{"id":4494,"date":"2026-10-08T09:00:00","date_gmt":"2026-10-08T09:00:00","guid":{"rendered":"https:\/\/www.paradigmcooling.co.za\/articles\/?p=4494"},"modified":"2026-10-08T09:00:00","modified_gmt":"2026-10-08T09:00:00","slug":"low-superheat-and-high-subcooling-reading-an-overcharged-system","status":"publish","type":"post","link":"https:\/\/www.paradigmcooling.co.za\/articles\/2026\/10\/08\/low-superheat-and-high-subcooling-reading-an-overcharged-system\/","title":{"rendered":"Low Superheat with High Subcooling: Reading an Overcharged System"},"content":{"rendered":"\n

Low superheat paired with high subcooling is the readback of a system carrying more refrigerant than it needs, and it is more dangerous to ignore than most technicians assume \u2014 liquid floodback erodes bearings and washes down cylinders long before a compressor actually fails outright, so the damage is already happening by the time you get the call. This article gives you the exact numbers that separate a genuine overcharge from an overfeeding metering device, the signs to check at the compressor before you touch a hose, and the correct procedure for removing excess charge so you leave the system at its design point rather than guessing your way there.<\/p>\n\n\n\n

What the Numbers Look Like<\/h2>\n\n\n\n

A system with a 6 K superheat target and a 7 K subcooling target that is genuinely overcharged typically reads something like this:<\/p>\n\n\n\n

Parameter<\/th>Design target<\/th>Overcharge reading<\/th>Deviation<\/th><\/tr><\/thead>
Superheat<\/td>6 K<\/td>1\u20133 K<\/td>3\u20135 K low<\/td><\/tr>
Subcooling<\/td>7 K<\/td>11\u201315 K<\/td>4\u20138 K high<\/td><\/tr>
Head pressure (at 25\u00b0C ambient, R-410A)<\/td>~2050 kPa<\/td>2250\u20132450 kPa<\/td>200\u2013400 kPa high<\/td><\/tr>
Suction pressure<\/td>Design value<\/td>At or slightly above design<\/td>Normal to slightly high<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The pattern is both numbers moving in the direction that “more refrigerant” would push them: subcooling climbs because there’s a larger reserve of liquid backed up in the condenser, and superheat falls because that excess liquid is also making it past the metering device faster than the evaporator can boil it off. When you see subcooling more than 3\u20134 K above target and superheat more than 2\u20133 K below target on the same call, treat overcharge as the leading suspect before anything else.<\/p>\n\n\n\n

Why Low Superheat Is Dangerous, Not “Just a Bit Rich”<\/h2>\n\n\n\n

Superheat exists to guarantee that only vapour reaches the compressor suction valve. Once superheat drops below roughly 3 K under load, you no longer have a reliable margin \u2014 a load swing, a defrost cycle, or a fan hiccup can put liquid straight into the suction line and from there into the compressor shell. Three things happen in sequence, and each is measurable:<\/p>\n\n\n\n

  1. Floodback dilutes the oil.<\/strong> Liquid refrigerant returning to the crankcase mixes with the lubricating oil, cutting its viscosity. A healthy compressor oil viscosity is compromised once even 10\u201315% by volume is liquid refrigerant, and at superheat readings under 2 K sustained over hours, that dilution level is easily reached.<\/li>
  2. Diluted oil stops protecting the bearings.<\/strong> Oil that’s been thinned by refrigerant can no longer maintain the film thickness bearings need under load, so metal-to-metal contact begins \u2014 this is bearing washout, and it’s abrasive wear, not a single event.<\/li>
  3. Wear accumulates until something lets go.<\/strong> Depending on run hours and load cycling, this shows up as a scored crankshaft, a knocking noise under load, or a compressor that eventually locks or grounds electrically. None of this is instantaneous \u2014 it is a wear-out process measured in weeks to months of continuous low-superheat operation, which is exactly why it gets missed: the system still cools acceptably on the day you’re called out.<\/li><\/ol>\n\n\n\n

    Signs at the Compressor<\/h2>\n\n\n\n

    Check these before you touch the charge, because they confirm floodback is actually happening rather than just being a theoretical risk at the current reading:<\/p>\n\n\n\n