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Superheat & Subcooling Calculator

Convert pressure and temperature readings into superheat and subcooling to diagnose charge.

System

Saturation data runs 11–461 psig for this refrigerant.

Metering device
Suction side

Read pressure at the service valve and line temperature with a clamp probe about six inches upstream, insulated from ambient air.

psig
°F
Liquid side
psig
°F
Conditions

These set the charted target superheat on a fixed-orifice system. On a TXV or EEV the valve sets superheat itself, so they are context only.

°F

Return air wet bulb at the indoor coil.

°F

Ambient at the condenser, out of direct sun.

Subcooling

10.3°Fmeasured

A TXV or EEV holds superheat itself, so subcooling is the charge indicator. Target is 8–12°F.
SuperheatTarget 8–14°F
10.1°F
Evaporator saturationInterpolated at 125 psig
44.9°F
Condensing saturationInterpolated at 380 psig
115.3°F
Condenser splitCondensing temp above ambient; 20–30°F is typical
20.3°F

Charge appears correct

10.1°F superheat and 10.3°F subcooling both land inside the expected window. If the system still is not cooling, the fault is somewhere other than the charge — check airflow, blower speed, duct leakage into unconditioned space, and the compressor's own capacity.

Verify airflow before you touch the charge

Low airflow produces readings that look exactly like an undercharge. Adding refrigerant to a system with a loaded filter, a dirty blower wheel or a collapsed flex duct floods the compressor with liquid and destroys it. Measure total external static pressure against the blower table and check the temperature split first — every single time, before this page means anything.

EPA Section 608 certification is legally required

Only a Section 608 certified technician may purchase, handle or recover refrigerant. Venting refrigerant to the atmosphere is a federal violation under the Clean Air Act, and the penalties fall on the individual as well as the company.
Reading the combinations
SuperheatSubcoolingWhat it points toWhere to look
In rangeIn rangeCharge is correctMove on to airflow, blower speed and duct static
HighLowUnderchargeWeigh in refrigerant after finding and repairing the leak
LowHighOverchargeRecover to the target; check for a previous top-off
HighHighLiquid line restrictionFilter drier, kinked liquid line, partially closed valve
LowLowMetering device overfeedingSensing bulb contact and insulation, valve superheat setting, wrong piston
HighIn rangeRestricted or undersized metering deviceCompare the piston size against the nameplate
In rangeLowNon-condensables or a flooded condenserCondenser coil, fan speed, and whether the system was evacuated properly

Saturation temperatures are interpolated linearly between published pressure-temperature points spaced five to ten degrees apart, which holds accuracy well inside the tolerance of a field gauge set. R-410A, R-454B and R-32 values are dew point, which is what a suction gauge reads; R-454B carries roughly 2°F of glide, so a bubble-point subcooling reading on that refrigerant runs slightly cooler than shown here. Always charge to the equipment manufacturer's own chart when one is on the unit.

R-410A charging targets quick reference

Fixed-orifice / piston systems: charge to target superheat from the standard field chart (indoor wet bulb × outdoor dry bulb). TXV and EEV systems: charge to subcooling at the outdoor unit — typically 8–12°F — not to a fixed superheat number.

Target superheat for R-410A fixed orifice systems by indoor wet bulb and outdoor dry bulb
Outdoor dry bulbIndoor WB 57°FIndoor WB 63°FIndoor WB 67°FIndoor WB 72°F
75°F8°F17°F23°F31°F
85°F3°F12°F18°F26°F
95°F-2°F7°F13°F21°F
105°F-7°F2°F8°F16°F
TXV systems subcooling and superheat bands
Metering typePrimary charge indicatorTypical targetNotes
Fixed orifice / pistonSuperheat at evaporator8–15°F (chart above)Verify airflow before adjusting charge
TXV / EEVSubcooling at outdoor liquid line8–12°FSuperheat held by valve (~8–14°F); do not chase superheat alone

Superheat targets use the common field formula (3 × indoor WB − 80 − outdoor DB) ÷ 2. Always follow the equipment manufacturer's charging chart when available. EPA Section 608 certification required to handle refrigerant.

Quick answer

On R-410A, target superheat on a fixed-orifice system is typically 8–15°F from the manufacturer charging chart (outdoor dry bulb and indoor wet bulb). On a TXV system, charge to subcooling at the outdoor liquid line — usually 8–12°F at steady state — not to a fixed superheat number. Verify airflow before adjusting charge: low airflow mimics undercharge. Enter suction and liquid pressures and line temperatures below; compare results to targets for your metering device type.

What this assumes

  • Saturation temperatures are interpolated from published pressure-temperature data for the selected refrigerant.
  • Fixed-orifice systems are evaluated on superheat; TXV and EEV systems are evaluated on subcooling.
  • Assumes airflow has been verified first — charge cannot be judged on a system with restricted airflow.

Important

Only EPA Section 608 certified technicians may purchase, handle or recover refrigerant. Venting refrigerant to the atmosphere is a federal violation under the Clean Air Act.

Understanding the result

Verify airflow before you touch the charge

Low airflow produces readings that look exactly like undercharge. Adding refrigerant to a system with a dirty filter or a collapsed flex duct floods the compressor and destroys it. Check static pressure and temperature split first, every time.

Which number to trust

On a fixed-orifice system, superheat reflects charge and you charge to a target superheat from a chart based on indoor wet bulb and outdoor dry bulb. On a TXV or EEV system the valve holds superheat constant, so subcooling is the meaningful indicator — typically 8 to 12°F.

Reading the combinations

High superheat with low subcooling points to undercharge. Low superheat with high subcooling points to overcharge. High superheat with high subcooling suggests a liquid line restriction. Low superheat with low subcooling usually means the metering device is overfeeding.

Ambient temperature changes the targets

Superheat and subcooling targets are not fixed numbers — they depend on outdoor dry bulb, indoor wet bulb, and the metering device type. A target superheat of 10°F at 95°F outdoor may be 15°F at 75°F outdoor. Always use the manufacturer's charging chart for the specific unit rather than a single rule of thumb.

Non-condensables contaminate readings

Air or nitrogen left in a system after service raises head pressure and distorts subcooling readings. If subcooling is high but the system is not cooling, suspect non-condensables before adding more refrigerant. Recovery, evacuation to 500 microns, and a fresh charge is the correct fix.

Common questions

What should superheat be on R-410A?
On a fixed-orifice system, target superheat is typically 8 to 15°F and is read from the manufacturer's charging chart based on outdoor dry bulb and indoor wet bulb. On a TXV system, superheat is controlled by the valve and you charge to subcooling instead — usually 8 to 12°F on the liquid line.
What is normal subcooling for R-410A?
Normal subcooling on a properly charged TXV system is 8 to 12°F at the outdoor unit's liquid line service valve. Below 5°F suggests undercharge or a restriction. Above 15°F suggests overcharge. Always measure at the outdoor unit with the system running at steady state for at least 10 minutes.
What does high superheat mean?
High superheat — above 20°F on a fixed-orifice system — means too little refrigerant is entering the evaporator. The refrigerant boils off before it reaches the end of the coil, leaving the suction line hot. Add refrigerant in small increments until superheat reaches the chart target.
What does low superheat mean?
Low superheat — below 5°F — means liquid refrigerant is reaching the compressor suction line. That floods the compressor and can cause mechanical damage. On a fixed-orifice system, recover refrigerant until superheat rises to the target. On a TXV system, low superheat usually indicates a failed or oversized valve.
How do you calculate superheat?
Superheat equals suction line temperature minus saturation temperature at suction pressure. If suction pressure is 118 psi on R-410A (saturation 45°F) and the suction line measures 55°F, superheat is 10°F. Use a clamp-on pipe thermometer on a clean, insulated section of the suction line.
How do you calculate subcooling?
Subcooling equals saturation temperature at liquid line pressure minus actual liquid line temperature. If liquid pressure is 390 psi on R-410A (saturation 115°F) and the liquid line measures 105°F, subcooling is 10°F. Measure on the liquid line at the outdoor unit, not at the indoor coil.
What superheat should a TXV have?
A properly functioning TXV maintains 5 to 15°F superheat at the evaporator outlet regardless of load. You do not charge a TXV system to superheat — you charge to subcooling at the condenser. If superheat is erratic or outside range with correct subcooling, the TXV itself is suspect.
What causes high subcooling?
High subcooling above 15°F most often means overcharge — too much refrigerant in the condenser. It can also indicate a restriction in the liquid line or filter drier, which backs up liquid in the condenser. Check subcooling before and after the restriction point to isolate the fault.
Can you charge AC by superheat alone?
On a fixed-orifice (piston) system, yes — superheat is the primary charging method. On a TXV or EEV system, charge to subcooling; superheat is a diagnostic check only. Charging a TXV system by superheat will overcharge it because the valve controls evaporator superheat automatically.
What is the superheat on an undercharged system?
An undercharged fixed-orifice system typically shows superheat above 20°F, warm suction line, and high subcooling below 5°F. The evaporator starves for refrigerant while the condenser runs short. Add refrigerant in 2-ounce increments until superheat matches the charging chart target for current conditions.

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