Please select a refrigerant.
Please select a mode.
Saturation temp will appear here
Please enter a valid pressure reading.
Use clamp thermometer on suction (or liquid) line
Please enter a valid line temperature.
Superheat
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⚠️ Disclaimer: This calculator uses standard PT chart reference data. Always wear PPE when working with refrigerants. Results are for diagnostic guidance only — verify against manufacturer specifications before making system adjustments.
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Sources & Methodology
PT chart saturation temperatures verified against ASHRAE refrigerant data and manufacturer PT references.
ASHRAE — Refrigerant Safety & Properties
ashrae.org — Authoritative source for refrigerant thermodynamic properties and PT chart data.
AC Service Tech — Superheat & Subcooling Methodology
acservicetech.com — Field methodology for measuring superheat and subcooling on residential HVAC systems.
Superheat = Actual Line Temp − Saturation Temp at measured pressure
Subcooling = Saturation Temp at measured pressure − Actual Line Temp
Saturation temperatures interpolated from standard PT chart reference points for each refrigerant. Gauge pressure (PSIG) is used.
Subcooling = Saturation Temp at measured pressure − Actual Line Temp
Saturation temperatures interpolated from standard PT chart reference points for each refrigerant. Gauge pressure (PSIG) is used.
Last reviewed: April 2026
How to Calculate HVAC Superheat and Subcooling
Superheat and subcooling are the two most important diagnostic measurements for HVAC refrigerant charge verification. Together they confirm whether a system is properly charged, overcharged, or undercharged — and can identify metering device problems, airflow issues, and other system faults without invasive inspection.
Superheat Formula
Superheat (SH) = T_line − T_sat
T_line = actual suction line temperature (clamp thermometer near evaporator outlet)
T_sat = saturation temperature at suction pressure (from PT chart)
Example — R-410A: Suction pressure 120 PSIG → T_sat ≈ 38°F. Actual line temp = 48°F. Superheat = 48 − 38 = 10°F (normal for TXV).
T_sat = saturation temperature at suction pressure (from PT chart)
Example — R-410A: Suction pressure 120 PSIG → T_sat ≈ 38°F. Actual line temp = 48°F. Superheat = 48 − 38 = 10°F (normal for TXV).
Normal Superheat and Subcooling Ranges
| Measurement | System Type | Normal Range | Action if Outside |
|---|---|---|---|
| Superheat | Fixed orifice / piston | 10°F – 25°F | Adjust charge based on target SH chart |
| Superheat | TXV / EEV | 8°F – 12°F | Adjust TXV superheat setting |
| Subcooling | TXV system | 10°F – 20°F | Add or remove refrigerant |
| Subcooling | Fixed orifice | Not primary | Use superheat method instead |
What High and Low Superheat Mean
High superheat (above 25°F on a fixed orifice system) typically signals low refrigerant charge, restricted metering device, low indoor airflow, or a refrigerant leak. The refrigerant boils off too quickly in the evaporator.
Low superheat (below 5°F) is a serious warning. Liquid refrigerant may reach the compressor — called liquid slugging — and can destroy it within minutes. Causes include overcharge, stuck-open TXV, or flooded evaporator. Never add refrigerant when superheat is already low.
Subcooling for TXV System Charging
For TXV systems, subcooling is the primary charging method. Measure pressure at the condenser outlet, convert to saturation temperature, then subtract the actual liquid line temperature. Target subcooling of 10–20°F confirms liquid refrigerant delivery to the TXV without flash gas.
💡 Pro Tip: Always run the system at least 15 minutes before taking superheat readings. A clogged filter causes artificially high superheat that looks like low charge. Verify airflow before adding refrigerant.
Frequently Asked Questions
Superheat is degrees of heat added to a refrigerant vapor above its boiling point at a given pressure. Measured at the suction line, it confirms refrigerant has fully vaporized before reaching the compressor. Normal range is 8-25 degrees F depending on system type.
1) Attach manifold gauge to suction line. 2) Read suction pressure in PSIG. 3) Convert to saturation temperature using the PT chart. 4) Measure actual suction line temperature with a clamp thermometer. 5) Superheat = Actual Temp minus Saturation Temp.
High superheat above 25 degrees F usually indicates low refrigerant charge, restricted metering device, or low airflow across the evaporator. Verify indoor filter and blower operation before adding refrigerant to avoid overcharging.
Low superheat below 5 degrees F means liquid refrigerant may reach the compressor, causing liquid slugging that can destroy it immediately. Causes include overcharge, stuck-open TXV, or excessive indoor airflow. Never add refrigerant if superheat is already low.
Superheat is measured on the suction (vapor) side at the evaporator outlet and confirms refrigerant is fully vaporized. Subcooling is measured on the liquid side at the condenser outlet and confirms refrigerant is fully liquid with no flash gas before the metering device.
For R-410A TXV systems, target superheat at the evaporator outlet is 8 to 12 degrees F. For fixed-orifice R-410A systems, target depends on indoor wet bulb and outdoor dry bulb temperatures, typically 10 to 25 degrees F.
R-410A at 120 PSIG suction pressure has a saturation temperature of approximately 38 degrees F. If your actual suction line temp is 48 F, superheat = 10 degrees F, which is normal for a TXV system.
Use a digital manifold gauge set with a built-in PT chart function. Modern manifolds display saturation temperature automatically when you select the refrigerant. This calculator also provides PT chart saturation temperatures for R-22, R-410A, and R-134a.
Subcooling is preferred for TXV system charging, not superheat. TXVs regulate their own superheat, so evaporator superheat is unreliable for charge diagnostics on TXV systems. Use liquid line subcooling of 10-20 degrees F instead.
Superheat fluctuates when the TXV is hunting, when variable-speed blowers change airflow, or during the first 15 minutes of system operation. Always let the system run 15-20 minutes before recording readings.
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