Superheat and subcooling are the two readings technicians use to verify a system's refrigerant charge by the numbers.
Superheat is suction line temperature minus saturated suction temperature; subcooling is saturated liquid temperature minus liquid line temperature — both definitions straight from Goodman's installation instructions.
Fixed-orifice systems are charged by superheat, TXV systems by subcooling, and both readings are judged against the equipment manufacturer's own target.
Here is what each measures, when each applies, and how to take them.
What is superheat?
Goodman's installation instructions for its GVXC20 condensing unit define superheat as suction line temperature minus saturated suction temperature.
In plain words: it is how far the vapor in the suction line has warmed past its saturation temperature.
The second half of that subtraction — the saturated suction temperature — is not something you measure with a thermometer on the line.
Saturation temperatures come from the refrigerant's pressure–temperature chart: match the suction pressure you read to the chart for the refrigerant in the system, and the chart returns the saturation temperature that goes with it.
(The cycle that makes one pressure map to one temperature is its own topic — our guide to how the refrigeration cycle works covers it.)
How to calculate superheat, step by step:
- Read the suction pressure.
- Convert that pressure to the saturated suction temperature using the pressure–temperature chart for the refrigerant in the system.
- Measure the suction line temperature.
- Subtract: suction line temperature minus saturated suction temperature is superheat.
On a fixed-orifice system, that number is also the adjustment gauge: Goodman's older instruction sheet for its split-system condensing units teaches the direction of the fix — adding refrigerant lowers superheat, and removing refrigerant raises it, until the reading matches the manufacturer's superheat table.
What is subcooling?
Subcooling is the mirror reading on the other side of the cycle.
The same Goodman instructions define it as saturated liquid temperature minus liquid line temperature — how far the liquid leaving the condenser has cooled below its saturation temperature.
Notice the two definitions run in opposite directions.
Superheat subtracts the saturation temperature from the line temperature; subcooling subtracts the line temperature from the saturation temperature.
Same pressure–temperature chart, same thermometer work, opposite order — which is why the two are easy to mix up, and why the subtraction direction is worth double-checking whenever a number looks wrong (more on that below).
| Reading | Formula (Goodman's definition) | Taken on | Where it sits in the cycle |
|---|---|---|---|
| Superheat | Suction line temp − saturated suction temp | Suction line | The suction side — vapor leaving the evaporator, headed back to the compressor |
| Subcooling | Saturated liquid temp − liquid line temp | Liquid line | The liquid side — refrigerant leaving the condenser, where it released the collected heat outdoors |
The component roles in that last column are DOE's: its Home Cooling 101 guide describes the evaporator removing heat and humidity from indoor air, the condenser releasing the collected heat outdoors, and the compressor pumping refrigerant between the two.
Neither reading means anything without a target to compare it to, and the target belongs to the equipment — which is what the next section sorts out.
When do you charge by superheat vs subcooling?
The metering device decides.
ACCA's installation-verification protocol, ANSI/ACCA 9 QIvp, requires the subcooling test on systems with thermal expansion valves and the superheat test on systems with fixed metering devices — a fixed orifice, capillary tube or piston — leaving the other test optional in each case.
Goodman's own instructions teach the same split from the manufacturer's side.
Its older instruction sheet for R-22 split-system condensing units says capillary-tube and fixed-orifice systems are charged by superheat using the manufacturer's superheat table, while expansion-valve systems are charged by liquid-line temperature below saturation — subcooling.
| Metering device | Required charge test (ACCA 9 QIvp) | Judged against |
|---|---|---|
| Thermal expansion valve (TXV) | Subcooling | OEM-specified subcooling, ±3°F |
| Fixed orifice · capillary tube · piston | Superheat | OEM-specified superheat, ±5°F |
Both tolerances are relative, not absolute: charge passes when the measured superheat is within ±5°F of the OEM-specified superheat, or the measured subcooling is within ±3°F of the OEM-specified subcooling — the value the manufacturer specifies for that model, which is why you look the target up in the equipment's documentation instead of memorizing one number.
Goodman's GVXC20 instructions, for example, give that model a target system subcooling of 8°F ± 1°F with a TXV indoor coil — 10°F ± 1°F for 5-ton units — and below 65°F outdoor ambient they say to weigh in the charge rather than set it by subcooling.
That is Goodman's number for that model line; the number for the unit you are on comes from its own documentation.
Charging by the numbers is one skill inside the trade's technical set — the HVAC technician skills guide maps the rest, from electrical troubleshooting to airflow.
The OEM spec sheet is the target
How do you measure superheat and subcooling?
The conditions around the measurement come first, because a charge reading taken at the wrong moment is a number you can't trust.
ACCA 9 QIvp's charge-verification form says to run the system at least 10 minutes — or until readings stabilize — before measuring, and not to test cooling mode below 60°F outdoor air for the subcooling test or below 55°F for the superheat test.
Its footnotes add that heat pumps below those outdoor temperatures may be checked in heating mode, per the OEM's instructions.
Goodman's subcooling procedure for the GVXC20 shows the measurement itself, step by step: read the liquid pressure, convert it to temperature with a pressure/temperature chart, measure the liquid line temperature at the liquid service valve, and subtract to get subcooling.
The superheat measurement mirrors it on the suction side — suction pressure to the chart for saturated suction temperature, suction line temperature at the line, subtract as above.
One more requirement sits on the same verification form: airflow.
ACCA 9 QIvp records contractor-recorded airflow on its own line, and it must be within 15% of design airflow — a separate check from the charge readings, on the same form.
That loop — set the conditions, take both readings, compare each against the OEM's number for the model — is what ACCA's installation-verification protocol formalizes, and it is a habit worth building from your first charged system as a working HVAC technician.
Inverter equipment needs its charge mode
Common mistakes apprentices make with the readings
The checks on ACCA's charge-verification form double as a list of the ways a reading goes wrong before it is even taken.
Each mistake below maps to a requirement the standards actually state — which makes them the right things to self-audit while you are learning.
- Charging to a memorized number. ACCA 9 QIvp grades superheat within ±5°F and subcooling within ±3°F of the OEM-specified value for the model — Goodman's GVXC20, for instance, targets 8°F ± 1°F subcooling, and 10°F ± 1°F on its 5-ton models. Look the target up in the OEM's instructions before you connect gauges.
- Subtracting in the wrong order. Goodman's two definitions run opposite ways: superheat is line temperature minus saturation; subcooling is saturation minus line temperature. Flip the order and you report a nonsense number with confidence.
- Measuring too early. ACCA 9 QIvp's form says to run the system at least 10 minutes — or until readings stabilize — before measuring.
- Charging a system in the cold. ACCA 9 QIvp's form says not to test cooling mode below 60°F outdoor air for subcooling or below 55°F for superheat; its footnotes let heat pumps be checked in heating mode per the OEM's instructions.
- Using the wrong test for the metering device. Under ACCA 9 QIvp, TXV systems take the subcooling test; fixed orifice, capillary tube and piston systems take the superheat test.
- Overlooking the airflow check. ACCA 9 QIvp's form records contractor-recorded airflow on its own line, and it must be within 15% of design airflow.
- Reading an inverter system at partial capacity. Set the OEM's charge mode first; Goodman's GVXC20 subcooling readings, for one, are valid only at 100% compressor speed.
One more thing separates apprentices from the rest of the crew on these calls: the refrigerant circuit itself is regulated, and EPA's rule lets you work on one without your own 608 certification only while you are closely and continually supervised by a certified technician — the supervising technician and the apprentice share responsibility for compliance.
For more on that supervised route into the trade, our guide for HVAC apprentices covers it.
Trade information, not legal advice: the numbers that govern a charge come from each equipment manufacturer's instructions for the specific model — confirm the target against the OEM documentation, and the required tests against the standards your employer works to.