Target Superheat Calculator (Fixed Orifice)
Free target superheat calculator for fixed-orifice (piston) HVAC systems. Enter indoor wet bulb and outdoor dry bulb to get the target superheat you should be charging to.
How target superheat works
A fixed-orifice (piston) metering device does not regulate superheat the way a thermostatic expansion valve does. As a result, the correct superheat for the system changes with the indoor load and the outdoor temperature. Instead of charging to a single number, you charge to a target superheat that you calculate from the current conditions.
A widely used field approximation is:
Target Superheat (°F) = (3 × Indoor Wet Bulb − 80 − Outdoor Dry Bulb) ÷ 2
You then compare your measured superheat (from the Superheat Calculator) to this target:
- Measured superheat above target → system is undercharged → add refrigerant.
- Measured superheat below target → system is overcharged → recover refrigerant.
Worked example
Indoor wet bulb is 63°F and outdoor dry bulb is 95°F:
(3 × 63 − 80 − 95) ÷ 2 = (189 − 175) ÷ 2 = 14 ÷ 2 = 7°F target superheat
If your measured superheat is 12°F against a 7°F target, the system is running about 5°F high — undercharged — so you’d add refrigerant and recheck.
Important cautions
- This formula is a field approximation. When the manufacturer publishes a target superheat charging chart, use it instead.
- The method is only valid for fixed-orifice / piston systems. Charge TXV/EEV systems to subcooling — see the Subcooling Calculator.
- Accurate wet-bulb and dry-bulb readings matter. At very low indoor loads or low outdoor temperatures the calculated target can fall near zero, which is a sign to stop and use the manufacturer chart.
How FieldPad helps in the field
Calculate the target here, then let FieldPad keep the whole charging record — measured conditions, target, final superheat — attached to the equipment and the client, ready to drop into a signed invoice. The next tech who pulls up this system sees exactly how it was charged last time.
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