How the superheat calculation works
Superheat = suction-line temperature − saturation temperature. The saturation temperature is the boiling point of the refrigerant at the pressure on your low-side gauge, read off the P-T chart. On R-410A, 118 psig is about 40°F; a suction line reading 52°F means the vapor picked up 12°F of heat after it finished boiling in the coil. That is superheat. It proves the coil is fully boiling the refrigerant (no liquid reaching the compressor) without being starved.
Subcooling = saturation temperature − liquid-line temperature. Same lookup on the high side: the condensing temperature at your liquid-line pressure, minus what the liquid line actually measures at the condenser outlet. It tells you how much fully-condensed liquid is stacked up in the condenser, which is why it tracks charge on a TXV system.
Both readings need the system running 15 minutes or more so pressures settle, and an insulated clamp probe on clean copper. A bare probe in sunlight will lie to you by several degrees.
Which reading to charge by
- TXV or EEV: charge by subcooling. The valve holds superheat near 8–12°F no matter what, so superheat barely moves as you add refrigerant. Read both anyway: high superheat plus high subcooling points at a restriction, not the charge.
- Fixed orifice (piston, cap tube): charge by superheat against the target for today's indoor wet bulb and outdoor dry bulb. The Target tab uses the [(3 × WB) − 80 − DB] ÷ 2 approximation, which lands close to manufacturer charts; use the unit's own chart when you have it.
- Always: the number printed on the outdoor unit's data plate or install manual beats any rule of thumb. Type it into the optional target field and the verdict checks against it.
Saturation temperatures at common gauge readings
Generated from the same tables the calculator uses (CoolProp 7.2.0; R-22 from the DuPont saturation table). R-454B uses the dew column for suction and the bubble column for liquid readings.
| Suction psig | R-410A dew °F | R-454B dew °F | R-32 dew °F | R-22 dew °F |
|---|---|---|---|---|
| 90 | 26 | 31 | 25 | 53 |
| 100 | 31 | 36 | 30 | 59 |
| 110 | 36 | 41 | 35 | 64 |
| 118 | 40 | 45 | 39 | 68 |
| 125 | 43 | 48 | 42 | 72 |
| 135 | 47 | 52 | 46 | 76 |
| 145 | 51 | 56 | 50 | 81 |
| 160 | 56 | 62 | 55 | 87 |
| Liquid psig | R-410A bubble °F | R-454B bubble °F | R-32 bubble °F | R-22 bubble °F |
|---|---|---|---|---|
| 250 | 84 | 87 | 82 | 117 |
| 300 | 96 | 99 | 94 | 131 |
| 325 | 101 | 105 | 100 | 137 |
| 350 | 107 | 110 | 105 | 143 |
| 375 | 112 | 115 | 110 | 149 |
| 400 | 116 | 120 | 115 | — |
| 425 | 121 | 125 | 119 | — |
| 450 | 125 | 129 | 124 | — |
A dash means the reading is outside that refrigerant's table (R-22 runs far lower than the A2L refrigerants at the same temperature).
Quick answers
What is normal superheat for an air conditioner?
It depends on the metering device. A TXV system self-regulates to roughly 8–12°F of superheat. A fixed-orifice (piston) system has no fixed number: the target moves with indoor wet bulb and outdoor dry bulb, usually landing between 5 and 25°F. Use the Target tab to get today's number.
What should subcooling be on an R-410A system?
Most residential R-410A TXV systems target 10–12°F of subcooling at the condenser outlet; some high-efficiency units spec up to 16°F. The number on the nameplate or charging chart overrides any rule of thumb, and you are in spec within about ±3°F of it.
Which column do I use for R-454B: bubble or dew?
Dew (vapor) for superheat on the suction line, bubble (liquid) for subcooling on the liquid line. R-454B is a zeotropic blend with about 1.5°F of glide, so the two columns differ. This calculator picks the right column for you.
Do I charge a TXV system by superheat or subcooling?
Subcooling. A TXV holds superheat roughly constant no matter how much charge is in the system, so superheat will not tell you where the charge sits. Fixed-orifice systems are the opposite: charge those by superheat against the target for today's conditions.
What does negative superheat mean?
Liquid refrigerant is leaving the evaporator and heading for the compressor (flood-back). Shut it down and look at charge, the metering device, and indoor airflow before it damages the compressor.
How accurate is the target superheat formula?
The [(3 × indoor WB) − 80 − outdoor DB] ÷ 2 approximation lands close to manufacturer charging charts across normal conditions. When the manufacturer publishes its own chart or slide rule for the unit, use that number instead.
Sources & standards
Read the method
How to Calculate Superheat
The formula, where to clamp, and target ranges for fixed-orifice and TXV systems.
Read the guide →What Should My Subcooling Be?
Typical 10–12°F targets, when the nameplate overrides, and what high or low readings mean.
Read the guide →TXV: Charge by Superheat or Subcooling?
Why TXV systems charge by subcooling, fixed-orifice by superheat — and why you read both.
Read the guide →R-454B PT Chart
Full pressure-temperature table with bubble and dew columns, plus how glide changes your readings.
Read the guide →