Free subcooling calculator
Enter the liquid line pressure and liquid line temperature and get actual subcooling for R-410A, R-454B, R-32, R-22 or R-134a. Add the target from the unit's data plate and it tells you whether the charge is high, low or right.
How to measure subcooling
- Run the system in cooling for at least 10 to 15 minutes so pressures settle.
- Read liquid line pressure on the high-side gauge at the liquid (smaller) line service valve.
- Clamp a pipe thermometer on the liquid line next to the service valve.
- Convert the pressure to saturation (condensing) temperature with the PT chart (the calculator does this).
- Subcooling = saturation temperature − liquid line temperature.
Worked example
An R-410A system reads 365 psig on the liquid line. That is a saturation temperature of about 110 °F. The liquid line is 95 °F, so subcooling is about 15 °F. If the data plate target is 10 °F, it is about 5 °F high, which usually means overcharged: recover a little and re-check.
Charging by subcooling
Systems with a TXV or EEV are charged by subcooling, to the target the manufacturer prints on the outdoor unit's data plate or in the install manual.
- Subcooling below target usually means undercharged.
- Subcooling above target usually means overcharged, or a restriction in the liquid line.
Fixed-orifice systems are charged by superheat instead.
Working on refrigerant circuits?
Opening the refrigerant circuit on stationary equipment using the refrigerants on this page requires EPA Section 608 certification, and suppliers can only sell you most refrigerants if you have it. See how to get EPA 608 certified or take the free EPA 608 practice tests.
FAQ
How do you calculate subcooling?
Subcooling = saturation temperature at the liquid line pressure − liquid line temperature. Read the high-side gauge at the liquid line service valve, convert it with a PT chart, and subtract the liquid line temperature measured next to the valve.
What should subcooling be?
Whatever the manufacturer specifies for that unit, printed on the data plate or in the install manual. Charge to that target, within a few degrees.
Does subcooling use the bubble point or dew point?
The bubble point (liquid column) for blends like R-454B and R-410A. The calculator picks the right one.
What does low subcooling mean?
Usually an undercharge: not enough liquid is stacking up in the condenser. High subcooling usually means overcharge or a liquid line restriction.
Is this tool free?
Yes. No account, no email, no download. It runs in your browser and works on your phone at the unit.