Superheat and subcooling: what they are and how to calculate them

The idea in one paragraph

A refrigerant boils and condenses at a temperature fixed by its pressure. That temperature is the saturation temperature, read from a pressure–temperature (PT) chart. Once all the liquid in the evaporator has boiled, the vapor keeps picking up heat and its temperature rises above saturation: that rise is superheat. In the condenser the opposite happens: once all the vapor has condensed, the liquid keeps cooling below saturation: that drop is subcooling.

Superheat  = suction line temperature − saturation temperature at suction pressure
Subcooling = saturation temperature at liquid pressure − liquid line temperature

Where to measure

• Evaporator superheat: pressure and temperature at the evaporator outlet. Total superheat: at the suction line near the compressor (or the outdoor unit service valve on a split system). Manufacturer charging charts for residential split systems usually work with the reading at the outdoor unit.
• Subcooling: pressure at the liquid line service valve and the liquid line temperature at the same point, leaving the condenser.
• Clamp the temperature probe to clean copper and insulate it from the air, or the reading follows the room instead of the pipe.
• Use gauge pressure (psig or bar gauge) with a gauge-pressure PT chart. Mixing absolute and gauge pressure shifts the saturation temperature by about 14.7 psi (1 bar) worth.

Worked examples

Superheat, R-410A: suction pressure 118 psig, suction line 52 °F. The PT chart gives a saturation temperature of 39.6 °F, so superheat = 52 − 39.6 = 12.4 °F.

Subcooling, R-410A: liquid pressure 340 psig, liquid line 95 °F. Saturation is 104.7 °F, so subcooling = 104.7 − 95 = 9.7 °F.

Metric, R-32: suction 8.5 bar gauge, suction line 12 °C. Saturation is 5.0 °C, so superheat is 7.0 K. A temperature difference in kelvin equals the same difference in °C; to convert a °F difference, multiply by 5/9.

Which number to charge by

The metering device decides it. A fixed orifice (piston or capillary) cannot adjust, so the evaporator superheat changes with the charge: those systems are charged by superheat. A thermostatic expansion valve (TXV) or electronic valve holds the evaporator superheat roughly constant on its own, so superheat says little about the charge and those systems are charged by subcooling.

For fixed-orifice systems the target superheat is not a single number: it comes from the manufacturer's charging chart, read from the indoor wet-bulb and outdoor dry-bulb temperatures. For TXV systems the target subcooling is usually printed on the outdoor unit's data plate or in the installation instructions.

Typical values give a sense of scale: a residential TXV usually holds superheat around 8–12 °F, according to a long-standing ACHR News technical article. But the target for a specific system is always the manufacturer's, and our calculator does not suggest one.

How charge changes the numbers

On a fixed-orifice system, adding refrigerant floods more of the evaporator and lowers superheat; removing refrigerant raises it. On a TXV system the valve keeps superheat steady, and the extra refrigerant backs up in the condenser instead: adding refrigerant raises subcooling, recovering it lowers subcooling.

Airflow, dirty coils and non-condensable gases move both numbers too, so check filters, blower and coils before adding or removing refrigerant.

Blends with glide (R-454B, R-407C)

Zeotropic blends do not boil at one temperature: at a given pressure the liquid starts boiling at the bubble point and finishes at the dew point. For superheat use the dew-point temperature; for subcooling use the bubble-point temperature. R-454B has a small glide of about 2 °F, so our calculator uses the midpoint of the two and says so on the page; for tight work use a PT chart with separate dew and bubble columns.

Common mistakes

• Taking readings before the system has stabilized. Let it run until pressures and line temperatures stop drifting.
• Reading the PT chart for the wrong refrigerant, or a chart in absolute pressure.
• Charging a TXV system by superheat.
• Adding refrigerant to fix low suction pressure caused by a dirty filter or blower problem.
• Using the liquid line temperature at the indoor unit for subcooling. Measure where the pressure is measured.

Questions people ask