Target Superheat

To determine the Target Superheat for an air conditioning system with a fixed orifice (such as a piston or capillary tube), measure the indoor WB (wet bulb) temperature with a digital psychrometer and the outdoor DB (dry bulb) temperature with a standard digital temperature reader. Input those temperatures into a superheat chart, calculator, app, or digital manifold to get the Target Superheat at that moment.

Remember that the target superheat will change as the building lowers in WB, and while charging refrigerant. The outdoor DB will generally stay the same while checking the charge, but it may fluctuate some.

Set the Actual Superheat as close to the Target Superheat as possible to have an accurate refrigerant charge.

Where to Measure

Measure the indoor WB as close as possible to the inlet of the evaporator coil, preferably in the return duct a few feet prior to the coil. Measure the outdoor DB roughly one foot away from the inlet of the outdoor coil, in the shade.

Target Superheat Chart

Example 1: Using the Chart

Indoor WB 66°F, Outdoor DB 90°F → Target Superheat of 13°F.

Find the target superheat on the chart, measure the actual superheat using the saturation temperature and the line temperature. If the actual superheat is too high, add refrigerant. If the actual superheat is too low, recover refrigerant. Remember that the target superheat will change as the system runs.

Example 2: Target Superheat Calculation

This calculation will get you close to the manufacturer superheat chart results:

Target Superheat = [(3 × WB) − 80 − DB] ÷ 2

With WB 64°F and DB 96°F:

[(3 × 64) − 80 − 96] ÷ 2
[192 − 80 − 96] ÷ 2
16 ÷ 2 = 8°F of Target Superheat

Videos

If you want to learn more about refrigerant charging methods, procedures, and troubleshooting, read the Refrigerant Charging and Service Procedures book by Craig Migliaccio.

See also the Target Superheat Calculator, the P/T Chart, and Charging Procedures.

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