Target Superheat Calculator

Target superheat is the superheat a fixed-orifice (piston or capillary-tube) air conditioner should read for the current operating load. It is set by two things: the indoor return-air wet-bulb temperature and the outdoor dry-bulb temperature entering the condenser. You add or recover refrigerant until the measured superheat matches the target. It applies to fixed-orifice metering only; TXV and EXV systems are charged by subcooling, not target superheat.

Target superheat is read from a charging chart indexed by indoor return-air wet-bulb and outdoor dry-bulb, not from a formula you keep in your head. The reverse-engineered field shortcut is [(3 x indoor wet-bulb) - 80 - outdoor dry-bulb] / 2, but it drifts up to about 7 degrees at the corners, so a chart is more accurate. This calculator interpolates the California Title 24 CEC RA3.2-2 target superheat table. Example: at 66F indoor wet-bulb and 90F outdoor dry-bulb, target superheat is about 13F.

Target superheat does not depend on the refrigerant. It is set by the load (indoor wet-bulb and outdoor dry-bulb), so the target for R-410A, R-454B, R-22, or R-32 on a fixed orifice is the same at the same conditions. What changes per refrigerant is how you read the measured superheat, because saturated suction temperature comes from that refrigerant's pressure-temperature chart. A chart labeled 410A target superheat is really the same load-based table with an R-410A PT scale attached.

There is no single good number: target superheat ranges from about 5F on a hot, dry day into the low 30s in mild, humid conditions with a high indoor wet-bulb, as the chart on this page shows. It rises as indoor wet-bulb rises (more latent load) and falls as outdoor dry-bulb rises. That is why a fixed-orifice system charged to a flat number like 12F is often wrong for the conditions. Charge to the chart target for the wet-bulb and dry-bulb you actually measure.

Actual (measured) superheat is what the system is doing right now: suction line temperature minus the saturated suction temperature from the refrigerant PT chart. Target superheat is what it should be doing for the current load. You compare the two: if measured superheat is more than about 5F above target the charge is low (add refrigerant after confirming airflow); if it is more than 5F below target the system is overcharged (recover).

No. A thermostatic or electronic expansion valve actively holds superheat near a fixed value on its own, so superheat tells you almost nothing about the charge on a TXV or EXV system. Charge those by subcooling instead. Target superheat is the charging method for fixed-orifice metering (pistons and capillary tubes), where superheat floats with the charge and the load.

As you add refrigerant the evaporator gets colder and pulls more moisture out of the indoor air, so the indoor return-air wet-bulb drops. A lower wet-bulb means a lower target superheat. Re-measure the wet-bulb and re-read the target after each addition rather than charging to the first number you looked up, or you will end up overcharged.

When the outdoor dry-bulb is high and the indoor wet-bulb is low, the CEC RA3.2-2 table has no target value because superheat would be below 5F, and a valid superheat charge check cannot be performed that close to floodback. The honest answer is to let the system run closer to design conditions, or charge by subcooling or weigh-in instead of guessing a low number. The calculator reports below minimum rather than inventing a value.

The values in this calculator come from the California Energy Commission 2022 Reference Appendices (publication CEC-400-2022-010-AP), Reference Residential Appendix RA3, Table RA3.2-2 Target Superheat. The whole 61-row table was parsed from the source PDF and checked against AC Service Tech's independent chart at 66F wet-bulb / 90F dry-bulb, where both read 13F. The fixed-metering superheat pass tolerance is plus or minus 5F per Table RA3.2-1.