About the Subcooling Calculator
Subcooling is the second key number for checking an air-conditioning or heat pump charge. In the outdoor condenser coil, hot refrigerant vapour gives up its heat and condenses into liquid. Once it is all liquid, it keeps cooling as it passes through the rest of the coil. Subcooling is how many degrees the liquid has cooled below its condensing (saturation) temperature, measured on the liquid line near the outdoor unit. It shows how much liquid is stacked up in the condenser, which tracks closely with the amount of refrigerant in the system.
This subcooling calculator converts the liquid line pressure into the saturation temperature for R-410A, R-22 or R-134a, subtracts the measured liquid line temperature, and compares the result with the target subcooling from the unit's data plate.
How to Use the Subcooling Calculator
Choose the refrigerant.
Enter the liquid line pressure from the high-side gauge and the liquid line temperature from a clamp thermometer near the liquid service valve.
Enter the target subcooling printed on the outdoor unit's data plate or in its manual.
The Formula
saturation temperature = P-T chart value at the liquid line pressure
subcooling = saturation temperature − liquid line temperature
difference = subcooling − targetStep-by-Step Example
An R-410A system at 317.4 psig with a 90 °F liquid line and a 10 °F target.
saturation R-410A at 317.4 psig = 100 °F
subcooling 100 − 90 = 10 °F
check equal to the 10 °F targetIf the liquid line read 96 °F, subcooling would be only 4 °F, six degrees short, which points to an undercharge.
Reading the Result
Subcooling above target usually means extra refrigerant has backed up in the condenser: an overcharge. Head pressure and power use rise, and the compressor works harder. A restriction just after the condenser can give the same reading, so compare with superheat before recovering any refrigerant.
Subcooling below target means little liquid is stacked in the condenser, which usually points to an undercharge, often from a leak. Negative subcooling means the liquid line contains vapour bubbles, or flash gas, which starves the metering device and sharply reduces capacity. Leaks should be found and repaired before topping up, rather than simply adding refrigerant each season.
When to Charge by Subcooling
Systems with a thermostatic expansion valve (TXV) or an electronic expansion valve are charged by subcooling, because the valve holds superheat roughly constant whatever the charge. Manufacturers print the target, often 8 to 12 °F and commonly 10 °F for residential R-410A units, on the outdoor unit. Fixed orifice systems are charged by superheat instead. Many technicians record both numbers, because together they distinguish an incorrect charge from airflow problems or restrictions.
Taking Good Readings
Let the system run for 15 minutes or more with normal indoor airflow and a clean outdoor coil. Clean the liquid line and clamp the thermometer about four to six inches from the service valve, insulated from sunlight and the air. Use the high-side pressure taken at the same time. Outdoor temperatures that are very low can make readings unreliable; manufacturers often give a minimum ambient temperature for charging.
Pressure–Temperature Charts
Each refrigerant has its own relationship between pressure and boiling temperature. This calculator uses standard published values every 10 °F and interpolates between them, which agrees with gauge manifold charts to within a fraction of a degree in the normal operating range. Blends such as R-410A have a very small temperature glide, so use the dew or bubble point your equipment maker specifies when precision matters, and always confirm with a calibrated digital manifold.
Superheat and Subcooling Together
Reading both numbers at the same time separates charge problems from other faults. Low superheat with high subcooling points to an overcharge. High superheat with low subcooling points to an undercharge or leak. High superheat with high subcooling suggests a restriction, such as a blocked filter drier or a kinked line, trapping refrigerant in the condenser while starving the evaporator. Low superheat with low subcooling often means poor indoor airflow or a metering device that is overfeeding. Recording both readings, with outdoor temperature and indoor wet bulb, gives a useful history for the next service visit.
Weighing In the Charge
After a repair that opens the system, technicians often recover the old charge and weigh in a fresh one, using the factory charge on the data plate plus an allowance for extra line length. Subcooling is then used to fine-tune the charge once the system is running.
Line Length
Long line sets hold extra refrigerant, so follow the manufacturer's added-charge rule for each foot of line beyond the standard length.
Understanding Your Result
The headline is the subcooling in degrees Fahrenheit.
The working line shows the saturation temperature and the subtraction.
The charge check line compares the result with the target.
When Should You Use This Calculator?
Use it when charging or checking a TXV or EEV system.
Use it alongside superheat to diagnose airflow or restriction problems.
Use it in training to practise reading high-side pressures.
Common Mistakes
Using the suction pressure. Subcooling uses the high-side liquid pressure.
Guessing the target. Use the value on the unit's data plate.
Adding refrigerant without finding the leak. A low charge usually has a cause.
Charging in very cold weather. Follow the manufacturer's minimum ambient temperature.