Subcooling is the liquid-side twin of superheat. In the condenser, hot refrigerant vapor gives up heat and turns back into liquid. Once it's all liquid, it can keep cooling below its condensing temperature. That extra drop is the subcooling, and it tells you how much liquid is backed up in the condenser.
Subcooling in plain English
The condenser holds a mix: vapor coming in at the top, liquid draining out at the bottom. The more refrigerant in the system, the more of the condenser fills with liquid, and the longer that liquid sits in the outdoor air cooling off. So more charge usually means more subcooling.
DOE's Building America measure guideline defines it as the difference between the liquid line refrigerant temperature and the liquid saturation temperature at the measured pressure1. A healthy number means the metering device is getting a solid column of liquid, not a mix of liquid and flash gas.
How do you calculate subcooling?
- 1
Run the system 15 minutes
Let pressures settle before reading, as ENERGY STAR's checklist and DOE's guide both require.
- 2
Read liquid line pressure
Connect the high-side gauge to the liquid line service port.
- 3
Convert to condensing saturation temperature
Use the PT chart for the refrigerant on the nameplate. On a blend, use the liquid (bubble) side unless the manufacturer says otherwise.
- 4
Clamp a thermometer on the liquid line
Near the service valve, on clean pipe, insulated from the air.
- 5
Subtract
Saturation temperature minus liquid line temperature is the actual subcooling.
Formula
Worked example (example numbers)
An R-32 system with a TXV. NIST's saturation data puts R-32 at about 105°F when the gauge reads about 350 psig (roughly 365 psia at sea level)2. The nameplate target is 10°F (example).
| Reading | Value |
|---|---|
| Liquid line pressure | 350 psig |
| Saturation temperature from PT data | 105°F |
| Liquid line temperature | 95°F |
| Subcooling | 105 − 95 = 10°F |
| Deviation from 10°F target | 0°F: pass |
If the line had read 99°F, subcooling would be 6°F, which is 4°F under target and outside ENERGY STAR's ±3°F band.
Free calculatorSuperheat, subcooling & target superheat calculatorEnter pressures and line temperatures; get superheat, subcooling and a target superheat check.Common mistakes in the math
- Flipping the subtraction. Superheat is line minus saturation. Subcooling is saturation minus line. A negative subcooling number usually means the subtraction was reversed or the line is full of flash gas.
- Wrong refrigerant on the gauge. R-410A and R-32 are close but not identical. Pick the one on the nameplate.
- Probe in the sun. A liquid line clamp in direct sun reads high, which shows up as low subcooling.
What should subcooling be?
TXV and EEV systems. Use the manufacturer's target, usually on the outdoor unit's nameplate or charging chart. ENERGY STAR's commissioning checklist passes a TXV system when subcooling is within ±3°F of that target3. DOE's guide gives the same band and the same direction: remove refrigerant if subcooling is more than 3°F above target, add if it's more than 3°F below1. It also notes California Energy Commission testing that found subcooling from 2°F under to 8°F over target cost no more than 5% of normalized sensible EER1.
Fixed-orifice systems. They're charged by superheat and often have no subcooling target. DOE's guide says about 10°F, plus or minus 5°F, is typical, and recommends measuring it anyway to make sure the piston is getting solid liquid1.
Weather matters. Subcooling falls as outdoor temperature rises. DOE's guide says it may drop to zero above 105°F and that about 25°F can be expected at 70°F outdoor on fixed-orifice systems1.
What causes high or low subcooling?
| Reading | Superheat | Likely cause |
|---|---|---|
| High subcooling | Low or normal | Overcharge |
| High subcooling | High | Restriction: filter drier, kinked liquid line, TXV not opening |
| Low subcooling | High | Undercharge, often a leak |
| Low subcooling | Low or normal | Possible bad TXV or very high outdoor temperature; check airflow and the condenser coil |
DOE's guide adds a third reading, condensing temperature over ambient (COA): condenser saturation temperature minus the air entering the condenser. A COA of 30°F or more can point to an overcharge, a liquid line restriction or non-condensables, and superheat and subcooling sort out which1.
Subcooling vs superheat
| Subcooling | Superheat | |
|---|---|---|
| Side of the system | High side, liquid line | Low side, suction line |
| Formula | Saturation temp − line temp | Line temp − saturation temp |
| Charge by it on | TXV and EEV systems | Fixed-orifice systems |
| Target from | Nameplate or install manual | Chart using indoor wet bulb and outdoor dry bulb |
| ENERGY STAR pass band | ±3°F | ±5°F |
| On a blend's PT chart | Bubble (liquid) side | Dew (vapor) side |
Source: ENERGY STAR National HVAC Commissioning Checklist, Rev. 14; DOE Building America measure guideline
See superheat for the vapor side, and temperature split for the quick airside check that comes before either.
Before you add or recover refrigerant
- Airflow is at least 300 CFM per ton (DOE's minimum for valid charge tests)
- Filter and evaporator coil are clean
- Condenser coil is clean and the fan is running
- Outdoor temperature is above 55°F, ideally above 70°F
- Both superheat and subcooling are recorded
- The amount added or recovered is written down for the refrigerant log
How subcooling shows up in TradesWork
In the TradesWork technician app, the subcooling calculator runs offline, and the reading can be saved to the equipment record next to superheat and temperature split. When a tech adds or recovers refrigerant, the amount goes into the refrigerant log for that unit, which calculates the leak rate on save. See warranty and refrigerant tracking. The manufacturer's charging chart always comes first.
Footnotes
-
D. Springer and B. Dakin, Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement, U.S. DOE Building America, DOE/GO-102013-3766, March 2013, pp. 15–31. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_air_cond_diagnostics.pdf ↩ ↩2 ↩3 ↩4 ↩5 ↩6
-
NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems, difluoromethane (R-32) saturation properties, retrieved Oct. 15, 2026. Gauge pressure assumes 14.7 psi atmospheric. https://webbook.nist.gov/chemistry/fluid/ ↩
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ENERGY STAR Single-Family New Homes, National HVAC Commissioning Checklist, Version 3.1 / 3.2 / 3.3 (Rev. 14), Section 2, revised Jan. 15, 2025. https://www.energystar.gov/sites/default/files/2025-01/National%20HVAC%20Commissioning%20Checklist_Rev%2014.pdf ↩
