Pressures alone do not tell you whether a system is charged right. Two temperature differences do most of the work, and every new tech needs to be able to measure them, read them and act on them without guessing. This is superheat and subcooling explained the way you will use them on a service call: what each number means, how to get it, which one to charge by, and what to suspect when the readings look wrong.
One rule before anything else: the manufacturer's charging instructions on the unit beat any general guidance here, including ranges quoted in this article.
Start With Saturation Temperature
Both measurements compare a real line temperature with a saturation temperature. Saturation temperature is the temperature at which the refrigerant is boiling or condensing at a given pressure. Your gauges or digital manifold convert pressure to saturation temperature for the refrigerant you select, which is why choosing the right refrigerant on the manifold matters.
- In the evaporator, refrigerant boils at the low-side saturation temperature.
- In the condenser, refrigerant condenses at the high-side saturation temperature.
As long as liquid and vapor are both present, the refrigerant sits at saturation. Superheat and subcooling measure how far past that point it goes once it is all vapor or all liquid.
What Superheat Is and How to Measure It
Superheat is the number of degrees the refrigerant vapor has been heated above its saturation temperature after it has fully boiled off. A 2025 HVAC School tech tip on target superheat describes it that way and notes it is generally measured on the suction line leaving the evaporator.
Superheat = suction line temperature minus low-side saturation temperature
In practice: clamp a pipe thermometer on the suction line, read the low-side pressure, let the manifold convert it to saturation temperature, and subtract.
What superheat tells you
- Whether liquid could be reaching the compressor. Any superheat above zero suggests only vapor is in the suction line, within the accuracy of your tools.
- How much of the evaporator is working. Lower superheat means liquid is feeding further through the coil, using more of it. Higher superheat means less of the coil is doing useful work.
- Whether the compressor is being cooled. Suction vapor carries heat away from the compressor motor.
What Subcooling Is and How to Measure It
Subcooling is the number of degrees the liquid refrigerant has been cooled below its condensing saturation temperature after it has fully condensed. You measure it at the liquid line.
Subcooling = high-side saturation temperature minus liquid line temperature
Clamp a thermometer on the liquid line near the service valve, read the high-side pressure, convert to saturation temperature, and subtract the line temperature from it.
Subcooling tells you how much liquid is backed up in the condenser. That liquid seal is what feeds the metering device a solid column of liquid instead of a liquid-vapor mix.
Superheat and Subcooling Explained by Metering Device
Which number you charge by depends on the metering device. Building Science Corporation's information sheet on refrigerant charge maps it simply: fixed orifice systems use the superheat method, and TXV systems use the subcooling method.
| Fixed orifice (piston, cap tube) | TXV / EEV | |
|---|---|---|
| Charge by | Superheat, against a target | Subcooling, against the manufacturer's spec |
| Where the target comes from | A target superheat chart using indoor wet bulb and outdoor conditions, or the manufacturer's chart | The unit's data plate or installation instructions |
| How superheat behaves as load changes | Moves with load; drops as load drops and can hit zero at low load | Stays fairly steady because the valve adjusts |
| What superheat is still good for | Setting the charge | Checking whether the valve is feeding correctly |
| To lower the reading | Add refrigerant to lower superheat | Remove refrigerant to lower subcooling |
Fixed orifice: why the target moves
A piston cannot adjust its flow, so superheat swings with heat load on the coil. That is why you cannot use one fixed superheat number. The HVAC School article points to a target superheat chart that requires the return air wet-bulb temperature, which accounts for moisture load, and it stresses that chart is only for fixed orifice systems. The BSC sheet adds two practical thresholds: if the target superheat comes out at 5°F or below, you cannot reliably judge the charge, so leave it alone; if it is above 5°F, adjust until you are within about 2°F of target.
TXV: check superheat anyway
On a TXV, the valve controls superheat, so you set charge by subcooling. But superheat still tells you how the valve is doing. HVAC School notes a TXV should hold its set superheat if it receives a full column of liquid at enough pressure difference, often cited as at least 100 PSIG above evaporator pressure. Low superheat suggests the valve is overfeeding; high superheat suggests it is underfeeding. It gives general ranges such as 6 to 14°F for air conditioning, with the manufacturer's number taking priority.
Reading the Numbers Together
Superheat and subcooling are most useful side by side. Here is a starting point for interpreting them, based on the charge symptoms described in the BSC sheet. Treat these as clues, not a diagnosis.
| Superheat | Subcooling | First thing to suspect | Check before acting |
|---|---|---|---|
| Low | High | Overcharge | Indoor airflow, dirty evaporator, filter |
| High | Low | Undercharge (possible leak) | Icing, airflow, then leak search |
| High | High | Restriction or underfeeding metering device | Filter drier, kinked line, TXV bulb mounting |
| Low | Low | Overfeeding metering device or low load | Indoor airflow, TXV bulb contact, operating conditions |
The icing trap: The BSC sheet warns that ice on the evaporator coil can produce high superheat even when the charge is fine, because ice blocks airflow and starves part of the coil. If it ices fully, liquid and vapor can return to the compressor. Thaw it and fix the airflow problem before you judge the charge. Adding refrigerant to an iced coil is a classic new-tech mistake.
For more mistakes like this, see our list of first-year HVAC tech mistakes.
Measurement Mistakes That Give You Bad Numbers
Superheat and subcooling are only as good as the readings behind them. HVAC School points out that even the zero-superheat threshold depends on the accuracy of your tools. Most bad readings come from a short list of avoidable errors:
- Wrong refrigerant selected on the manifold. The saturation temperature will be wrong, so every calculation after it is wrong. Check the setting every time you connect, especially when you move between systems.
- Poor thermometer contact. Clamping over insulation, paint, oxidation or a fitting gives a reading that is off. Use clean, bare copper.
- Measuring in the wrong place. Superheat is taken on the suction line leaving the evaporator or at the outdoor unit as the manufacturer specifies; subcooling is taken on the liquid line. Keep thermometer and pressure readings close to each other.
- Not letting the system stabilize. Readings right after startup or right after adding refrigerant will drift.
- Uncalibrated tools. Check your thermometers against each other and your pressure sensors at atmospheric pressure before you trust them.
Why Getting Charge Right Matters
The BSC sheet explains the stakes. Overcharging can send liquid back to the compressor and drive up head pressure and temperature, which can overheat it and shorten its life. Undercharging lowers both capacity and efficiency. Either way, the customer pays: in comfort, in utility bills or in a compressor replacement.
It also matters for your career. Charging by feel or by pressure alone is one of the fastest ways to lose the trust of a lead tech. Being able to explain your superheat and subcooling readings, and why you did or did not adjust charge, is a big part of how you move up the HVAC career ladder.
Charge-Check Checklist
- Confirm the metering device type (piston, cap tube, TXV or EEV).
- Find the manufacturer's charging method and targets on the unit or in the install manual.
- Check the filter, indoor coil and blower, and make sure the coil is not iced.
- Run the system long enough to stabilize before taking readings.
- Select the correct refrigerant on your manifold.
- Clamp clean, accurate thermometers on bare pipe at the suction and liquid lines.
- Record suction pressure, liquid pressure, both line temperatures, superheat and subcooling.
- Fixed orifice: find target superheat from the chart and compare. TXV: compare subcooling to spec, and check that superheat is in a reasonable range.
- Adjust in small steps, let it settle, and re-measure.
- Write the final readings on the ticket.
The Bottom Line
Superheat tells you about the evaporator side; subcooling tells you about the condenser side. Charge fixed orifice systems by superheat against a real target, charge TXV systems by subcooling against the manufacturer's spec, and check both numbers on every call. Before you touch the charge, rule out airflow and icing. If the numbers do not make sense together, that is a diagnosis waiting to happen, not a reason to add refrigerant.