What is Superheat?

Superheat is the temperature difference between the actual temperature of the refrigerant vapor and its boiling point (saturation temperature) at the same pressure. A positive superheat value confirms that all liquid refrigerant has fully boiled to vapor before reaching the compressor.

Compressors are designed to compress vapor, not liquid. Even a small amount of liquid refrigerant entering the compressor causes slugging, which can bend connecting rods and destroy valve reeds. Adequate superheat is the primary safeguard.

What is Subcooling?

Subcooling is the temperature difference between the condensing (saturation) temperature at the condenser pressure and the actual liquid line temperature. A positive subcooling value confirms that all refrigerant vapor has fully condensed to liquid before reaching the metering device.

Without adequate subcooling, refrigerant can flash to vapor in the liquid line or at the inlet of the metering device, reducing capacity and causing erratic TXV operation. Subcooling also provides a buffer for pressure drop in the liquid line piping.

Normal Ranges

Superheat target ranges:

• TXV / EXV systems: 8–14°F at the evaporator outlet (valve controls to its own setpoint, typically 10°F)
• Fixed-orifice / piston systems: 15–25°F measured at the service valve (varies with outdoor temp and load)
• Always check the equipment manufacturer’s specifications — some TXV systems target as low as 6°F

Subcooling target ranges:

• Typical systems: 8–14°F at the liquid service valve
• Systems with long liquid line runs: target the higher end (12–16°F) to compensate for line pressure drop
• Manufacturer spec sheets often list target subcooling; use them when available

Diagnostic Guide

Low Superheat (<5°F):

• Possible refrigerant overcharge
• TXV stuck open or set too low
• Restricted airflow across evaporator (dirty coil, blower issue)
• Oversized metering device

High Superheat (>20°F):

• Possible refrigerant undercharge (most common cause)
• TXV stuck closed or hunting
• Restricted liquid line, filter-drier, or metering device
• Low evaporator airflow starving the coil

Low Subcooling (<5°F):

• Possible refrigerant undercharge
• Condenser fan issue (dirty coil, low RPM, failed motor)
• Excessive ambient temperature at condenser

High Subcooling (>20°F):

• Possible refrigerant overcharge
• Restricted metering device (low superheat accompanies this)
• TXV bulb not making contact with suction line

Heat Pumps in Heating Mode

The reversing valve swaps the two coils. In heating the outdoor coil is the evaporator — it boils refrigerant by pulling heat out of cold outdoor air — and the indoor coil is the condenser, rejecting that heat into the house. Everything a cooling-mode diagnosis says about "the evaporator" now applies outdoors, and everything it says about "the condenser" applies indoors.

This moves the gauge ports. The vapor service valve at the outdoor unit carries hot discharge gas in heating, so a gauge on it reads the high side, not suction. Suction pressure and suction line temperature have to come from the compressor suction line or the accumulator port. Reading the outdoor vapor valve and calling it suction is the most common way a heating-mode superheat number ends up wrong by fifty degrees.

The arithmetic does not change: superheat is still line temperature minus saturation at suction pressure, subcooling is still saturation at liquid pressure minus liquid line temperature. What changes is the interpretation. Cooling-mode targets do not apply in heating. Treat the verdict as advisory: superheat at the compressor typically runs about 5–15°F, and subcooling measured at the outdoor unit runs higher than a cooling target and is not a charging target at all. Verify charge in cooling mode, by the manufacturer’s heating chart, or by weight. The one reading that is not advisory is superheat under 3°F — that is liquid heading for the compressor and it needs attention in any mode.

Outdoor coil frost and defrost cycles make heating readings unstable. A coil that is frosted, or a unit that has just come out of defrost, is not at steady state and nothing measured on it means much until it settles.

Zeotropic Refrigerants & Temperature Glide

Pure refrigerants and azeotropes (like R-410A) boil and condense at a single, constant temperature at a given pressure. Zeotropic blends (like R-407C) behave differently: they have a temperature glide, meaning the vapor and liquid phases exist at different temperatures at the same pressure.

For zeotropic refrigerants, two saturation temperatures are defined:

• Dew point — the temperature at which the last drop of liquid vaporizes (used for superheat)
• Bubble point — the temperature at which the first bubble of vapor forms (used for subcooling)

This calculator automatically uses the dew point for R-407C superheat calculations and the bubble point for subcooling calculations, matching industry-standard practice.

Safety Reminders

• Always wear safety glasses when working with refrigerant gauges
• Use calibrated manifold gauges appropriate for the refrigerant pressure range
• Never mix different refrigerants — always identify the refrigerant type before connecting gauges
• Recover refrigerant before opening any system — venting is illegal under EPA Section 608
• R-410A operates at nearly twice the pressure of R-22 — use high-pressure rated equipment
• Let the system run for at least 10–15 minutes under normal load before taking readings