On a TXV or EEV system, liquid-line subcooling is the measurement that tells you whether the charge is right. This calculator converts your high-side gauge pressure to condensing saturation temperature with the correct refrigerant chart, subtracts the measured liquid-line temperature, and compares the result against the manufacturer’s target to flag overcharge or undercharge.
How it works
Subcooling is the gap between the condensing saturation temperature and the actual liquid temperature:
satTemp = PT-chart lookup(refrigerant, high-side psig)
subcooling = satTemp − liquidLineTemp
verdict = compare subcooling to manufacturer target (± tolerance)The pressure-temperature lookup uses the refrigerant’s saturation curve. Because the refrigerant is condensing at constant pressure, its temperature is fixed by that pressure; the difference between that saturation temperature and the cooler liquid leaving the condenser is the subcooling.
Worked example: R-410A residential split
Conditions measured on the service call:
- Refrigerant: R-410A
- High-side (liquid line) gauge pressure: 395 psig
- Liquid line temperature (at the sensing point): 107°F
- Manufacturer’s target subcooling: 10°F (from the data plate)
Step 1 — convert pressure to saturation temperature using the R-410A chart:
395 psig corresponds to a condensing saturation temperature of approximately 116°F.
Step 2 — calculate subcooling:
116°F − 107°F = 9°F
Step 3 — compare to target:
9°F versus a 10°F target, within the typical ±2°F tolerance → charge on target.
If the liquid line had measured 120°F instead, subcooling would be 116 − 120 = −4°F — a clearly undercharged condition, or a restriction upstream of the measurement point.
Interpreting the result
| Subcooling vs. target | Likely diagnosis |
|---|---|
| Well above target (+5°F or more) | Overcharged — excessive refrigerant backing up in the condenser |
| Slightly above (+2–4°F) | Borderline — confirm the reading is stable and ambient is typical |
| On target (±2°F) | Charge is correct at current operating conditions |
| Below target (−2–5°F) | Undercharged or a liquid-line restriction |
| Well below target (−5°F+) | Significant undercharge, liquid-line restriction, or TXV issue |
Why R-410A pressures are so different from R-22
R-410A operates at roughly 60% higher pressure than R-22 at the same condensing temperature. On a hot day when the condensing temperature might be 115°F, R-410A sits around 380–400 psig while R-22 sits around 225–240 psig. Using the wrong refrigerant’s PT chart gives a saturation temperature several degrees off — enough to misdiagnose a charge that is actually correct. Always confirm the refrigerant type on the data plate before reading pressures.
When not to use subcooling
Subcooling is the correct method for systems with thermostatic expansion valves (TXV) or electronic expansion valves (EEV). Fixed-orifice systems — those using a piston or capillary tube as the metering device — must be charged by superheat instead. On a fixed-orifice system, subcooling varies significantly with outdoor temperature and load, so a subcooling reading has no fixed target and cannot be used reliably. Always identify the metering device type before choosing a charging method.