# Superheat and Subcooling, Explained with Worked Examples

Superheat is how many degrees the suction vapor is above its saturation (boiling) temperature. Subcooling is how many degrees the liquid line is below its saturation (condensing) temperature. Measure pressure, convert it to a saturation temperature with a PT chart, then compare it with a line temperature. Together they tell you whether the charge and metering are right.

## Key takeaways

- Superheat = suction line temperature − saturation temperature at suction pressure.
- Subcooling = saturation temperature at liquid line pressure − liquid line temperature.
- On blends with glide like R-454B, use the dew column for superheat and the bubble column for subcooling.
- The target numbers come from the manufacturer's charging chart, not from a rule of thumb.
- Low superheat risks liquid reaching the compressor; high superheat means the evaporator is starved.

## Why superheat and subcooling matter on a service call

A gauge set gives you two pressures. On their own they don't tell you much: suction pressure moves with indoor load, head pressure moves with outdoor temperature. Superheat and subcooling turn those pressures into temperatures you can compare against the pipe. That comparison is how you tell an undercharge from a restriction, and a dirty coil from an overcharge, without guessing or topping off.

## What is superheat?

Superheat is the temperature of refrigerant vapor above the point where it finished boiling. Inside the evaporator, liquid refrigerant boils at a fixed saturation temperature set by the pressure. Once the last drop has boiled, any more heat the vapor picks up raises its temperature. That extra rise, measured on the suction line, is superheat.

**Superheat**

```
Superheat = T(suction line) − T(sat, dew) at suction pressure
```

Where:
- T(suction line) = pipe temperature near the outdoor unit's suction service valve
- T(sat, dew) = saturation temperature at suction pressure, from the PT chart's dew (vapor) column

## What is subcooling?

Subcooling is the temperature of liquid refrigerant below the point where it finished condensing. In the condenser, vapor gives up heat and condenses at the saturation temperature for the head pressure. Once it's all liquid, any more cooling drops its temperature below saturation. That drop, measured on the liquid line, is subcooling.

**Subcooling**

```
Subcooling = T(sat, bubble) at liquid pressure − T(liquid line)
```

Where:
- T(sat, bubble) = saturation temperature at liquid line pressure, from the PT chart's bubble (liquid) column
- T(liquid line) = pipe temperature near the liquid service valve

**Where each reading is taken**

|  | Superheat | Subcooling |
| --- | --- | --- |
| Pressure | Suction (low side, larger line) | Liquid line (high side, smaller line) |
| Temperature | Suction line, a few inches from the service valve | Liquid line, a few inches from the service valve |
| PT column for blends | Dew (vapor) | Bubble (liquid) |
| Usually used to charge | Fixed-orifice / piston systems | TXV and EEV systems |
| Target comes from | Manufacturer chart (indoor wet bulb, outdoor dry bulb) | Unit nameplate or installation instructions |

_Follow the equipment manufacturer's charging method when it differs from these general rules._

## Worked example: superheat on an R-454B system

> **Superheat (illustrative readings)**
>
> Suction pressure: **110 psig**. Suction line temperature: **52°F**.
>
> On the [R-454B PT chart](https://www.brix.so/reference/refrigerant-pt-chart/r-454b), 110 psig in the dew (vapor) column is a saturation temperature of 41.2°F.
>
> Superheat = 52 − 41.2 = **10.8°F**.

Now the mistake. If you read the bubble column instead, 110 psig converts to 38.5°F, and superheat comes out at 13.5°F. That 2.7°F error is R-454B's temperature glide (about 2.7°F at this pressure). On a fixed-orifice system charged to a superheat target, it could lead you to add refrigerant the system doesn't need.

## Worked example: subcooling on an R-454B system

> **Subcooling (illustrative readings)**
>
> Liquid line pressure: **330 psig**. Liquid line temperature: **98°F**.
>
> On the bubble (liquid) column, 330 psig is a saturation temperature of 106.0°F.
>
> Subcooling = 106.0 − 98 = **8.0°F**. Compare that with the subcooling target on the unit's nameplate or charging chart.

## What about single-component refrigerants?

R-32 and R-22 are single-component refrigerants, so the bubble and dew points are the same and there's only one column. At 125 psig, R-32 saturates at 41.7°F, so a 52°F suction line means 10.3°F of superheat whichever way you read it. R-410A is a blend too, but its glide is a fraction of a degree, which is why older charts showed one column.

## What do high and low readings mean?

**Common patterns (confirm with airflow and a full set of readings)**

| Reading | Superheat | Subcooling | Often points to |
| --- | --- | --- | --- |
| Undercharge | High | Low | A leak. Find and fix it before adding refrigerant. |
| Overcharge | Low | High | Too much refrigerant, often from topping off by pressure alone. |
| Restriction at the metering device | High | High | Liquid backing up before a blocked or underfed TXV, piston or filter drier. |
| Low indoor airflow | Low | Normal to high | Dirty filter or coil, slow blower. Check airflow before touching the charge. |

_These are general diagnostic patterns taught in HVAC programs, not rules. The manufacturer's troubleshooting chart for the unit takes precedence._

## Common mistakes

- **Charging by pressure alone.** Pressure changes with load and weather. Without the line temperature you don't know the charge.
- **Reading the wrong PT column on a blend.** Dew for superheat, bubble for subcooling. On R-454B the error is a couple of degrees.
- **Using the wrong refrigerant chart.** R-454B runs lower pressures than R-410A. Check the nameplate.
- **Clamping the probe in the wrong place.** Clean the pipe, insulate the probe, and measure close to the service valve where you read pressure.
- **Skipping airflow.** A dirty filter or coil changes both readings. Fix airflow first, then judge the charge.
- **Not letting the system settle.** Let it run long enough to stabilize before you record readings.

## Check yourself: 5 questions

1. An R-454B system reads 118 psig suction with a 55°F suction line. What's the superheat?
2. The same system reads 400 psig on the liquid line, and the liquid line is 112°F. What's the subcooling?
3. In question 1, what superheat would you get if you read the bubble column by mistake?
4. An R-22 system reads 70 psig suction with a 52°F suction line. What's the superheat?
5. Superheat is near zero and subcooling is high. Which is more likely: an undercharge or an overcharge?

> **Answers**
>
> 1. Dew point at 118 psig is 44.9°F, so superheat is 55 − 44.9 = **10.1°F**.
>
> 2. Bubble point at 400 psig is 120.3°F, so subcooling is 120.3 − 112 = **8.3°F**.
>
> 3. The bubble point is 42.1°F, giving **12.9°F**, which overstates superheat by the glide.
>
> 4. R-22 saturates at 40.9°F at 70 psig, so superheat is **11.1°F**. One column, no glide.
>
> 5. **Overcharge** (or very low indoor airflow, so check that first). Low superheat with high subcooling means too much liquid in the system.

### Cheat sheet: superheat and subcooling

**Superheat (suction side)**

Superheat = suction line temp − dew point at suction pressure.
High: starved evaporator (undercharge, restriction). Low: flooding risk (overcharge, low airflow).

**Subcooling (liquid side)**

Subcooling = bubble point at liquid pressure − liquid line temp.
High: liquid stacking in the condenser (overcharge, restriction). Low: undercharge.

**Blends with glide (R-454B)**

Dew column for superheat. Bubble column for subcooling.
Charge blends as liquid unless the manufacturer says otherwise.

**Always**

Targets come from the manufacturer's charging chart.
Fix airflow before judging the charge.

## Practice, licensing and what comes next

Refrigerant handling is regulated: EPA requires technicians who maintain, service, repair or dispose of equipment that could release refrigerant to hold Section 608 certification (40 CFR Part 82, Subpart F). Work through the [EPA 608 Core practice questions](https://www.brix.so/practice/epa-608/core) next, then see what your state requires. In Texas, for example, TDLR licenses air conditioning and refrigeration contractors and registers and certifies technicians: see [HVAC licensing in Texas](https://www.brix.so/licensing/hvac/texas).

### Keep going

- [R-454B PT chart](https://www.brix.so/reference/refrigerant-pt-chart/r-454b): Bubble and dew pressures with a quick lookup.
- [EPA 608 Core practice test](https://www.brix.so/practice/epa-608/core): Free questions with explanations.
- [HVAC licensing in Texas](https://www.brix.so/licensing/hvac/texas): The state licensing page pattern, with official sources.
- [HVAC From Scratch track](https://www.brix.so/academy/hvac): All modules and lessons in order.

Next in this module: recovery, evacuation and charging. Planning to run your own shop one day? The Tech-to-Owner path starts with [how to start an HVAC business](https://www.brix.so/start/hvac): licensing, startup costs, pricing and getting the phone to ring.

## Frequently asked questions

### What is the difference between superheat and subcooling?

Superheat is measured on the suction side and tells you how far the vapor is above its boiling point. Subcooling is measured on the liquid line and tells you how far the liquid is below its condensing point. Superheat describes the evaporator; subcooling describes the condenser.

### Do I use superheat or subcooling to charge a system?

Follow the manufacturer. As a general rule, fixed-orifice (piston) systems are charged by superheat using a target from indoor wet bulb and outdoor dry bulb, and TXV or EEV systems are charged by subcooling to the target on the nameplate or installation instructions.

### Why does R-454B have two saturation temperatures?

R-454B is a zeotropic blend of R-32 and R-1234yf. As it boils, the mix changes and the boiling temperature rises, so the bubble point (start of boiling) and dew point (end of boiling) differ by about 2.7°F at typical suction pressures. Use dew for superheat and bubble for subcooling.

### What is a normal superheat or subcooling?

There isn't one number for every system. The correct superheat depends on indoor and outdoor conditions and the metering device, and the correct subcooling is set by the manufacturer for each unit. Use the charging chart on the unit or in its installation instructions.

### Is superheat on the EPA 608 exam?

EPA's Section 608 certification covers refrigerant handling under the Clean Air Act, with Core, Type 1, Type 2 and Type 3 sections. Superheat and subcooling are core service skills taught alongside it in HVAC programs. Practice the refrigerant-handling questions on our free EPA 608 Core practice test.

## Related

- [R-454B PT Chart (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart/r-454b)
- [HVAC From Scratch: A Free HVAC Course](https://www.brix.so/academy/hvac)
- [Free EPA 608 Core Practice Test](https://www.brix.so/practice/epa-608/core)
- [How to Get an HVAC License in Texas: TDLR Registration, Certification and Contractor Classes](https://www.brix.so/licensing/hvac/texas)
- [EPA 608 Certification: Who Needs It and How to Get It](https://www.brix.so/certifications/epa-608)
- [How to Start an HVAC Business in 2026 (Costs, Licenses and First Customers)](https://www.brix.so/start/hvac)
- [R-410A PT Chart (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart/r-410a)
- [HVAC Software That Answers the Phone, Books the Tech and Keeps the Books](https://www.brix.so/for/hvac)

## Sources

- [U.S. EPA: Section 608 Technician Certification (40 CFR Part 82, Subpart F)](https://www.epa.gov/section608/section-608-technician-certification-0) (checked 2026-10-24)
- [Texas Department of Licensing and Regulation: Air Conditioning and Refrigeration](https://www.tdlr.texas.gov/acr/acr.htm) (checked 2026-10-24)
- [CoolProp 8.0.0 thermophysical property library (source of the PT values used in the examples)](https://coolprop.org/coolprop/HighLevelAPI.html) (checked 2026-10-24)
- [Daikin Comfort Technologies: A1 vs. A2L Pressure Temperature Chart (R-454B composition and cross-check)](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/academy/hvac/refrigeration/superheat-subcooling