Superheat Calculator
By BRIX Editorial Team · Updated
Superheat is the suction line temperature minus the refrigerant's saturation (dew) temperature at suction pressure. For example, R-410A at 115 psig saturates at 38.5°F, so a 58°F suction line means 19.5°F of superheat. Compare it with the target on the unit's charging chart, not a generic number.
- Saturation (dew) temperature at suction
- 38.5°F
- Suction line temperature
- 58.0°F
- Your target
- 10.0°F ± 3°F
- Measured minus target
- +9.5°F
- If you'd read the bubble column (wrong)
- 19.7°F
High superheat usually means the evaporator is starved: often an undercharge (look for the leak) or a restriction at the metering device or filter drier. Check subcooling to tell them apart. Rule of thumb only: follow the manufacturer's charging chart for the target and the method.
Key takeaways
- Superheat = suction line temperature − dew-point saturation temperature at suction pressure.
- On blends (R-454B, R-410A) use the dew (vapor) column. On R-32 and R-22 there's only one column.
- High superheat points to a starved evaporator; low superheat risks liquid reaching the compressor.
- The target comes from the manufacturer's charging chart. This calculator is a rule-of-thumb check, not a charging procedure.
How do you calculate superheat?
Measure suction pressure at the service valve, convert it to a saturation temperature with the refrigerant's PT chart, and subtract that from the suction line temperature measured at the same spot. The result is how many degrees the vapor has warmed after the last liquid boiled off in the evaporator.
Superheat (°F) = Suction line temperature − Saturation (dew) temperature at suction pressure
- Suction line temperature
- measured with a clamp probe near the suction service valve
- Saturation (dew) temperature
- from the PT chart at the suction gauge pressure; dew column for blends
- 1
Let the system run.
Give it time to stabilize with the indoor and outdoor units running and normal airflow.
- 2
Read suction pressure.
Attach the low-side gauge at the suction service valve and note the psig.
- 3
Measure the suction line.
Clamp a temperature probe on clean pipe near the service valve and insulate it from ambient air.
- 4
Convert pressure to saturation.
Look up the dew-point temperature for that pressure on the right refrigerant's PT chart, or let the calculator do it.
- 5
Subtract and compare.
Line temperature minus saturation temperature is superheat. Compare it with the target from the unit's charging chart.
Why the refrigerant matters
The same gauge reading means a different saturation temperature on each refrigerant. Here is 118 psig with a 52°F suction line, run through each chart:
| Refrigerant | Saturation (dew) temperature | Superheat |
|---|---|---|
| R-410A | 39.8°F | 12.2°F |
| R-454B | 44.9°F | 7.1°F |
| R-32 | 38.7°F | 13.3°F |
| R-22 | 68.3°F | -16.3°F |
What do high and low superheat mean?
| Reading | Superheat | Subcooling | Often points to |
|---|---|---|---|
| Undercharge | High | Low | A leak. Find and repair it before adding refrigerant. |
| Restriction (metering device, drier) | High | High | Liquid backing up before the restriction. |
| Overcharge | Low | High | Too much refrigerant, often from charging by pressure alone. |
| Low indoor airflow | Low | Normal to high | Dirty filter or coil, slow blower. Fix airflow first. |
Superheat on fixed-orifice vs. TXV systems
On a fixed-orifice (piston) system, superheat is usually the charging method: the target changes with indoor wet-bulb and outdoor dry-bulb temperature, and the unit's charging chart gives it. A TXV or EEV holds superheat roughly steady by itself, so those systems are usually charged by subcooling instead, with superheat as a check that the valve is feeding properly. Either way, use the method and targets in the installation instructions.
Common superheat mistakes
- Wrong chart. R-454B runs lower pressures than R-410A; R-32 runs slightly higher. Use the chart for the nameplate refrigerant.
- Wrong column on a blend. Superheat uses the dew (vapor) column.
- Probe in the wrong place. Measure the line temperature where you read pressure, not at the evaporator outlet with the gauge at the condenser.
- Not correcting for altitude. Gauges at elevation read higher than the sea-level chart. The refrigerant PT calculator has an altitude correction.
- Adding refrigerant to fix high superheat without checking subcooling. A restriction also causes high superheat, and more refrigerant makes it worse.
Frequently asked questions
What is a good superheat reading?
Whatever the unit's charging chart says for current conditions. On fixed-orifice systems the target changes with indoor wet bulb and outdoor dry bulb; on TXV systems superheat is usually a check, not the charging target. There's no single correct number for every system.
Do I use the dew or bubble temperature for superheat?
Dew. Superheat is measured on the suction line, where the refrigerant has finished boiling, so the dew-point saturation temperature applies. On single-component refrigerants like R-32 and R-22 the two are the same.
What causes high superheat?
A starved evaporator: most often an undercharge from a leak, or a restriction at the metering device or filter drier. Check subcooling to tell them apart. Low subcooling suggests an undercharge, high subcooling suggests a restriction.
What causes low superheat?
Too much liquid reaching the end of the evaporator, which can flood the compressor. Common causes are low indoor airflow (dirty filter or coil, slow blower) and overcharge. Check airflow before recovering any refrigerant.
Where do I measure suction line temperature?
On the suction line near the service valve where you read pressure, on clean copper, with the probe insulated from ambient air. Measuring far from the pressure tap adds error from line heat gain and pressure drop.
Does this superheat calculator work for R-454B?
Yes. It uses the dew column of our R-454B PT chart, which accounts for the blend's glide. It also covers R-410A, R-32 and R-22. Values are computed with CoolProp and checked against published manufacturer charts.
Related
- Subcooling CalculatorFree subcooling calculator: enter liquid line pressure and temperature for R-410A, R-454B, R-32 or R-22, using the bubble column, and check against target.
- Refrigerant PT Calculator (Pressure ↔ Temperature)Convert refrigerant pressure to saturation temperature and back for R-410A, R-454B, R-32 and R-22, with bubble and dew points and an altitude correction.
- Refrigerant PT Charts (Pressure-Temperature)Refrigerant pressure-temperature charts for R-410A, R-454B, R-32 and R-22, computed with CoolProp, plus a side-by-side comparison and how to read them.
- R-410A PT Chart (Pressure-Temperature)R-410A pressure-temperature chart from −40°F to 150°F in psig, computed with CoolProp, plus a quick lookup and what the 2026 EPA rule change means.
- R-454B PT Chart (Pressure-Temperature)R-454B pressure-temperature chart from −40°F to 150°F with bubble (liquid) and dew (vapor) psig, a quick lookup, °F/°C toggle and how to read glide.
- Superheat and Subcooling, Explained with Worked ExamplesSuperheat and subcooling explained with worked R-454B and R-32 examples: which PT column to use, the formulas, common mistakes and five check questions.
- AC Tonnage CalculatorRough AC tonnage calculator using the square-feet-per-ton rule of thumb, with a Manual J reality check. Ballpark only; size systems with ACCA Manual J.
- HVAC Software That Answers the Phone, Books the Tech and Keeps the BooksHVAC software that answers every no-heat and no-cool call, books the tech, follows up on replacement quotes, chases invoices and keeps QuickBooks clean.
- BTU CalculatorFree BTU calculator built on the ENERGY STAR room AC sizing chart, with sun, occupant and kitchen adjustments. A rule of thumb, not a Manual J load calc.
- HVAC Answering Service That Triages, Books and Pages Your On-Call TechAn AI answering service for HVAC that triages no-heat and no-cool calls, screens for gas smells and books the job instead of taking a message.
Sources
- CoolProp 8.0.0: thermophysical property library, used to compute the PT tables (checked October 24, 2026)
- Daikin Comfort Technologies: A1 vs. A2L Pressure Temperature Chart, USA (PM-A2LPTC-USA_04-24), used to check the tables (checked October 24, 2026)