Formula
Superheat is the suction-line temperature minus the dew-point temperature at the suction pressure. Subcooling is the bubble-point temperature at the liquid-line pressure minus the liquid-line temperature. A zeotropic blend does not boil at one temperature. Liquid first appears at the bubble point and the last vapor disappears at the dew point. The gap is the glide. Superheat has to use the dew point, because that is the saturated vapor. Subcooling has to use the bubble point, because that is the saturated liquid. R-32, R-22, and R-134a are single-component fluids, so the two temperatures are the same. R-410A glide is about 0.2 °F. R-454B glide on the published chart is about 2 to 3 °F. R-407C glide is several degrees. The result panel shows both temperatures and names the one that was subtracted.
Superheat = T_line − T_dew(P)
Subcooling = T_bubble(P) − T_line
Target SH = (3 × WB − 80 − DB) / 2
Where the pressures come from
R-410A, R-32, R-22, R-134a, and R-407C are saturation pressures at each integer degree from −40 to 160 °F, generated with CoolProp 8.0.0 and interpolated in pressure. The checkpoints published against CoolProp 6.x land on these tables within the rounding those checkpoints used. R-454B is not a predefined CoolProp fluid. Its table is the Chemours Opteon XL41 chart C-XL41PT-ENG (7/23), saturation temperature at each integer psig from −2 to 400. A CoolProp mixture at 68.9/31.1 percent R-32/R-1234yf by weight differs by about half a degree, which is inside ±1 °F, so the published chart is the lookup. Pressure outside the table is an error. The page does not extrapolate. Line temperature has to sit between −40 and 200 °F.
Published PT charts are gauge pressure on top of 14.696 psia (101.325 kPa). A manifold gauge reads pressure above whatever atmosphere it is in, so chart psig = gauge psig + local psia − 14.696. At sea level that correction is zero. Altitude uses p = 101.325 (1 − 2.25577×10⁻⁵ Z)^5.2559 kPa, the same expression as the psychrometric page, then converts to psia. A typed barometer replaces both. kPa(g) and psig convert by 101.325/14.696. The atmosphere offset cancels, so a gauge reading converted to kPa(g) and back is the same superheat.
What the number is not
A negative superheat is shown, with a warning that liquid may be present at the measurement point. A negative subcooling is shown the same way. Neither warning says to add or remove a weight of refrigerant. The plain-language line only describes the band the number fell in. Fixed-orifice target superheat is (3 × indoor wet-bulb − 80 − outdoor dry-bulb) / 2. Indoor wet-bulb 63 °F and outdoor dry-bulb 85 °F give (189 − 80 − 85) / 2 = 12 °F. The page flags a result under about 5 °F, and it flags wet-bulb outside about 50–76 °F or outdoor dry-bulb outside about 55–115 °F. Those flags are not an OEM chart. TXV and EEV equipment is charged by subcooling from the installation instructions. R-410A is being phased down under the AIM Act. R-32 and R-454B are A2L refrigerants in new equipment. Anyone who opens the circuit needs EPA Section 608 certification.
Worked examples
R-410A superheat, sea level. Suction 118 psig, dew point 39.8 °F. Suction line 52 °F. Superheat 12.2 °F (6.8 °C). Bubble point on the same pressure is 39.7 °F.
R-410A subcooling, sea level. Liquid 365 psig, bubble point 109.7 °F. Liquid line 99 °F. Subcooling 10.7 °F. Dew point is 109.9 °F. Subcooling uses the bubble point.
R-32. 130 psig is about 43.8 °F. A 54 °F suction line is about 10.2 °F of superheat. 400 psig is about 114.7 °F. A 104 °F liquid line is about 10.7 °F of subcooling. Dew and bubble match.
R-22, older systems only. 68 psig is the classic point near 40 °F. 118 psig is about 68.4 °F.
R-454B glide. At 118 psig the chart dew point is 45.1 °F and the bubble point is 42.7 °F. At 365 psig the bubble point is 114.0 °F. Dew is warmer than bubble. Those chart temperatures are within 1 °F of a CoolProp mixture model of the same blend.
Target, fixed orifice. Indoor wet-bulb 63 °F and outdoor dry-bulb 85 °F give 12 °F. The same R-410A superheat example entered as kPa(g) and °C returns the same 12.2 °F, which is 6.8 °C of temperature difference.
Reference
Dew / bubble, °F, at sea-level gauge pressure. R-410A and R-32 are CoolProp 8.0.0. R-454B is Chemours Opteon XL41, C-XL41PT-ENG (7/23).
| psig | R-410A dew / bubble | R-32 | R-454B dew / bubble |
|---|---|---|---|
| 68 | 13.8 / 13.6 | 12.9 / 12.9 | 18.6 / 16.3 |
| 118 | 39.8 / 39.7 | 38.7 / 38.7 | 45.1 / 42.7 |
| 130 | 45.0 / 44.8 | 43.8 / 43.8 | 50.3 / 47.9 |
| 365 | 109.9 / 109.7 | 108.1 / 108.1 | 116.2 / 114.0 |
| 400 | 116.7 / 116.4 | 114.7 / 114.7 | 123.0 / 120.8 |
Questions
What is superheat?
Superheat is how many degrees the suction-line vapor is warmer than the dew-point saturation temperature at the suction pressure. Measure the pressure and the line temperature near the evaporator outlet or the service valve. Superheat = line temperature − dew point. It describes the reading. It is not an instruction to add or remove refrigerant.
What is subcooling?
Subcooling is how many degrees the liquid line is cooler than the bubble-point saturation temperature at the liquid-line pressure. Measure near the condenser outlet. Subcooling = bubble point − liquid-line temperature. TXV and EEV systems are charged by subcooling on the nameplate, not by the fixed-orifice target formula on this page.
What should superheat be?
It depends on the system and the manufacturer’s charging chart. A fixed-orifice or piston system is often checked with a target from indoor wet-bulb and outdoor dry-bulb: (3 × indoor wet-bulb − 80 − outdoor dry-bulb) / 2. At 63 °F wet-bulb and 85 °F outdoors that is 12 °F. Results under about 5 °F are flagged. That formula is not the charge method for a TXV or an EEV, and it is not a substitute for the chart shipped with the equipment.
Does a TXV charge by superheat or by subcooling?
By subcooling, using the manufacturer’s installation instructions and the nameplate. The TXV or EEV is what holds the superheat. A fixed-orifice or piston system is the one the target-superheat formula is written for. In both cases the manufacturer chart wins.
Why does R-454B have two saturation temperatures?
R-454B is a blend of R-32 and R-1234yf, 68.9/31.1 percent by weight, with about 2 to 3 °F of glide. At one pressure the liquid starts to boil at the bubble point and the last vapor condenses at the dew point. Superheat uses the dew point. Subcooling uses the bubble point. R-32 is a pure fluid, so those two temperatures match. R-410A glide is only about 0.2 °F. This page shows both and says which one it used.
What is a PT chart?
A pressure-temperature chart lists the saturation temperature of a refrigerant at each pressure. Gauges read psig, so the chart has to know the atmosphere it was built at. These tables use 14.696 psia (101.325 kPa). At altitude, the same gauge reading is a lower absolute pressure, and this page shifts the lookup by the difference. R-410A, R-32, R-22, R-134a, and R-407C are CoolProp 8.0.0 tables. R-454B is the Chemours Opteon XL41 chart C-XL41PT-ENG (7/23).
Do I need EPA 608 certification to work on refrigerant?
Yes. Under Clean Air Act Section 608, anyone who maintains, services, repairs, or disposes of equipment that could release refrigerant into the air must be certified. R-32 and R-454B are A2L, mildly flammable, refrigerants used in new equipment as R-410A is phased down under the AIM Act. Follow the manufacturer’s instructions and the local code. This page does not say how much refrigerant to add.
Sources
- Bell, Wronski, Quoilin, and Lemort, “Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp,” Industrial & Engineering Chemistry Research, 2014. Tables for R-410A, R-32, R-22, R-134a, and R-407C were generated with CoolProp 8.0.0 at 14.696 psia. NIST REFPROP is the property model those fluids are aligned to. This page does not ship REFPROP.
- Chemours Opteon XL41 pressure-temperature chart, document C-XL41PT-ENG (7/23). R-454B saturation temperatures are read from that chart. Composition stated there is R-32/R-1234yf 68.9/31.1 percent by weight.
- U.S. EPA Section 608 refrigerant management, Clean Air Act. Certification is required for anyone who opens a refrigerant circuit. EPA Technology Transitions under the AIM Act cover the R-410A phasedown and the move to lower-GWP refrigerants, including A2L refrigerants such as R-32 and R-454B.
- Indoor wet-bulb for the target formula can come from the psychrometric calculator. Room load estimates stay on the BTU, tonnage, and mini-split pages, and none of those is a charging chart.