High & Low Superheat / Subcooling Troubleshooting

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Read Superheat and Subcooling Together

One number is a clue. Two numbers pick a side. You will not always have both (piston techs live on SH; TXV techs live on SC), but when you can take the other, do it.

Low and High Superheat Causes

Low superheat (flooding)

• Overcharge on a piston. Extra liquid in the evaporator, SH collapses, compressor sees liquid. Recover to target SH after you confirm airflow.
• Low indoor airflow / low load. Dirty filter, closed supplies, undersized return, blower on the wrong tap. Not enough heat to boil the feed. SH drops. This is the one techs miss when they “add two pounds” on a 95°F day with a clogged media cabinet.
• Oversized orifice or a piston that was swapped wrong after a coil change.
• TXV stuck open (if this is actually a TXV system — then you should not have been charging by SH in the first place).

High superheat (starving)

• Undercharge / leak. High SH, usually low SC, low suction. Sniff before you keep filling. High SH plus a stained evaporator drain is not a mystery.
• Restricted metering device, filter-drier, or liquid line kink. High SH and often high SC — liquid is backing up before the restriction. Replacing the drier beats adding gas.
• High indoor load (hot attic return, oversized home, first hour after setback). SH runs high until load drops. Do not finish a charge on a 110°F return without saying so on the ticket.
• Wrong refrigerant selected on the manifold. Garbage sat temp, garbage SH. Confirm R-410A vs R-22 vs R-32 on the bottle and the nameplate.

Causes that live in the duct, not the bottle, belong on ESP troubleshooting and CFM troubleshooting.

Low and High Subcooling Causes

Low subcooling

• Undercharge / leak — the usual TXV story. Weigh toward the OEM target (often 8–12°F).
• Liquid-line restriction upstream of the port so your “liquid” pressure is not really condenser outlet pressure.
• Fan cycle / low outdoor ambient dropping head so there is little liquid to subcool. Some units need a low-ambient kit; charging to 10°F SC at 55°F outdoor will overcharge the next heat wave.

High subcooling

• Overcharge. Extra liquid stacked in the condenser. Recover to the plate. Head pressure often up with it.
• Dirty condenser or blocked outdoor airflow. Head up, SC can look high. Wash the coil before you recover 3 pounds that were never extra.
• Restriction after the condenser (liquid drier, kink) — liquid backs up, SC high, SH high. Same row as the matrix above.

How to Tell Overcharge from Undercharge from Airflow Restriction

1. Compute SH or SC in the calculator. Write the flag down before you unscrew a bottle.
2. Airflow first when SH is low on a piston and the house is still hot, or the coil is icing. Measure TESP vs nameplate with the ESP calculator. If TESP is high, you found the restriction. Delta-T (return − supply in cooling) running 22–28°F with low SH is the same story: too little air, too much temperature drop.
3. Charge when SH and SC agree. High SH + low SC = add (after leak search). Low SH + high SC = recover. Do not use one number against a duct problem.
4. Restriction when both are high. Temperature drop across the drier, frost on a liquid line, piston inlet screen. Replace / clear, then re-charge to target.
5. If rooms are short after charge is right, size or prove airflow with the duct CFM calculator and, if the run is small, the ductulator.

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