Superheat and Subcooling Calculator

What Superheat and Subcooling Tell You

Refrigerant boils in the evaporator and condenses in the condenser at a temperature set by its pressure: the saturation temperature on a pressure-temperature (PT) chart. Superheat and subcooling measure how far the refrigerant is from that saturation point where it leaves each coil.

Superheat is how much warmer the suction line is than the refrigerant’s boiling point at suction pressure. It shows how much of the evaporator is doing work and protects the compressor: zero superheat means liquid may be reaching it. Subcooling is how much cooler the liquid line is than the condensing temperature. It shows that a solid column of liquid is leaving the condenser and feeding the metering device.

Which number you charge by depends on the metering device. Fixed orifice (piston) systems are charged by superheat against a target that changes with indoor humidity and outdoor temperature. TXV and EEV systems hold their own superheat, so they are charged by subcooling against the manufacturer’s target.

Dew Point, Bubble Point and Glide

R-22 and R-32 are single refrigerants: they boil and condense at one temperature for each pressure. R-410A is a near-azeotropic blend with almost no glide. R-454B (R-32 and R-1234yf) has a small glide: at the same pressure, it starts boiling (bubble point) about 2 to 3°F below where it finishes (dew point).

For blends, use the dew point (saturated vapor) for superheat and the bubble point (saturated liquid) for subcooling. This calculator does that automatically. Using the wrong column on R-454B throws the reading off by the full glide.

Superheat and Subcooling Formulas

Saturation temperatures come from the refrigerant’s PT chart at the measured gauge pressure. Line temperatures come from a pipe clamp on the suction line and the liquid line.

SH

Superheat in °F

SC

Subcooling in °F

WB

Indoor wet bulb at the return, °F

DB

Outdoor dry bulb at the condenser, °F

The target formula approximates manufacturer charging charts and usually lands within a degree or two of them. Charge to within about ±5°F of the target, and use the manufacturer’s chart when the unit has one. Targets below 5°F are too low to charge to.

Worked Example: R-410A Piston System

An R-410A system with a fixed orifice. Suction pressure is 118.4 psig and the suction line is 58°F. Indoor wet bulb is 64°F and it is 96°F outside.

1. Dew point at 118.4 psig: 40.0°F.
2. Superheat: 58 − 40.0 = 18.0°F.
3. Target: (3 × 64 − 80 − 96) ÷ 2 = 8°F, so the acceptable range is 3 to 13°F.

Result: 18°F is 10 degrees over target, which points to a low charge. Confirm the filter, blower and coils are clean first, then add refrigerant slowly and let the system settle between readings.

Worked Example: R-454B TXV Subcooling

An R-454B system with a TXV. Liquid pressure is 375 psig and the liquid line is 105°F.

1. Bubble point at 375 psig: 115.3°F.
2. Subcooling: 115.3 − 105 = 10.3°F.

Result: 10.3°F of subcooling. Compare it with the target on the outdoor unit’s data plate.

Worked Example: Reading R-454B on an R-410A Chart

A tech reads 118 psig suction and a 50°F suction line on an R-454B system, but the gauge is still set to R-410A.

1. R-410A dew point at 118 psig: 39.8°F, so superheat reads 10.2°F.
2. R-454B dew point at 118 psig: 44.8°F, so the real superheat is 5.2°F.

Result: the wrong refrigerant setting overstates superheat by 5°F and could lead to overcharging. Set the manifold to the refrigerant on the nameplate.

Pressure Temperature Chart (psig)

Saturation pressure in psig. For R-410A and R-454B, the first number is the liquid (bubble) pressure and the second is the vapor (dew) pressure.

Target Superheat Chart for Fixed Orifice Systems

Target superheat in °F by indoor wet bulb (rows) and outdoor dry bulb (columns). A dash means the target is under 5°F, too low to charge to.

Field Tips

Practical advice from HVAC School, HVAC Today, ACHR News, AC Service Tech and Ferguson’s technical guides.

1. 1

   Fix airflow before touching the charge

   Superheat and subcooling shift with airflow even when the charge is right. Restricted airflow lowers superheat and can make a correctly charged system look overcharged. Check the filter and ductwork and measure airflow before you add or recover refrigerant.

2. 2

   Let it settle

   Run the system until the readings stabilize, about ten minutes, before you record them, and wait again after every adjustment. Readings taken right after startup or right after adding refrigerant are not stable.

3. 3

   Charge by the right number

   Piston systems are charged by superheat. TXV and EEV systems hold their own superheat, so charging them by superheat does not work; use subcooling and the manufacturer’s target.

4. 4

   Charge blends as liquid

   R-454B, R-410A and other blends should come out of the cylinder as liquid so the mix stays right. Throttle the liquid in on the suction side through the manifold so it flashes before reaching the compressor.

5. 5

   Dry climates can push the target too low

   In very dry air, the fixed orifice target can fall below 5°F. Charging to that risks liquid reaching the compressor, so techs in places like Arizona keep superheat higher than the chart, or the system gets a TXV.

Sources: Ferguson: HVAC superheat and subcooling, ACHR News: Charging refrigeration systems with blended refrigerants, HVAC School: Glide, blends and why you can’t mix refrigerants, AC Service Tech: Target superheat, HVAC Today: Airflow gives refrigerant readings their meaning, HVAC School podcast: How to charge an AC