Physics of Buildings

Superheat and subcooling calculator

Gauge pressure and line temperature in; saturation temperature, superheat and subcooling out, for R-410A or R-32.

Defaults: R-410A at 118 psig suction with a 50°F suction line, and 356.5 psig liquid line at 98°F. Fill either pair, or both. Saturation temperatures come from published equations of state (dew point for superheat, bubble point for subcooling). The result is a measurement, not a charging target: compare it with the equipment maker’s data.

How it is calculated

A gauge reads pressure above the atmosphere. The property tables use absolute pressure, so the calculator first adds a standard atmosphere, 14.696 psi or 101.325 kPa [1]. It then looks up the temperature at which the refrigerant boils or condenses at that pressure: the saturation temperature, the number on a pressure–temperature chart.

superheat = Tsuction line − Tdew(Psuction)(1)
subcooling = Tbubble(Pliquid) − Tliquid line(2)

Superheat uses the dew point, the temperature at which the last liquid boils off; subcooling uses the bubble point, the temperature at which the first bubble forms. For R-32, a single substance, the two are the same. For R-410A, a blend, they differ by about 0.1 K, or 0.2°F [2], so either is fine in practice, but the calculator uses the right one for each side.

The saturation temperatures come from Lemmon’s 2003 equation of state for R-410A, with its dew- and bubble-pressure equations, and from Tillner-Roth and Yokozeki’s 1997 equation for R-32 [2][3], evaluated with CoolProp [4] and tabulated every 1 K. The table reproduces the equations’ saturation pressures to better than 0.001%, which is far below any gauge’s error.

Worked example

An R-410A system reads 118 psig on the suction line, and a clamp on the suction line reads 50°F. 118 psig is 132.7 psia. At that pressure R-410A’s dew point is 39.8°F, so the superheat is 50 − 39.8 = 10.2°F. On the liquid line, 356.5 psig is the pressure at which R-410A condenses at 108°F; a training example from HVAC School uses exactly this pair for a 15 SEER system on an 88°F day [5]. With the liquid line at 98°F, the subcooling is 108 − 98 = 10°F.

The same readings on an R-32 system give a different answer: at 118 psig R-32 boils at 38.7°F, so the same 50°F line means 11.3°F of superheat. Selecting the wrong refrigerant is one of the easiest ways to get a misleading number.

What the result does and does not tell you

Superheat and subcooling describe the state of the refrigerant at the point where they are measured. Whether a value is right depends on the system. Systems with a thermostatic expansion valve are usually checked by subcooling against the maker’s figure; Carrier’s heat pump labels, for example, give the required subcooling on the unit’s rating plate, within ±3°F [6]. Systems with a fixed orifice are usually checked by superheat, against a target that depends on the indoor wet-bulb and outdoor temperatures and can run from 5 to 40°F [7]. A typical TXV system shows 8 to 12°F of subcooling [7], but the equipment maker’s data decides. The essay explains what high and low combinations commonly point to; those are common causes, not a diagnosis.

Assumptions and valid range

  • Refrigerants: R-410A and R-32 only. R-454B, R-22 and others are not supported; their pressures and temperatures differ, and entering their readings here gives wrong answers.
  • Pressures are gauge pressures, converted with the sea-level standard atmosphere. At altitude the air pressure is lower, so the same gauge reading means a slightly lower absolute pressure than the calculator assumes; the error is small at the pressures in these systems, larger on the suction side than on the liquid side.
  • Accepted inputs: pressures from −14.6 to 800 psig (−100 to 5,515 kPa(g)) and temperatures from −60 to 200°F (−51 to 93°C). Saturation lookups cover about 3 to 630 psig for R-410A and 3 to 720 psig for R-32 (−54°F up to about 152°F or 161°F); outside that the calculator says so.
  • A reading on the wrong side of saturation (a suction line colder than the boiling point, a liquid line warmer than the condensing point) is reported in words rather than as a negative number, because it means liquid or vapour where there should be none, or a measurement problem.
  • This is a measurement aid for understanding, not a charging procedure or a diagnosis. Equipment makers’ instructions decide targets.

Gauges go on only in certified hands. 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; refrigerant may not be knowingly released, and only certified technicians may buy it [8]. If you are a homeowner, use these numbers to understand what your technician tells you.

Sources

  1. Thompson, A. & Taylor, B. N. (2008). Guide for the Use of the International System of Units (SI), NIST SP 811, Appendix B.8 Factors for units listed alphabetically. National Institute of Standards and Technology. standard Standard atmosphere = 1.01325 × 10⁵ Pa; 1 psi = 6.894757 × 10³ Pa (so 1 atm = 14.696 psi); T/K = (t/°F + 459.67)/1.8; 1 Btu_IT/lb = 2326 J/kg.
  2. Lemmon, E. W. (2003). Pseudo-Pure Fluid Equations of State for the Refrigerant Blends R-410A, R-404A, R-507A, and R-407C. International Journal of Thermophysics 24(4): 991–1006 (copy hosted by NIST). journal article R-410A = 50% R-32 / 50% R-125 by mass; Helmholtz-energy equation fitted to the full mixture model (densities within 0.1% except near the critical point; heat capacities 0.1% average, 0.5% max); dew and bubble pressures from ancillary Eq. 13 (Table VI), within 0.02–0.05% of the mixture model; valid 200–450 K; Table V check values (300 K: bubble 1.7404 MPa, dew 1.7351 MPa); dew and bubble curves of R-410A nearly coincident.
  3. Tillner-Roth, R. & Yokozeki, A. (1997). An International Standard Equation of State for Difluoromethane (R-32) for Temperatures from the Triple Point at 136.34 K to 435 K and Pressures up to 70 MPa. Journal of Physical and Chemical Reference Data 26(6): 1273–1328. doi:10.1063/1.556002. journal article Read via the NIST Chemistry WebBook fluid page for R-32, which uses this equation and states its estimated uncertainties: 0.05% for density, 0.02% for vapor pressure, 0.5–1% for liquid heat capacity and speed of sound.
  4. Bell, I. H., Wronski, J., Quoilin, S. & Lemort, V. (2014). Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp. Industrial & Engineering Chemistry Research 53(6): 2498–2508. doi:10.1021/ie4033999. software CoolProp (MIT licence), version 8.0.0, implements the Lemmon (2003) R-410A and Tillner-Roth & Yokozeki (1997) R-32 equations used to build this site's property tables.
  5. Saleeby, K. (HVAC School) (2021). “The Pressures Look Good.” How Do You Know?. HVAC School. trade training Evaporator design temperature difference (DTD) about 35 °F ± 3 below return air; condensing temperature over ambient (CTOA) 30 °F (6–9 SEER), 25 °F (10–12), 20 °F (13–16), 15 °F (17+); worked example for R-410A: 74 °F return, 88 °F outdoor → 39 °F evaporating (116.1 psig) and 108 °F condensing (356.5 psig).
  6. Carrier (2015). 25HBB5 Base Series Heat Pump with Puron Refrigerant: wiring diagrams and charging label. Carrier Corporation. manufacturer manual Required subcooling is given on the unit rating plate, with a ±3 °F tolerance; reversing valve solenoid and defrost control in the ladder diagram; relieve pressure and recover all refrigerant before system repair or final disposal.
  7. TruTech Tools (2012). A/C-Heat Pump System Troubleshooting Guide for the Testo 550. TruTech Tools, Akron, OH. trade training Tables of suction pressure, liquid pressure, superheat and subcooling direction for common faults on fixed-orifice and TXV systems (overcharge, undercharge, liquid restriction, low evaporator airflow, dirty condenser, low ambient, inefficient compressor, non-condensables, TXV bulb faults); typical TXV subcooling 8–12 °F, evaporator superheat 6–12 °F; fixed-orifice target superheat 5–40 °F depending on indoor wet bulb and outdoor dry bulb; DTD 35 °F, condenser +20 to +30 °F; check airflow first; measure both superheat and subcooling.
  8. U.S. Environmental Protection Agency (2020). EPA’s Refrigerant Management Requirements: What Technicians Need to Know (Section 608 fact sheet). U.S. EPA. government guidance Section 608 prohibits knowingly releasing refrigerant during maintenance, service, repair or disposal; requirements cover HCFCs and non-exempt substitutes such as HFCs and HFOs; only certified technicians can use non-exempt refrigerants to service AC and refrigeration equipment; refrigerant sales restricted to certified technicians; technicians must pass an approved certification exam.