Free superheat calculator: actual and target superheat
Enter the suction pressure and suction line temperature and get actual superheat for R-410A, R-454B, R-32, R-22 or R-134a. Add indoor wet bulb and outdoor dry bulb to get the target superheat for a fixed-orifice (piston) system and whether to add or recover refrigerant.
How to measure superheat
- Run the system in cooling for at least 10 to 15 minutes so pressures settle.
- Read suction pressure on the low-side gauge at the outdoor unit's vapor (larger) line service valve.
- Clamp a pipe thermometer on the suction line right next to the suction service valve, insulated from outdoor air.
- Convert suction pressure to saturation temperature with the PT chart (the calculator does this).
- Superheat = suction line temperature − saturation temperature.
Worked example
An R-410A system reads 118 psig on the suction side. That is a saturation temperature of about 40 °F. The suction line is 52 °F, so superheat is about 12 °F. With 63 °F indoor wet bulb and 85 °F outdoor dry bulb, the target is (3 × 63 − 80 − 85) ÷ 2 = 12 °F, so the charge is right.
Target superheat: fixed orifice only
On a fixed-orifice (piston or capillary) system, superheat changes with the charge, so you charge to a target that depends on indoor wet bulb and outdoor dry bulb. The calculator uses the common field formula, (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2, which approximates the manufacturers' charging charts. If the unit has its own chart, use that.
- Superheat above target usually means undercharged, or a restriction.
- Superheat below target usually means overcharged, or low indoor airflow (dirty filter or coil).
On TXV or EEV systems the valve holds superheat steady, so superheat says little about the charge. Charge those by subcooling.
Working on refrigerant circuits?
Opening the refrigerant circuit on stationary equipment using the refrigerants on this page requires EPA Section 608 certification, and suppliers can only sell you most refrigerants if you have it. See how to get EPA 608 certified or take the free EPA 608 practice tests.
FAQ
How do you calculate superheat?
Superheat = suction line temperature − saturation temperature at the suction pressure. Read suction pressure on the low-side gauge, convert it with a PT chart, and subtract it from the suction line temperature measured next to the suction service valve.
What should superheat be?
On a fixed-orifice system it depends on conditions: work out the target from indoor wet bulb and outdoor dry bulb, or use the manufacturer's chart. On TXV and EEV systems the valve controls superheat, so check the charge by subcooling instead.
Does superheat use the dew point or bubble point?
The dew point (vapor column) for blends like R-454B and R-410A. The calculator picks the right one.
What does high superheat mean?
Not enough refrigerant reaching the evaporator: usually an undercharge, or a restriction such as a clogged filter drier. Low superheat usually means overcharge or low airflow across the indoor coil.
Is this tool free?
Yes. No account, no email, no download. It runs in your browser and works on your phone at the unit.