Subcool (subcooling) is the temperature difference between liquid refrigerant leaving the condenser and the refrigerant’s condensation point at that pressure. It’s measured in degrees F, typically 8-12°F for a properly charged AC system using a TXV.
While superheat tells us about the evaporator side of the system, subcool tells us about the condenser side. Together they confirm the system has the right refrigerant charge for the conditions.
What subcool readings tell us
- 3-5°F subcool — undercharged. System needs more refrigerant.
- 8-12°F subcool — normal, target range for TXV systems
- 15-20°F subcool — overcharged. Refrigerant needs to be removed.
- 25+°F subcool — significantly overcharged or condenser airflow problem
For systems using a fixed-orifice piston (older equipment) rather than a TXV, superheat is the primary charging metric and subcool isn’t used the same way. For all modern systems, both numbers matter.
Related: Superheat, Refrigerant, Condenser Coil, TXV.
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