Superheat too high or too low on TXV system - Carrier split systems

What is going on

Charge looks right by subcooling but superheat at the outdoor unit is off; decide if the TXV or the system is the problem.

Likely causes

• Low indoor airflow or dirty coil
• TXV bulb loose, misplaced or not insulated
• TXV stuck open or closed
• Restriction in liquid line, drier, distributor or suction line
• Wrong refrigerant valve (R-22 vs R-410A)
• Weak compressor
• High indoor load or return leaks

Check in this order

1. Verify filter, dampers, indoor coil and outdoor coil airflow first.
2. Set subcooling to the rating plate value (long line: plate value or 10 F, whichever is higher).
3. Superheat at the vapor valve under 2 F = low superheat path; 2-20 F = valve probably fine; over 20 F = work out evaporator outlet superheat using the suction line drop.
4. Low superheat with both readings under 2 F: valve likely stuck open; first confirm the bulb is tight and wrapped, then put the bulb in ice for about 1 min - superheat should rise.
5. High superheat with normal or low suction: look for kinks, a plugged drier, a broken power element cap tube, a blocked equalizer or (R-22) inlet screen before replacing the valve.
6. Uneven circuit outlet temperatures (over 15 F apart) point to the distributor or coil.
7. Hunting (swings over 10 F) check distributor tubes, valve size and bulb position; moving the bulb further down the suction line may help.
8. Suction reference R-410A about 135 psig, R-22 about 80 psig separates normal/low from high.

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