Superheat calculator

How it works

Superheat is the suction line temperature minus the saturation temperature at the suction pressure. CEC Title 24 writes it as Actual Superheat = T(suction) − T(evaporator, sat) in Reference Appendix RA3.2.

The tool takes the gauge pressure, looks up the saturation temperature in the PT data for the refrigerant you pick and subtracts it from the line temperature. The data are 1°F tables computed from the equation of state, interpolated linearly between rows.

For blends it reads the dew point (saturated vapor) column. Refrigerant suppliers say the same thing: the Chemours PT guide marks saturated vapor as the value to “calculate Superheat”, and Honeywell’s procedure has you take pressure and temperature at the evaporator outlet and read the dew column. Single-component refrigerants have one saturation temperature, so there is no column choice.

The inputs:

• Refrigerant: any of the 62 on the site.
• Suction pressure in psig, read at sea level or corrected with the elevation field.
• Suction line temperature in °F, measured at the same point as the pressure.
• Metering device: fixed orifice/piston or TXV/EEV. This changes how the result is judged.
• Target superheat (fixed orifice only): from the target superheat calculator or the manufacturer’s chart.
• Elevation in feet. Gauges read against local air pressure, but PT charts in psig assume sea level. The tool converts your reading to the sea-level pressure the chart uses.

Worked example

The tool opens with R-410A at 118 psig suction and a 52°F suction line.

1. In the R-410A data the dew pressure is 116.09 psig at 39°F and 118.35 psig at 40°F.
2. Interpolate: 39 + (118 − 116.09) ÷ (118.35 − 116.09) = 39 + 1.91 ÷ 2.26 = 39.8°F dew.
3. Superheat = 52 − 39.8 = 12.2°F.

Now judge it on a fixed-orifice system. At 63°F indoor wet bulb and 85°F outdoor dry bulb the CEC target is 10.3°F. 12.2 − 10.3 = 1.9°F above target, which is inside ±5°F, so the charge passes at those conditions.

The same reading at 5,000 ft is different. Local air pressure there is about 12.23 psia against 14.70 at sea level, so 118 psig on the gauge equals 115.5 psig on a sea-level chart. That gives a dew point of 38.7°F and 13.3°F of superheat, a 1.1°F shift from elevation alone.

Blends with real glide make the column choice matter. Honeywell’s R-448A example uses 40 psig suction and a 26°F line. In our R-448A data the dew point at 40 psig is 16.5°F, so superheat is 26 − 16.5 = 9.5°F. Read the bubble column (5.4°F) by mistake and you get 20.6°F. That error is large enough to make you add refrigerant to a system that does not need it.

Reading the result

Fixed orifice or piston. Compare with the target. CEC allows ±5°F of target for the installer and ±8°F for the HERS/ECC Rater. Above target points to an undercharge or a restriction. Below target points to an overcharge or low indoor airflow. Check the filter, blower and coil before you move refrigerant: CEC’s correction rule (too high, add; too low, remove) assumes airflow is right.

TXV or EEV. The valve holds superheat roughly constant, so superheat does not tell you the charge. Charge by subcooling to the manufacturer’s target. Superheat is only a check that the valve works: CEC’s installer check passes at the manufacturer’s spec or between 4°F and 25°F. The tool flags very low superheat as a floodback risk and very high superheat as a starved evaporator.

Negative superheat means the line is colder than saturation. Either liquid is reaching the compressor or a reading is wrong: the wrong refrigerant selected, a probe not touching the pipe, or a gauge error.

No valid verdict. The superheat method only applies between 55°F and 115°F outdoors with return air at 70°F dry bulb or warmer and 76°F wet bulb or less, after 15 minutes of running. Below 55°F outdoors, CEC says to weigh in the charge. If the target table shows a dash, the target is below 5°F and the test cannot be done.

Common mistakes

• Using the bubble column on a blend. On R-407C, R-448A, R-449A and similar blends this overstates superheat by roughly the glide. See the temperature glide calculator.
• Charging a TXV system by superheat. Use subcooling.
• Taking temperature and pressure at different places. Measure both at the same point on the suction line.
• Ignoring gauge tolerance. CEC requires digital gauges accurate to ±3.5 psi on the suction side. Around 40°F on R-410A, 1°F of saturation is about 2.3 psi, so that tolerance alone is worth about ±1.5°F.
• Reading too soon. CEC wants 15 minutes of steady cooling before measuring and again after each adjustment.
• Forgetting elevation. Above a few thousand feet, an uncorrected gauge reads low against a sea-level chart. See how to read a PT chart.