How it works
What do superheat and subcooling tell a technician?
Two temperature differences, read at the outdoor unit, that show how far through each coil the refrigerant finished boiling and condensing. Drag the point where the last liquid boils off.
Superheat and subcooling are two temperature differences. Superheat is how much warmer the refrigerant vapour is than the temperature at which it boils at its pressure. Subcooling is how much cooler the refrigerant liquid is than the temperature at which it condenses at its pressure. Each one tells you where, inside a coil you cannot see into, the refrigerant finished changing phase. That is why technicians read both on almost every service call [1].
Superheat: vapour warmer than its boiling point
In the evaporator the refrigerant enters as a cold mix of liquid and vapour and boils as it takes heat from the air. While any liquid is left, the temperature stays pinned at the boiling point for that pressure: 40°F for R-410A at 118 psig [2][3]. The heat goes into boiling, not into warming. Once the last drop has boiled off, there is nothing to hold the temperature down, and the vapour starts to warm toward the air temperature for the rest of the coil.
The figure at the top shows that. Drag the point where the last liquid boils off. Move it back toward the inlet, as happens when the coil is fed too little refrigerant, and the vapour has a long stretch of coil to warm up in: the superheat rises. Move it to the very end, as happens when the coil is fed too much, and the vapour leaves still at its boiling point, possibly with liquid in it: the superheat falls to zero. A superheat reading is a way of seeing where that boiling ended without cutting the coil open.
That matters because the compressor is built to raise the pressure of a vapour [4]. A healthy superheat is a margin: proof that what is heading for the compressor is vapour. It is also a measure of how much of the coil is doing the most useful work. Boiling refrigerant takes in heat at a steady 40°F; superheated vapour, warming toward the air, takes in much less. Too much superheat means part of the evaporator is being wasted.
In practice superheat is read on the suction line at the outdoor unit: the suction pressure gives the boiling point (from a pressure–temperature chart or a calculator), and a clamp-on thermometer gives the line temperature. That “total” superheat includes whatever the vapour picked up in the suction line on its way outside, so it reads higher than the superheat right at the evaporator outlet; the Testo guide lists typical evaporator superheat as 6 to 12°F and total superheat as 8 to 20°F on TXV systems [1].
Subcooling: liquid cooler than its condensing point
The condenser runs the same story backwards. Hot gas from the compressor first cools to the condensing temperature, then condenses at that one temperature, 115°F for R-410A at 392 psig in the figure below, as it gives its heat to the outdoor air. Once the last vapour has condensed, the liquid cools a little further toward the air temperature before it leaves. The difference between the condensing temperature and the liquid line temperature is the subcooling.
Subcooling, then, is a measure of how much of the condenser is full of liquid. If the system is short of refrigerant, condensing finishes late, close to the outlet, and the subcooling falls; at zero, vapour bubbles can leave with the liquid. If the system holds too much, liquid backs up, condensing finishes early, and the subcooling rises. The liquid going to the metering device should be entirely liquid: vapour bubbles reaching it take up room the liquid needs, and the vapour that arrives can carry no more heat.
Subcooling is read at the outdoor unit too, on the small liquid line: the liquid line pressure gives the condensing temperature, and a clamp gives the line temperature. Typical TXV systems run 8 to 12°F of subcooling [1], and many manufacturers print the required value on the unit’s rating plate; Carrier’s heat pump labels give it with a ±3°F tolerance [5].
Why technicians measure both
Neither number alone tells you much. Superheat says how the evaporator is being fed; subcooling says how full the condenser is. Together they bracket where the refrigerant charge is sitting, and the pair moves in recognisable patterns when something is wrong. The Testo troubleshooting guide puts it plainly: watch both, because you cannot make any evaluation without checking both sides of the system [1].
Which one a technician leans on depends on the metering device. A thermostatic expansion valve (TXV) adjusts its opening to hold the evaporator superheat roughly steady, so on a TXV system the superheat stays near normal across a range of faults and the subcooling becomes the main indicator of charge [1][5]. A fixed orifice (a piston or capillary) cannot adjust, so its superheat swings with the charge and with the conditions; there the maker’s chart gives a target superheat for the indoor wet-bulb and outdoor temperatures, anywhere from about 5 to 40°F [1].
Both readings also depend on airflow and load. The same guide tells technicians to set and verify the airflow across the indoor coil first, and notes that low indoor load makes readings unreliable [1]. A wrong reading is a symptom of the whole system, not just of the refrigerant.
Superheat and subcooling on the P-h chart
On a pressure–enthalpy chart, superheat moves point ① to the right of the dome’s vapour edge along the low-pressure line, and subcooling moves point ③ to the left of the liquid edge along the high-pressure line. Both look like small changes. They are not free.
More subcooling pushes ③ left, so after the metering device ④ starts further left inside the dome: less of the refrigerant flashes to vapour, and each kilogram can soak up more heat in the evaporator. In the simulation, going from 10 to 20°F of subcooling raises the heat collected per kilogram by about 6% (with R-410A at the default conditions). More superheat adds a little heat collected in the evaporator, but it raises the temperature of the gas entering the compressor and so the discharge temperature, and in a real coil it means less of the coil is boiling. Neither number is something to maximise: the right values are the ones the equipment was designed for.
High and low: common causes, not a diagnosis
The patterns below come from a technician’s troubleshooting guide that lists how suction pressure, liquid pressure, superheat and subcooling move for common faults, separately for fixed-orifice and TXV systems [1]. Several faults share each pattern, the first cause in a row is only the most common one, and normal operating conditions shift the readings too. Each line is a list of things a technician checks, not a verdict; only measurements on the actual system, with correct airflow and enough load, narrow it down.
| Superheat | Subcooling | Common causes to check |
|---|---|---|
| High | Low | The classic pattern of too little refrigerant (an undercharge, usually from a leak); on a TXV system the valve may hold the superheat near normal while the subcooling falls. Also check: on a fixed-orifice system, whether the conditions call for a high target superheat (the target runs from about 5 to 40°F with indoor wet-bulb and outdoor temperature); heat picked up by a poorly insulated suction line, which raises the total superheat read outdoors; mixed or wrong refrigerant, or a reading taken with the wrong refrigerant’s chart [1]. |
| Low | High | The classic pattern of too much refrigerant (an overcharge); on a TXV system the superheat may read normal and the subcooling high. Also check: on a fixed-orifice system, whether the conditions call for a low target superheat (the target moves with indoor wet-bulb and outdoor temperature); mixed or wrong refrigerant, or a reading taken with the wrong refrigerant’s chart [1]. |
| High | High | A restriction in the liquid line, such as a blocked filter-drier; a metering device feeding too little, such as a TXV whose sensing bulb has lost its charge; a compressor that pumps poorly. On a fixed-orifice system, a cool outdoor temperature also raises both [1]. |
| Low | Low | On a fixed-orifice system, too little air across the indoor coil (a dirty filter or coil, a slow blower, a light load) or a dirty outdoor coil. On a TXV system, a sensing bulb that is loose or poorly insulated, so the valve overfeeds [1]. |
Two cautions. First, readings in the normal range do not prove a system is healthy: on a TXV system, low airflow across the indoor coil can show normal superheat and subcooling with low pressures [1]. Second, the “normal” range itself depends on the metering device, the outdoor and indoor conditions and the maker’s design, which is why the manufacturer’s chart or rating plate is the reference, not a rule of thumb [1][5].
Reading is not repairing. Connecting gauges to a refrigerant circuit, and adding or removing refrigerant, is work for a technician certified under Section 608 of the Clean Air Act; knowingly releasing refrigerant is prohibited, and only certified technicians may buy it [6]. Nothing on this page is a charging procedure. If your system is not cooling or heating as it should, a licensed HVAC technician confirms the cause before anything is changed.
Working the numbers out
Both readings need the saturation temperature at the measured pressure, from a pressure–temperature chart for the right refrigerant. Gauges read pressure above the atmosphere, and the property data use absolute pressure, so 14.7 psi is added first [7]. For a blend such as R-410A, superheat uses the dew point and subcooling the bubble point; for R-410A they differ by about 0.1 K (0.2°F) [2].
For R-410A at 118 psig the saturation temperature is 39.8°F, so a 50°F suction line means 10.2°F of superheat. At 356.5 psig it is 108°F, so a 98°F liquid line means 10°F of subcooling [3]. The superheat and subcooling calculator does this for R-410A and R-32 with the working shown, and the tour of the refrigeration cycle shows where in the loop each reading is taken.