Superheat and subcooling are the two numbers that tell you whether a system is correctly charged. Both are temperature differences — a measured line temperature compared with the saturation temperature the gauge pressure implies. Get comfortable with the pair and you can diagnose most charge faults on the roof.
Superheat
How far the suction vapour has warmed above its boiling (saturation) temperature at the measured low-side pressure — proof that all the liquid boiled off before leaving the evaporator. Measure the suction line temperature near the outdoor unit, read the low-side gauge, and superheat = line temperature − saturation temperature.
Subcooling
How far the liquid leaving the condenser has cooled below its condensing temperature at the measured high-side pressure — proof there is a solid column of liquid at the metering device. Measure the liquid line temperature at the condenser outlet, read the high-side gauge, and subcooling = saturation temperature − line temperature.
Superheat & Subcooling Calculator
Enter gauge pressure (psi) and line temperature to check the charge — R-410A, R-32, R-22, R-134a.
What should they be?
Always use the manufacturer’s figures when you have them. As broad rules: systems with a TXV/EEV hold superheat around 9–22 °F (5–12 °C) and are charged to a subcooling target, typically 7–14 °F (4–8 °C). Fixed-orifice systems are charged to a superheat target instead, which varies with indoor and outdoor conditions — look it up on a charging chart rather than assuming a number.
Reading the results
| Reading | Usual meaning |
|---|---|
| High superheat + low subcooling | Undercharge — evaporator starved |
| Very low superheat | Liquid returning to compressor (floodback) |
| High subcooling | Overcharge or liquid-line restriction |
Diagnose with airflow, filters and coils checked first — charge problems and airflow problems mimic each other closely.
How accurate is the PT data?
Saturation temperatures are interpolated from standard pressure–temperature tables and assume sea-level atmospheric pressure. R-410A and R-32 have negligible temperature glide, so one curve serves both sides and the values land within a fraction of a degree of published charts. For blends with real glide (like R-407C), or for legal record work, use the manufacturer’s tables.