Quick answer
Subcooling is the number of degrees by which liquid refrigerant leaving the condenser is cooler than its condensing (saturation) temperature at the measured liquid-line pressure, found as saturated liquid temperature minus liquid line temperature.
It shows how much liquid is backed up in the condenser. Systems with a thermostatic expansion valve (TXV) are charged to a target subcooling, and the number also helps diagnose restrictions and overcharge.
Quick facts
The key facts about Subcooling, with sources:
- Formula
- Subcooling = saturated liquid line temperature − liquid line temperature 1
- Maker example
- 417 psi → 120°F saturated; line 109°F; subcooling 11°F 1
- Used to charge
- Systems with a TXV ("charged to subcooling only") 1
- Example design value
- 8.0°F ± 1°F for one Goodman TXV model at 82°F outdoor, low stage 1
- R-32 saturation at 110°F (NIST)
- 389.58 psia (about 375 psig) 2
- Partner measurement
- Superheat, on the suction side
Key takeaways
- Subcooling measures how far liquid refrigerant is cooled below its condensing point.
- It equals saturated liquid temperature minus liquid line temperature 1.
- TXV systems are charged by subcooling; piston systems by superheat 1.
- Targets are model-specific and tight; one Goodman table allows ±1°F 1.
- Reading it requires connecting to the refrigerant circuit, which is EPA 608 certified work 3.
What subcooling means
In the condenser, hot refrigerant vapor gives up heat and turns to liquid at its condensation temperature. While vapor and liquid are mixed, the temperature stays at that point. Once it is all liquid, further cooling drops its temperature below the condensing point. That drop is subcooling.
A little subcooling is needed. It makes sure a solid column of liquid, not bubbles, reaches the expansion valve. Too much means excess liquid is stacked up in the condenser, often from overcharge or a restriction. Too little means the condenser is short of liquid, often from low charge.
Subcooling is the high-side partner of superheat.
How subcooling is measured
These steps follow a Goodman R-32 installation manual 1. Only a certified technician should connect to the circuit.
- Run the system in cooling until pressures settle, with correct airflow confirmed.
- Read the liquid line pressure at the service port.
- Convert that pressure to the saturation temperature for this refrigerant.
- Measure the liquid line temperature near the service valve.
- Subcooling = saturated liquid line temperature − liquid line temperature.
- Compare with the maker's design subcooling for that model and conditions.
The manual's example: liquid line pressure 417 psi, corresponding temperature 120°F, liquid line 109°F, so subcooling is 11°F 1.
Where subcooling happens
- Compressor → hot vapor enters the condenser
- → vapor cools to the condensing temperature
- → vapor and liquid mix at constant temperature while condensing
- → all liquid; further cooling below condensing point: subcooling
- → liquid line → expansion valve (needs bubble-free liquid)
Kinds of subcooling readings
| Reading | Measured at | Used for |
|---|---|---|
| Condenser (system) subcooling | Liquid line at the outdoor unit | Charging TXV systems 1 |
| Liquid line subcooling at the indoor coil | Before the TXV | Checking for line losses or flash gas |
| Design subcooling | Maker's table | The target value 1 |
Worked example: an R-32 subcooling reading
Saturation data for pure R-32 from NIST 2; formula and the ±1°F tolerance style from the Goodman R-32 manual 1. The gauge reading, line temperature and target are assumed.
| Step | Value | Source |
|---|---|---|
| Liquid line pressure (assumed) | 375 psig ≈ 389.58 psia | Gauge |
| Saturated liquid temperature | 110°F | NIST R-32 table 2 |
| Liquid line temperature (assumed) | 101°F | Clamp thermometer |
| Subcooling | 110 − 101 = 9°F | Formula 1 |
| Maker target (assumed) | 8°F ± 1°F | Design table |
| Verdict | 9°F is within 7–9°F | In range |
A cross-check worth knowing: NIST lists pure R-32 at about 429 psig at 120°F, while the manual's worked example pairs 417 psi with 120°F 2 1. The manual's numbers illustrate the method; for real readings, techs use the refrigerant maker's or instrument's pressure-temperature data for the exact refrigerant.
Diagnosis grid: reading subcooling and superheat together
Technicians rarely read one number alone. This grid shows the usual first suspects when airflow has already been checked. It is a starting point for a certified tech, not a home repair guide.
| Subcooling | Superheat | Usual first suspect |
|---|---|---|
| Low | High | Low charge, often a leak |
| High | Low | Overcharge |
| High | High | Restriction: filter drier, kinked line, stuck metering device |
| Low | Low | Metering device overfeeding, or low indoor load or airflow |
| Normal | Normal | Charge and metering likely fine; look elsewhere |
Why the charging method depends on the valve
A thermostatic expansion valve senses suction line temperature and opens or closes to hold superheat near its setting. Add refrigerant to a TXV system and the valve simply throttles back; superheat barely moves.
The extra refrigerant collects as liquid in the condenser, and subcooling rises. That makes subcooling the honest gauge of charge on a TXV system, which is why the Goodman manual says units with a TXV should be charged to subcooling only 1.
A fixed orifice cannot throttle. Extra refrigerant floods the evaporator and lowers superheat, so piston systems are charged by superheat instead. Getting this backwards is a common reason a system ends up overcharged.
Sources: [1]
Where you see subcooling
- On the outdoor unit's data plate or charging chart, as a target.
- In the maker's design superheat and subcooling table 1.
- On a start-up or service ticket.
- In digital gauge readouts that compute it.
What subcooling tells a technician
What it does well
- Whether a TXV system has the right charge.
- Whether solid liquid reaches the metering device.
- Clues to restrictions and non-condensables.
- Less sensitive to indoor load than superheat on a piston system.
Where it falls short
- Needs stable conditions and correct airflow.
- Only meaningful against the maker's target.
- Not the right charging method for piston systems.
Limits of a subcooling reading
Subcooling is reliable only after airflow is right and the system has settled. The Goodman manual also warns against adjusting charge from suction pressure except in a gross undercharge, and says to recover, re-evacuate and recharge by the data plate when an undercharge is suspected 1.
Blends add a wrinkle. R-454B has a temperature glide of about 1.5 K, so its condensing and boiling points differ slightly at the same pressure 5. Techs use the bubble-point data for subcooling with blends. Chemours notes the glide is low enough that R-454B can be topped off after leaks 5.
Why it matters for cost
Charging by subcooling on a TXV system gets the efficiency the unit was rated for. A wrong charge wastes energy and can stress the compressor. The check is part of a normal service call.
If subcooling points to low charge, the leak repair is the real cost. HyreHVAC does not print repair prices here; see refrigerant leak repair and the repair cost calculator.
What a good tech does with subcooling
- Confirm airflow and let the system settle.
- Measure subcooling and superheat together.
- Compare with the design table for the model and stage 1.
- If low, leak-search before adding refrigerant.
- Adjust charge in small steps and wait between readings.
- Record both numbers on the ticket.
Owner tasks that support a good charge
- Keep the outdoor coil clean; a dirty coil raises condensing pressure and skews the reading.
- Keep plants and fences at least a couple of feet from the outdoor unit so it can move air freely.
- Change indoor filters.
- Ask for subcooling and superheat on every service ticket.
- Do not add refrigerant or connect gauges yourself 3.
What high or low subcooling can mean
- Low subcooling: low charge (leak), or a condenser that is not full of liquid.
- High subcooling: overcharge, or a restriction such as a clogged filter drier or a stuck TXV holding liquid back.
- High subcooling with high superheat often points to a restriction rather than a charge problem.
- Signs at home: weak cooling, high bills, ice on lines; call a certified tech.
Rules around refrigerant service
Technicians who maintain, service, repair or dispose of equipment that could release refrigerants must be certified under EPA Section 608 3. EPA prohibits knowingly venting refrigerants during service, with narrow exceptions 4.
For R-32 and other A2L systems, the Goodman manual adds that personnel must be certified to work on units with flammable refrigerants, and that evacuation must reach below 500 microns and hold 1.
Subcooling vs superheat
| Measure | Side | Formula | Charges |
|---|---|---|---|
| Subcooling | Liquid (high) | Saturated liquid temp − line temp | TXV systems 1 |
| Superheat | Suction (low) | Line temp − saturated suction temp | Fixed-orifice systems 1 |
| Condensation temperature | High side | From liquid pressure | Input to subcooling |
| Refrigeration cycle | Whole loop | — | Context for both |
Common misconceptions
- Myth More subcooling is better.
- Reality Too much can mean overcharge or a restriction.
- Myth Subcooling works for every system.
- Reality Piston systems are charged by superheat 1.
- Myth A charge top-up fixes low subcooling.
- Reality The leak causing it must be found and fixed.
Outdoor temperature and the target
Makers state subcooling targets at a given outdoor temperature and stage; the Goodman TXV row lists 82°F for low stage 1. On a 105°F day in Texas or Arizona, readings shift, so a tech uses the maker's chart for actual conditions rather than one number from memory.
When this matters to you
If your system has a TXV and a tech says it is low on refrigerant, ask for the subcooling reading and the target. If they did not leak-search, ask why.
Next step: compare repair and replacement with the repair-or-replace calculator if leaks keep coming back.
Questions about Subcooling
What is subcooling in an AC?
It is how many degrees the liquid refrigerant leaving the condenser is below its condensing temperature at the measured pressure. It shows how much liquid is in the condenser and is used to charge TXV systems.
How do you calculate subcooling?
Read the liquid line pressure, convert it to the saturation temperature for that refrigerant, and subtract the measured liquid line temperature. A Goodman manual's example: 120°F saturated minus 109°F line equals 11°F.
What should subcooling be?
It depends on the model. Makers list a design value and tolerance; one Goodman TXV model lists 8°F plus or minus 1°F at 82°F outdoor on low stage. Always use the maker's chart or data plate.
What does low subcooling mean?
Usually the system is low on charge, often from a leak, so the condenser holds little liquid. A technician confirms with superheat and a leak search before adding refrigerant.
What does high subcooling mean?
Often overcharge, or a restriction such as a clogged filter drier or a metering device holding liquid back. High subcooling with high superheat points toward a restriction.
Why are TXV systems charged by subcooling?
A TXV adjusts itself to hold superheat fairly steady, so superheat says little about charge. Subcooling reflects how much liquid is stored in the condenser, which tracks the charge. The Goodman R-32 manual says TXV units should be charged to subcooling only.
Can I check subcooling myself?
No, not by connecting gauges. EPA requires Section 608 certification for refrigerant service, and A2L systems need technicians trained for flammable refrigerants. Ask your technician to record the reading.
Does subcooling apply to heat pumps in heating?
Yes, but the indoor coil is the condenser in heating, so the reading and target differ. Many makers recommend checking charge in cooling mode when weather allows.
Can a dirty outdoor coil change subcooling?
Yes. A dirty condenser coil sheds heat poorly, so condensing pressure and temperature rise and readings shift. A technician cleans or checks the coil and confirms outdoor airflow before judging the charge from subcooling.
Is subcooling the same on every refrigerant?
The method is the same, but the pressure-temperature data differ. Each refrigerant has its own saturation curve, and blends such as R-454B have a small glide. Using the wrong chart gives a wrong answer.
Sources
- Goodman / ACiQ, A/GPHM3 13.4 SEER2 M-Series packaged heat pump with R-32, installation instructions (IOG-3035B, 2025), retrieved .
- NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems: difluoromethane (R-32) saturation properties, 30°F–120°F, retrieved .
- U.S. EPA, Section 608 Technician Certification, retrieved .
- U.S. EPA, Stationary Refrigeration: Prohibition on Venting Refrigerants, retrieved .
- Chemours, Opteon XL41 (R-454B) Product Information (C-11001, 9/2016), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Formula, example, design table value and charging rule copied from the Goodman R-32 manual; R-32 saturation data from NIST; glide and top-off note from Chemours' R-454B sheet; certification and venting rules from EPA, read 10 October 2026. Gauge readings and the target in the worked example are assumed.
Suggest a correction. We fix errors and say what changed.