Subcooling
How many degrees the liquid refrigerant has been cooled below its condensing temperature. Together with superheat it indicates whether the charge and the metering device are working as designed, which is why both are recorded during commissioning.
What is subcooling?
Subcooling is how many degrees the liquid refrigerant has been cooled below its condensing temperature. It indicates how much liquid the condenser is holding, which is closely tied to charge.
Subcooling: how it works
Once vapour has fully condensed in the condenser, any further heat removal cools the liquid below the temperature it condensed at, and that difference is subcooling. Because it depends on how much liquid is backed up in the condenser, it responds directly to how much refrigerant is in the system, which is what makes it the charge indicator on systems with an expansion valve.
Where you meet the term
- On a commissioning sheet, alongside superheat, as the pair of readings the system was handed over at.
- In a charge adjustment, where subcooling rather than pressure alone is what the technician is targeting.
- In a diagnosis of a system with correct charge but poor capacity, where normal subcooling shifts attention to metering or airflow.
How to check it
- Read liquid line pressure and convert it. The saturation temperature at that pressure is the condensing temperature the measurement is taken against. Technician
- Measure liquid line temperature at the specified point. Clamp position matters, and the probe is insulated from ambient air so it reads the pipe rather than the surroundings. Technician
Subcooling: what it is not
- Subcooling is not interchangeable with superheat. One looks at the liquid leaving the condenser, the other at the vapour leaving the evaporator.
- A fixed orifice system does not read charge the same way. Subcooling is the charging method on valve-metered systems, and manufacturers state which method applies.
Subcooling questions
What does low subcooling mean?
That little liquid is backed up in the condenser, which points at an undercharge or at refrigerant being pulled through faster than the condenser can fully condense it. Superheat read at the same time narrows it.
What does high subcooling mean?
That liquid is accumulating in the condenser, which points at an overcharge or at a restriction downstream holding refrigerant back. Overcharge raises pressure and can reach protection limits.
Why are superheat and subcooling always quoted together?
Each on its own has several explanations. Together they separate a charge problem from a metering problem, which is the distinction that decides what gets adjusted or replaced.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE TerminologyIndustry reference definitions for heating, ventilation, air conditioning, and refrigeration vocabulary.