# Superheat Calculator

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.

## 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.

> **Rule of thumb: follow the manufacturer's charging chart**
>
> This tool does the arithmetic from our CoolProp PT tables. The target superheat, the tolerance and the charging method for a specific unit come from its installation instructions and charging chart, which always take precedence. Readings are assumed at sea level.

## 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**

```
Superheat (°F) = Suction line temperature − Saturation (dew) temperature at suction pressure
```

Where:
- 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.

> **Worked example (illustrative readings)**
>
> An R-410A system reads 115 psig at the suction valve with a 58°F suction line. The charging chart calls for 10°F ± 3°F.
>
> 115 psig on the R-410A dew column is 38.5°F. Superheat = 58 − 38.5 = **19.5°F**, 9.5°F above target.
>
> That's high: the evaporator is starved. Before adding refrigerant, check subcooling. Low subcooling too points to an undercharge and a leak to find; high subcooling points to a restriction.
>
> R-410A's glide is tiny: reading the bubble column instead gives 19.7°F. On R-454B the same mistake costs about 2.7°F.

## 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:

**Superheat from the same reading (118 psig, 52°F line) on four refrigerants**

| 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 |

_Computed with the calculator above from our CoolProp tables. R-410A, R-32 and R-454B land within about 6°F of each other, close enough that the wrong chart looks plausible and far enough to flip a charging decision. On R-22 the same reading gives negative superheat, which a running system can't have: a sure sign of the wrong chart. Check the nameplate._

## What do high and low superheat mean?

**Common patterns (confirm with subcooling and airflow before acting)**

| 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. |

_General diagnostic patterns taught in HVAC programs, not rules. The manufacturer's troubleshooting chart takes precedence._

## 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](https://www.brix.so/tools/subcooling-calculator) 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](https://www.brix.so/tools/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.

While you're on the roof reading gauges, BRIX answers the calls you can't, books the next job on your calendar and texts back anyone you miss.

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## 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 Calculator](https://www.brix.so/tools/subcooling-calculator)
- [Refrigerant PT Calculator (Pressure ↔ Temperature)](https://www.brix.so/tools/refrigerant-pt-calculator)
- [Refrigerant PT Charts (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart)
- [R-410A PT Chart (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart/r-410a)
- [R-454B PT Chart (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart/r-454b)
- [Superheat and Subcooling, Explained with Worked Examples](https://www.brix.so/academy/hvac/refrigeration/superheat-subcooling)
- [AC Tonnage Calculator](https://www.brix.so/tools/hvac-tonnage-calculator)
- [HVAC Software That Answers the Phone, Books the Tech and Keeps the Books](https://www.brix.so/for/hvac)
- [BTU Calculator](https://www.brix.so/tools/btu-calculator)
- [HVAC Answering Service That Triages, Books and Pages Your On-Call Tech](https://www.brix.so/for/hvac/ai-receptionist)

## Sources

- [CoolProp 8.0.0: thermophysical property library, used to compute the PT tables](https://coolprop.org/coolprop/HighLevelAPI.html) (checked 2026-10-24)
- [Daikin Comfort Technologies: A1 vs. A2L Pressure Temperature Chart, USA (PM-A2LPTC-USA_04-24), used to check the tables](https://www.chadwellsupply.com/globalassets/articles-pk/a2l/a2lptc-usa_04-24.pdf) (checked 2026-10-24)

Talk to BRIX: call or text (518) 588-1122, email eeharris2004@gmail.com, or book a 15-minute call at https://cal.com/ethanharris/15min.

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Written by BRIX Editorial Team. Last updated 2026-10-24.
Canonical: https://www.brix.so/tools/superheat-calculator