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Superheat & Subcooling Calculator
Convert pressure and temperature readings into superheat and subcooling to diagnose charge.
Subcooling
10.3°Fmeasured
- SuperheatTarget 8–14°F
- 10.1°F
- Evaporator saturationInterpolated at 125 psig
- 44.9°F
- Condensing saturationInterpolated at 380 psig
- 115.3°F
- Condenser splitCondensing temp above ambient; 20–30°F is typical
- 20.3°F
Charge appears correct
Verify airflow before you touch the charge
EPA Section 608 certification is legally required
| Superheat | Subcooling | What it points to | Where to look |
|---|---|---|---|
| In range | In range | Charge is correct | Move on to airflow, blower speed and duct static |
| High | Low | Undercharge | Weigh in refrigerant after finding and repairing the leak |
| Low | High | Overcharge | Recover to the target; check for a previous top-off |
| High | High | Liquid line restriction | Filter drier, kinked liquid line, partially closed valve |
| Low | Low | Metering device overfeeding | Sensing bulb contact and insulation, valve superheat setting, wrong piston |
| High | In range | Restricted or undersized metering device | Compare the piston size against the nameplate |
| In range | Low | Non-condensables or a flooded condenser | Condenser coil, fan speed, and whether the system was evacuated properly |
Saturation temperatures are interpolated linearly between published pressure-temperature points spaced five to ten degrees apart, which holds accuracy well inside the tolerance of a field gauge set. R-410A, R-454B and R-32 values are dew point, which is what a suction gauge reads; R-454B carries roughly 2°F of glide, so a bubble-point subcooling reading on that refrigerant runs slightly cooler than shown here. Always charge to the equipment manufacturer's own chart when one is on the unit.
What this assumes
- Saturation temperatures are interpolated from published pressure-temperature data for the selected refrigerant.
- Fixed-orifice systems are evaluated on superheat; TXV and EEV systems are evaluated on subcooling.
- Assumes airflow has been verified first — charge cannot be judged on a system with restricted airflow.
Important
Only EPA Section 608 certified technicians may purchase, handle or recover refrigerant. Venting refrigerant to the atmosphere is a federal violation under the Clean Air Act.
Understanding the result
Verify airflow before you touch the charge
Low airflow produces readings that look exactly like undercharge. Adding refrigerant to a system with a dirty filter or a collapsed flex duct floods the compressor and destroys it. Check static pressure and temperature split first, every time.
Which number to trust
On a fixed-orifice system, superheat reflects charge and you charge to a target superheat from a chart based on indoor wet bulb and outdoor dry bulb. On a TXV or EEV system the valve holds superheat constant, so subcooling is the meaningful indicator — typically 8 to 12°F.
Reading the combinations
High superheat with low subcooling points to undercharge. Low superheat with high subcooling points to overcharge. High superheat with high subcooling suggests a liquid line restriction. Low superheat with low subcooling usually means the metering device is overfeeding.
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