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Calculator · Refrigerant charge
Superheat and subcooling calculator
Enter the gauge pressures and line temperatures from the outdoor unit. The calculator finds the saturation temperature on the correct line for the refrigerant, returns superheat and subcooling, and compares them with a fixed-orifice target or the nameplate subcooling. It runs in your browser with no daily limit.
- Select the refrigerant on the outdoor unit data plate.
- Enter the suction pressure and the suction line temperature measured at the outdoor unit.
- Enter the liquid line pressure and temperature for subcooling, and the nameplate target if the unit has a TXV or EEV.
- For a fixed orifice, add indoor wet-bulb and outdoor dry-bulb and compare the measured superheat with the target.
Example result for the default inputs
R-410A · sea-level gauge pressures · PT data from CoolProp
Superheat
12.2 °F
Subcooling
7.6 °F
| Suction saturation (dew point) | 39.8°F at 118.0 psig |
|---|---|
| Suction line temperature | 52.0°F |
| Liquid saturation (bubble point) | 104.6°F at 340.0 psig |
| Liquid line temperature | 97.0°F |
| Fixed-orifice target superheat | 12.0°F |
| Measured minus target | +0.2°F |
| Nameplate target subcooling | 10.0°F |
| Measured minus nameplate | -2.4°F |
- R-410A is a blend. Superheat uses the dew point line and subcooling the bubble point line.
For estimation purposes only. Consult a licensed engineer for final designs.
Free without signup, 3 questions a day
How superheat and subcooling are calculated
The calculator converts each gauge pressure to a saturation temperature and subtracts the pipe temperature you measured. Saturation temperatures come from pressure-temperature tables generated with CoolProp, an open thermophysical property library, for every whole degree from -40 to 140°F and interpolated in between. Blends get two lines. Superheat reads the dew point line and subcooling the bubble point line, the same convention manufacturer PT charts use.
- Superheat
SH = T_suction line - T_sat,dew(P_suction)Suction line temperature minus dew point saturation temperature, both taken at the suction service valve on the outdoor unit.- Subcooling
SC = T_sat,bubble(P_liquid) - T_liquid lineBubble point saturation temperature minus liquid line temperature, both taken at the liquid service valve.- Fixed-orifice target
SH_target = (3 x WB_indoor - 80 - DB_outdoor) / 2A field approximation of the fixed-orifice charging charts printed by equipment manufacturers. Those charts leave the target blank at very low values, and the calculator uses 5°F as that cutoff.- TXV and EEV systems
charge to nameplate SC; SH is set by the valveA metering valve holds its own superheat, so the charge is checked by subcooling against the value on the outdoor unit or in the installation instructions.- Gauge pressure
P_abs = P_gauge + 14.696 psiThe tables assume a sea-level atmosphere. At altitude a gauge reads higher for the same absolute pressure.
Worked example on an R-410A fixed-orifice system
A 3-ton R-410A split with a piston on a mild afternoon. Return air reads 63°F wet-bulb and the air entering the condenser is 85°F. On the low side the gauge shows 118 psig with the suction line at 52°F, and the high side shows 340 psig with the liquid line at 97°F.
R-410A · 118 psig suction · 340 psig liquid
- Suction saturation 39.8°F, superheat 12.2°F
- Liquid saturation 104.6°F, subcooling 7.6°F
- Fixed-orifice target superheat 12.0°F
Computed by the calculator above
Measured superheat is 0.2°F above the 12.0°F target. Check the tolerance on the manufacturer's charging chart for this unit before you call the charge correct. Subcooling is a cross-check here, because a fixed orifice is charged by superheat.
Saturation pressures used by the calculator, in psig at sea level. Blends show the dew point line first.
| Saturation | R-410A dew / bubble | R-454B dew / bubble | R-32 | R-22 |
|---|---|---|---|---|
| 20°F | 78.4 / 78.7 | 70.6 / 74.8 | 80.0 | 43.1 |
| 30°F | 97.0 / 97.4 | 87.8 / 92.7 | 99.1 | 55.0 |
| 40°F | 118.4 / 118.8 | 107.5 / 113.2 | 121.0 | 68.6 |
| 45°F | 130.1 / 130.6 | 118.3 / 124.5 | 133.0 | 76.1 |
| 50°F | 142.6 / 143.1 | 129.9 / 136.4 | 145.8 | 84.1 |
| 90°F | 274.5 / 275.3 | 251.9 / 262.3 | 281.3 | 168.4 |
| 100°F | 317.6 / 318.5 | 291.9 / 303.3 | 325.7 | 195.9 |
| 110°F | 365.4 / 366.5 | 336.3 / 348.6 | 374.9 | 226.4 |
| 120°F | 418.3 / 419.5 | 385.5 / 398.6 | 429.3 | 260.0 |
Standards and data sources
- ANSI/ACCA 5 QI, HVAC Quality Installation Specification - requires the refrigerant charge to be verified against the manufacturer's method, superheat for fixed metering devices and subcooling for TXV or EEV systems.
- Manufacturer installation instructions and charging charts - the target superheat or subcooling for the specific outdoor unit. They override any generic target on this page.
- CoolProp thermophysical property library - saturation pressures for R-410A, R-454B, R-32 and R-22 (Bell, Wronski, Quoilin and Lemort, Industrial and Engineering Chemistry Research, 2014).
- EPA Clean Air Act Section 608 - technician certification and refrigerant handling rules that apply whenever gauges go on a system.
- ASHRAE Standard 34 and UL 60335-2-40 - safety group A2L for R-454B and R-32, and the equipment requirements for mildly flammable refrigerants.
Limitations of this calculator
- Sea-level gauge pressures. At 5,000 ft the standard atmosphere in ASHRAE Handbook Fundamentals, Chapter 1, is about 2.5 psi lower, so a gauge reads about 2.5 psi high and the calculated superheat comes out low. On R-410A at a typical suction pressure that is about 1°F. On R-22 it is about 1.7°F.
- The fixed-orifice target is an approximation of manufacturer charts. When the unit has its own chart, charge to that chart.
- Readings are only as good as the clamp. A probe on insulation, on a fitting or in the sun can be off by several degrees.
- Non-condensables, a restricted filter drier or low indoor airflow change both numbers. The calculator reports what the gauges say and cannot tell those faults from a charge problem.
- PT data ends at 140°F saturation, which is about 540 psig on R-410A, 555 psig on R-32, 514 psig on the R-454B bubble line and 337 psig on R-22. Higher readings are outside the table and the calculator refuses them.
- When a reading here and the manufacturer's charging chart disagree, charge to the chart.
Common charging mistakes
- Superheat as the charge check on a TXV system. The valve holds its own superheat, so the number says little about the charge. On a TXV or EEV system, charge to the nameplate subcooling and use superheat to check the valve.
- The bubble line used for superheat on a blend. At one pressure the bubble point sits below the dew point, by about 2.7°F on R-454B at 110 psig in the table on this page. Superheat read off the bubble line comes out high by that much, and a tech charging a fixed orifice adds refrigerant the system does not need.
- Readings right after startup. Run the system for the stabilization time in the manufacturer's charging instructions, with the doors closed, before you record anything.
- Low indoor airflow mistaken for low charge. A dirty filter or a slow blower drops suction pressure. Check airflow and the coil before adding refrigerant.
- A clamp far from the gauge. Take the line temperature on clean bare copper within a few inches of the service valve where the gauge connects, insulated from ambient air.
Questions about superheat and subcooling
What should superheat be on a fixed-orifice system?
It depends on the indoor wet-bulb and the outdoor dry-bulb. The calculator uses the field approximation of the manufacturer charts, (3 x indoor wet-bulb - 80 - outdoor dry-bulb) / 2. At 63°F indoor wet-bulb and 85°F outdoor that gives 12°F, and at 67°F and 95°F it gives 13°F. Below 5°F the calculator shows no target, so check the unit's own chart before charging by superheat.
What should subcooling be?
Charge to the subcooling on that model's data plate or in its installation instructions. Enter it in the nameplate field and the calculator shows how far your reading is from it.
Why does R-454B have two saturation temperatures?
It is a zeotropic blend of R-32 and R-1234yf, so it boils over a range of temperatures at one pressure. At 40°F the dew point line is about 107.5 psig and the bubble point line about 113 psig in the tables on this page. Superheat uses the dew point and subcooling uses the bubble point.
Is R-410A a blend too?
Yes, but its glide is a fraction of a degree, so the dew and bubble lines sit within about 1 psi of each other at 40°F. The calculator still uses the correct line for each reading.
What does negative superheat mean?
Liquid is reaching the suction line or a reading is wrong. Check the clamp, the gauge and the refrigerant selected before you recover any charge, because a flooded compressor needs attention first.
Can I use these numbers at high altitude?
Yes, with a correction. Gauges read relative to local air pressure, which is about 2.5 psi lower at 5,000 ft. Subtract that from the gauge reading before you enter it, or expect the superheat on this page to read about 1°F low on R-410A and about 1.7°F low on R-22.
Does this calculator cost anything?
No. It runs in your browser with no daily limit. The BuildSolver chat is free to try without signup, and Pro adds room-by-room Manual J and Manual S equipment selection.
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