What it is

On a system with a thermostatic expansion valve (TXV), you charge by subcooling, not superheat. The TXV actively controls evaporator superheat — it modulates to hold superheat steady regardless of how much refrigerant you put in the system. So superheat barely tells you anything about charge on a TXV. Subcooling does, because extra charge backs up in the condenser and raises subcooling directly.

You add or remove refrigerant until the subcooling at the condenser outlet matches the manufacturer's target, usually found on the unit's data tag.

How it works

A TXV's whole job is to maintain a set evaporator superheat. Add refrigerant and the TXV throttles to keep superheat where it wants it. Remove some and it opens up to compensate. Within its operating range, the valve hides the charge from the superheat reading.

But the condenser can't hide charge. Refrigerant that isn't actively circulating stacks up as liquid in the bottom of the condenser. The more charge you add, the more liquid stacks, the more of the condenser surface is flooded, and the lower the liquid temperature drops below saturation — that's subcooling going up. Remove charge and subcooling falls. So subcooling tracks the charge cleanly. That's why it's the lever for a TXV.

In the field

  1. Run and stabilize the system 15–20 minutes with steady conditions. Higher outdoor temps give a more reliable subcooling charge; very cool weather makes it harder (see notes).
  2. Find the target subcooling on the unit's data tag or manufacturer spec. Don't guess — many units call for a specific value.
  3. Measure subcooling at the condenser outlet: read liquid-line pressure → saturation temp; clamp a thermometer on the liquid line near the condenser outlet; subtract actual from saturation.
  4. Compare and adjust:
  • Actual subcooling LOWER than target → undercharged → add refrigerant.
  • Actual subcooling HIGHER than target → overcharged → recover refrigerant.
  1. Adjust in small increments, let it settle a few minutes, re-measure. Walk it to the target.
  2. Sanity-check superheat too. It shouldn't be a charging target, but if superheat is wildly high or low while subcooling is on target, the TXV or airflow may have a problem.

Normal values & targets

  • Typical TXV subcooling target: generally 8–12°F, but the nameplate value wins. Some units spec a precise figure (e.g., 10°F). Variable-capacity and communicating systems may have their own commissioning procedure.
  • Evaporator superheat (controlled by the TXV): usually around 8–12°F at the coil; you don't set this, the valve does.
  • Conditions: the warmer it is outdoors, the more trustworthy the subcooling charge. Many manufacturers want ~65°F+ outdoor before charging by subcooling.

Common faults & what they mean

  • Subcooling on target but low capacity / high superheat: suspect the TXV (undersized, stuck, lost bulb charge, or screen restricted), or low indoor airflow. The charge is right; the valve or airflow is wrong.
  • Subcooling very high with high head pressure: overcharged, or a restriction downstream of the condenser stacking liquid. Recover charge; if it persists, look for a plugged drier.
  • Subcooling near zero / can't build it: undercharged, or the condenser can't reject heat (dirty coil, weak fan). Confirm the condenser is clean before dumping in more refrigerant.
  • Subcooling reads fine but system was just topped repeatedly: make sure no one chased superheat earlier and overshot — re-verify against nameplate subcooling.

Tech tips & gotchas

  • Don't charge a TXV system by superheat. This is the classic mistake. Because the valve holds superheat steady, you can be significantly overcharged and still see "normal" superheat. You'll keep adding to chase a number that won't move and end up badly overcharged with sky-high head pressure. Subcooling is the only honest lever on a TXV.
  • Verify the condenser is clean and the fan works before charging. A dirty condenser raises head pressure and skews subcooling, fooling you into removing charge you actually need (or masking an overcharge). Clean first, charge second.
  • The TXV needs solid liquid to work. That's the other reason subcooling matters — proper subcooling guarantees the valve is fed pure liquid, not a flashing mix.
  • Mind the conditions and the nameplate. Cool weather, a long hot liquid-line run, or a non-standard coil match can all shift things. The data tag is your authority; rules of thumb are the backup.
  • If a system has both a TXV and you're unsure, confirm by looking — a TXV has a sensing bulb strapped to the suction line and usually an external equalizer line. A piston has neither.

Safety / code notes

  • Charge blends as liquid to avoid fractionation; meter liquid into the low side or weigh it in.
  • Recover per EPA 608 when removing charge.
  • Don't overcharge to hit a number — high head pressure trips safeties and shortens compressor life.