Refrigerant Subcooling

Subcooling is superheat's mirror image at the other end of the circuit. Refrigerant condenses at a fixed temperature for its pressure, and once the last vapour bubble collapses the liquid can be cooled further by the remaining condenser surface. Those extra degrees are subcooling, and they are proof of a solid column of liquid feeding the metering device — which matters, because a TXV fed with bubbles cannot control anything. On a TXV system, subcooling is the primary charging measurement; 8–12 °F is the usual target, with the manufacturer's plate having the final word.

Read the liquid line pressure, convert to saturation temperature, and subtract the clamped liquid-line temperature: 110 °F saturation with a 100 °F liquid line is 10 °F of subcooling. Low subcooling (under about 5 °F) points to undercharge or a condenser that is not rejecting heat; high subcooling (over 15 °F) points to overcharge, a restricted metering device, or liquid stacking in a dirty condenser. The trap is diagnosing on one number: a system can show good subcooling and terrible superheat at the same time, which localises the fault to the metering device rather than the charge. Take both, always, and take them after the system has run at least 15 minutes. On R-410A remember that the blend's small glide means you read the bubble point for subcooling and the dew point for superheat — mixing them up costs you a degree or two of accuracy.