How to Measure AC Unit Subcooling


This article explains how to measure AC condenser unit subcooling.

Things You'll Need

  • Refrigeration gauge manifold and hoses, digital temperature thermometer
  • Remove the cap on the liquid line access fitting. Attach the high pressure (red) gauge hose on the refrigeration gauge manifold. Purge the air from the hose by momentarily opening the high side gauge.

  • Turn on the A.C. unit and allow to operate for 10 minutes. Record the high side pressure on the red gauge. This measurement is in psig or pounds per square inch gauge pressure.

  • Place the digital temperature thermometer on the liquid line near the condensing unit. Put tubing insulation around the thermometer to more accurately measure the copper tubing temperature. Record this temperature.

  • Using a pressure temperature chart, find the saturation temperature for the pressure you recorded in step 1. The difference between the measured temperature and the temperature from the pressure temperature chart is the subcooling. Adequate subcooling is needed to overcome any liquid line pressure drops. A.C. charging charts for TXV systems need subcooling readings to properly charge these units.

Tips & Warnings

  • Always use eye protection and gloves when working with refrigerant. You need to be EPA Refrigeration certified to handle CFC and HCFC refrigerants. For information on online refrigerant certification classes contact

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