a condenser that is not condensing fully |
Always confirm airflow first
Where it is used
Fixed orifice systems are charged by superheat using a charging chart that accounts for indoor wet bulb and outdoor dry bulb. Systems with a thermostatic or electronic expansion valve are normally charged by subcooling to the manufacturer target.
Symptoms
Practice & Mastery
- Questions4
- Quiz1
- Superheat and Subcooling Troubleshooting Scenarios2
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