Subcooling Calculator
Free subcooling calculator for HVAC techs. Enter the liquid line saturation temperature and the liquid line temperature to get measured subcooling in °F, with target ranges for charging TXV systems.
How the subcooling calculation works
Subcooling is the number of degrees the liquid refrigerant has been cooled below its saturation (condensing) temperature at the high-side pressure. It tells you how much liquid is “stacking up” in the condenser — which is a direct indicator of system charge on a TXV system.
The formula is:
Subcooling (°F) = Liquid Saturation Temperature − Liquid Line Temperature
The liquid saturation temperature comes from converting the high-side (head) pressure with the refrigerant’s P/T chart. The liquid line temperature is measured with a clamp thermometer at the condenser outlet.
Worked example
A R-410A system shows a head pressure that converts to a 105°F liquid saturation temperature, and the liquid line measures 95°F:
105 − 95 = 10°F of subcooling
If the data tag calls for 10°F, the charge is right on target.
Charging to subcooling
On a TXV or EEV system, subcooling is the primary charging target because the metering valve already controls superheat. Add refrigerant to raise subcooling, recover refrigerant to lower it, and always charge to the value printed on the equipment data tag rather than a generic number.
| Reading | Likely meaning |
|---|---|
| Low subcooling (under ~5°F) | Undercharge, or condenser airflow / TXV overfeed |
| On target (per data tag, often 8–12°F) | Correct charge |
| High subcooling (over ~15°F) | Overcharge, or a liquid-line restriction |
Read subcooling together with superheat. On a fixed-orifice system you charge to superheat instead — use the Superheat Calculator and the Target Superheat Calculator.
How FieldPad helps in the field
This calculator gives you the number instantly. FieldPad keeps the charging record with the equipment: log the subcooling reading against the system and the client, attach it to the job, and roll it straight into a signed invoice. Next season, the system’s charging history is one tap away on the same record.
Stop Re-Running These Numbers by Hand
FieldPad is the all-in-one iPhone CRM for solo HVAC techs — clients, scheduling, estimates, invoices with signatures, inventory, and equipment history. Keep your readings on the job, the client, and the equipment, all in one offline-ready app. EPA Section 608 and Subpart C compliance included.
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