A technician who only checks superheat is diagnosing half the refrigeration circuit. Subcooling is the reading that tells you what is happening at the condenser and how much liquid refrigerant charge is actually in the system \u2014 information superheat cannot give you on its own. This article sets out the exact measurement procedure, the arithmetic, a worked example, and why this particular number belongs to the high side of the system rather than the low side.<\/p>\n\n\n\n
Subcooling is the saturation temperature corresponding to the measured liquid line pressure, minus the measured liquid line temperature:<\/p>\n\n\n\n
Subcooling (K) = Saturation temperature at measured liquid pressure (\u00b0C) \u2212 Measured liquid line temperature (\u00b0C)<\/strong><\/p>\n\n\n\n Refrigerant leaving the condenser as saturated liquid sits exactly at the saturation temperature for the high-side pressure \u2014 subcooling reads 0 K at that point. As the liquid continues to lose heat to the condenser coil past full condensation, its actual temperature drops below the saturation point, and that drop, in kelvin, is subcooling. A healthy positive subcooling number confirms two things at once: that the condenser has enough surface area and airflow to fully condense the refrigerant, and that there is sufficient charge in the system to keep a liquid seal ahead of the metering device.<\/p>\n\n\n\n As with superheat, take no reading until the system has run at least 10\u201315 minutes at a steady load. On a hot Cape Town afternoon (condensing unit ambient 30\u00b0C or higher) a system cycling on after being off will show climbing head pressure and a moving subcooling value for the first several minutes as the condenser catches up. Do not measure during a defrost cycle, during a hot gas bypass event, or in the first 5 minutes after compressor start.<\/p>\n\n\n\n Connect to the high-side service port \u2014 ideally as close to the condenser outlet or the liquid line service valve as your access allows, and definitely before the filter drier if you also intend to check for a pressure drop across it (covered in the diagnostic articles linked below). Record the pressure in kPa or bar gauge, and pull the saturation temperature for that pressure from the correct refrigerant’s P-T chart. For R410A at 2,400 kPa gauge, saturation temperature runs around 42\u00b0C; for R32 at the same pressure it is close to 40\u00b0C. A 2 K gap between refrigerants at the same pressure is enough to turn a borderline reading into a wrong diagnosis if you use the wrong chart.<\/p>\n\n\n\n Clean the pipe to bare copper at the exact point you read pressure, clamp the probe with full contact and even tension, then insulate both the probe and a few centimetres of pipe on each side. Liquid line temperatures in direct Cape Town summer sun can run several degrees above the refrigerant temperature if the probe is left exposed \u2014 expect a 1\u20133 K high-side error from an uninsulated clamp in full sun on a black-jacketed line. Allow 60 seconds for the reading to settle before recording it.<\/p>\n\n\n\n Subcooling = saturation temperature from Step 2 \u2212 measured liquid line temperature from Step 3. Record both raw values alongside the result.<\/p>\n\n\n\n Liquid line pressure at the service valve: 2,500 kPa gauge. On the R410A chart, 2,500 kPa corresponds to a saturation temperature of 44\u00b0C. The clamped, insulated probe at the same point reads 36\u00b0C after settling.<\/p>\n\n\n\n Subcooling = 44\u00b0C \u2212 36\u00b0C = 8 K<\/strong>.<\/p>\n\n\n\n That falls at the top of the generally accepted 4\u20138 K band for most split and packaged systems \u2014 but “generally accepted” is a starting point, not the answer. The number that actually matters is the manufacturer’s design subcooling figure for that specific unit, which can run anywhere from 3 K to 12 K depending on condenser sizing and the metering device fitted. Always check the nameplate or commissioning documentation before treating an 8 K reading as either “fine” or “a problem.”<\/p>\n\n\n\n Superheat tells you about the evaporator and the metering device \u2014 whether the low side is being fed correctly. Subcooling tells you about the condenser and the charge \u2014 whether the high side is rejecting heat properly and whether there’s enough refrigerant mass in the system to keep the liquid line full ahead of the TXV or orifice. The two readings are diagnosing different halves of the same circuit, which is exactly why neither one, alone, gives you a full picture.<\/p>\n\n\n\nWhat You Need<\/h2>\n\n\n\n
Step 1: Let the System Reach Steady State<\/h2>\n\n\n\n
Step 2: Measure Liquid Line Pressure and Find Saturation Temperature<\/h2>\n\n\n\n
Step 3: Clamp and Insulate the Probe on the Liquid Line<\/h2>\n\n\n\n
Step 4: Subtract<\/h2>\n\n\n\n
Worked Example<\/h2>\n\n\n\n
Why This Reading Is a High-Side, Not Low-Side, Diagnostic<\/h2>\n\n\n\n