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How to Use the Subcooling and Superheat Combo Calculator
Select the refrigerant from the nameplate and say whether the system has a TXV or EEV, or a fixed orifice or piston. The device sets the default targets, and you can change any of them.
Type the suction and discharge pressures from the manifold and the suction line and liquid line temperatures from clamp probes. Choose psig, psia or bar, and degrees Fahrenheit or Celsius.
Enter the target superheat and subcooling from the nameplate or the charging chart, and how far either side counts as on target. Add the site elevation if the gauges read above the local air.
Check superheat, subcooling, the pattern and the steps suggested. Add the project details and use PDF, CSV, Copy or Print.
Reading superheat and subcooling together
Superheat tells you what is happening at the evaporator outlet, and subcooling tells you what is happening at the condenser outlet. Either number alone can mislead, because several different faults push a single reading the same way. Read as a pair against targets, they separate a low charge from a restriction and an overcharge from a metering problem. This page takes the four readings you can get with a manifold and two clamp probes and does the arithmetic, the saturation lookups and the pattern match for you.
The four readings and where to take them
Suction pressure and suction line temperature give superheat. Discharge pressure and liquid line temperature give subcooling. Take the suction temperature near the compressor if you want total superheat, or at the evaporator outlet for evaporator superheat, and say which one you used when you compare it with a target. Clamp the probe to bare, clean copper and insulate it, and let the system run until the readings stop moving before you write them down. A system that has just started, or that is still pulling down, gives numbers that mean little. The same discipline applies at startup, and the HVAC commissioning checklist lists the readings to record on a new installation so there is a baseline to compare with later.
Charge by subcooling or by superheat
The metering device decides which number to trust. ACHR News on charging methods explains that a TXV system is charged to the subcooling leaving the condenser because the valve holds superheat, which it gives as fixed at 8°F to 12°F in most residential air conditioning. A fixed-orifice system is charged to the superheat leaving the evaporator instead. For a TXV, a low charge gives low subcooling and an overcharge gives high subcooling. For an orifice, a low charge gives high superheat and an overcharge gives low superheat. The calculator starts with a 10°F target for each on a TXV and leaves both blank for an orifice, because the right values come from the nameplate or the charging chart for that unit. The tolerance field sets how far either side of a target still counts as on target, and it starts at 2°F.
What the four patterns look like
In the normal example, R-410A at 118 psig suction and 360 psig discharge has a dew point of 39.8°F and a bubble point of 108.7°F. A suction line at 49°F and a liquid line at 100°F give 9.2°F of superheat and 8.7°F of subcooling, both within 2°F of a 10°F target. In the undercharge example the suction pressure falls to 100 psig and the discharge to 330 psig, the suction line warms to 52°F and the liquid line reads 98°F. Superheat is now 20.5°F and subcooling 4.5°F, which is high superheat with low subcooling. The overcharge pattern is the mirror image, and the two remaining patterns are a high reading on both, which points to a restriction that starves the evaporator and backs liquid up in the condenser, and a low reading on both, which points to an overfeeding valve or a very light load. These are patterns, not proof, so the page names a check for each before you move any charge.
Dew point, bubble point and glide
Blends do not boil at one temperature, so the lookup has to use the right end of the range. Sporlan's guide to refrigerant blends says to use the bubble point to calculate subcooling and the dew point to calculate superheat, and the calculator follows that rule. For R-410A at 118 psig the two points are 39.8°F and 39.7°F, so the choice hardly matters. R-404A at 60 psig has 23.5°F and 22.5°F. R-407C at 70 psig is 44.5°F against 33.6°F, and R-448A at the same pressure is 39.2°F against 28.4°F. On those blends, reading the wrong end shifts superheat or subcooling by about 11°F, which is far larger than a 2°F tolerance band. The saturation data comes from CoolProp 8, the same source as the saturation temperature calculator on this site.
How much gauge and probe error is acceptable
Superheat and subcooling are small differences between large readings, so small errors show up. A 3 psi error on a low-side R-410A reading near 118 psig moves the saturation temperature by 1.3°F, and the same error near 360 psig moves it by 0.6°F. On R-134a near 35 psig, 3 psi is worth 3.0°F, because low-pressure refrigerants change temperature faster with pressure. Add a probe that reads 1 to 2°F off, and a result that looks 3°F out of target may be within the instrument error. Zero the gauges to the air and use the elevation field when the site is high above sea level. Also check the pressure reading before you believe a very low superheat, since liquid reaching the compressor is what the compressor diagnosis tool helps you find after the damage is done.
Frequently Asked Questions
They are the usual pattern of an undercharge. With too little refrigerant, the evaporator is starved, so the vapour leaving it is more superheated, and the condenser holds less liquid, so subcooling falls. ACHR News makes the same point for a TXV system: undercharging decreases subcooling and increases superheat. In the example, an R-410A system reads 20.5°F of superheat and 4.5°F of subcooling against targets of 10°F. Find any leak before adding charge, and add it in small steps.
They point to an overcharge. Extra refrigerant backs liquid up in the condenser, which raises subcooling and the discharge pressure, and it can carry liquid into the suction line, which lowers superheat. In the example, 128 psig suction and 420 psig discharge on R-410A give 0.9°F of superheat and 14.1°F of subcooling. Superheat near zero means liquid may reach the compressor, so recover charge in small steps and watch the compressor.
Charge a TXV or EEV system by subcooling and a fixed orifice or piston system by superheat. ACHR News explains that a TXV system is charged to the subcooling leaving the condenser because the valve holds superheat, and that a fixed-orifice system is charged to the superheat leaving the evaporator. It gives typical TXV superheat as 8°F to 12°F in most residential air conditioning. The targets themselves come from the nameplate or the manufacturer's charging chart, so enter them in the calculator.
It uses the dew point of the suction pressure for superheat and the bubble point of the discharge pressure for subcooling. For R-410A, R-404A and R-507A the two points are within about a degree, but for a blend with glide the choice matters. At 70 psig, R-407C has a dew point of 44.5°F and a bubble point of 33.6°F, a gap of 11°F. The table also shows both points at your readings, so you can see the glide for any refrigerant.