Superheat and Subcooling Calculator (R-454B, R-32, R-410A, R-22, R-134a)

Choose the refrigerant first. Then fill in the suction pressure and the suction line temperature for superheat, and the liquid line pressure and liquid line temperature for subcooling. Both answers update as you type, along with the saturation temperature each one was measured against. On R-134a the suction gauge can read vacuum: set Gauge reads under Low side (suction) to inHg vacuum and type it as the gauge shows it.

Worked example: an R-410A system

Show the math

The calculator opens on R-410A with 120 psig on the suction side and a 52°F suction line. On the R-410A chart, the dew pressure is 118.36 psig at 40°F and 120.65 psig at 41°F. The reading is (120 − 118.36) ÷ (120.65 − 118.36) = 1.64 ÷ 2.29, or about 0.72 of the way up that one-degree step, so the dew point is 40.7°F. The suction line is warmer than that by 52 − 40.7 = 11.3°F, which is the superheat.

The liquid side reads 320 psig with a 92°F liquid line. The bubble pressure is 318.53 psig at 100°F and 323.11 psig at 101°F, so 320 psig lands 1.47 ÷ 4.58, about 0.32, of the way along. That gives a bubble point of 100.3°F, and the liquid line is 8.3°F below it: that's the subcooling.

Worked example: R-454B, where glide matters

Show the math

Switch the refrigerant to R-454B and enter a suction pressure of 108.545 psig, exactly halfway between the 40°F and 41°F dew rows, with a 52°F suction line. The dew point is 40.5°F and the superheat is 11.5°F.

For the liquid line, enter 305.44 psig, halfway between the 100°F and 101°F bubble rows, and a line temperature of 92.3°F. The bubble point is 100.5°F and the subcooling is 8.2°F. Every row behind these numbers is on the R-454B chart.

Why superheat uses the dew point

In a blend, the two saturation curves sit apart. At 120 psig, R-454B's bubble point is 43.1°F and its dew point 45.7°F. Measuring the suction line against the bubble point instead of the dew point would inflate superheat by the whole 2.6°F difference, and measuring the liquid line against the dew point would inflate subcooling the same way.

The vapor leaving the evaporator has finished boiling, so the dew point is the right reference there. The liquid leaving the condenser hasn't started to boil, so the bubble point is the right reference for it. R-410A's two curves are only about 0.2°F apart, so the choice barely moves its numbers. R-32, R-22 and R-134a have just one curve, so the choice doesn't arise.

Targets come from the manufacturer

The calculator reports the numbers and stops there. It doesn't say whether a system is overcharged, undercharged or correct, and it has no built-in target. The equipment manufacturer's charging instructions set the superheat or subcooling a given system should run at, and those depend on the unit and the conditions.

When a result is negative

A negative figure means the line temperature you entered is on the wrong side of saturation: a suction line colder than its dew point, or a liquid line warmer than its bubble point. Check the reading itself, and where and how the temperature probe is attached, before trusting it. The calculator shows the negative number with a note rather than rounding it up to zero, so nothing is hidden.

How it's calculated

For each pressure, the calculator finds the two whole-degree rows on either side and interpolates in a straight line between them. It rounds each saturation temperature to 0.1°F before subtracting, so the saturation temperature on screen and the superheat or subcooling next to it are worked from the same rounded figure. A pressure above or below a refrigerant's chart gives no result, only a note saying it's out of range.

The saturation data is the same CoolProp data used on the pressure–temperature chart pages. The table under the calculator lists the dew and bubble temperatures at common gauge pressures for all five refrigerants, for a quick check without typing anything.

Not sure which refrigerant's page to start from? Which PT chart do I need? routes all five, plus this calculator, from one table.