High Subcooling, High Superheat on a TXV System
A 3-ton R-410A residential split system with a TXV metering device serves a house where the owner reports the unit "runs all day but the house sits at 78°F." Conditions at the call: outdoor 95°F DB, indoor 75°F DB / 63°F WB, clean filter, blower on high speed. Measured values: suction pressure 100 psig, suction line temperature 55°F at the service valve, discharge pressure 335 psig, liquid line temperature 82°F at the condenser outlet and 68°F at the outlet of the filter drier. The sight glass is clear. Compressor current is 0.8 A below nameplate RLA. The filter drier was replaced eight months ago during a compressor change, and the line set was shortened at that time with a new elbow brazed in near the outdoor unit.
1. Convert the gauge readings to superheat and subcooling, and state what the combination is telling you.
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Superheat: R-410A saturation at 100 psig is about 29°F, so SH = 55 − 29 = 26°F. Target with a TXV is roughly 8–12°F. Subcooling: saturation at 335 psig is about 103°F, so SC = 103 − 82 = 21°F. Target is roughly 8–12°F.
The pattern is low suction / high superheat / high subcooling / slightly low head. With a TXV, high superheat means the evaporator is starved; high subcooling means liquid is stacking in the condenser and cannot get through the liquid line fast enough. That combination is the signature of a liquid line restriction upstream of the TXV - not a charge problem. The 14°F temperature drop between the drier inlet and outlet localizes it to the drier, and the recently brazed elbow is a second candidate for a flow-restricting kink or excess solder.
2. The homeowner asks you to "top off the refrigerant." Explain what each of three plausible diagnoses would have looked like in the readings, and why adding charge is the wrong move.
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- Undercharge (leak): low suction, high superheat, but subcooling would be low or near zero (little liquid left to stack in the condenser), and there would be no temperature drop across the drier - the line would be uniformly warm or cool. Head would be noticeably low.
- Overcharge: high subcooling, but suction would be normal to high and superheat normal - the evaporator would be flooded, not starved.
- Restriction (this call): high subcooling, high superheat, low suction, and a measurable ΔT across the liquid line component.
Adding refrigerant to a restricted system raises head pressure and stacks even more liquid behind the restriction; the evaporator stays starved, so the house never gets cooler and the compressor is pushed harder. Charge is verified after the restriction is removed, by subcooling (or weigh-in), never before.
3. Describe the repair sequence and the charging/verification criteria you would use before leaving.
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- Recover the refrigerant per EPA Section 608 practice; do not vent.
- Cut out and replace the filter drier; inspect the new elbow for a kink or excessive solder; confirm the correct drier type (bi-flow if it is a heat pump, flare or braze as required).
- Flow dry nitrogen through the line set while brazing, then pressure test with nitrogen (typically 300–500 psig, per manufacturer) and leak check at joints.
- Evacuate to 500 microns or below and hold a decay test (e.g., under 500 microns after 10–15 minutes with the pump isolated) to confirm no moisture or leak remains.
- Weigh in the nameplate charge plus the manufacturer's adjustment for actual line-set length and coil volume - on a fixed-charge/weigh-in basis, not by guesswork.
- Start up and verify: 8–12°F superheat at the suction service valve, 8–12°F subcooling at the liquid line, suction saturation roughly 35–40°F at these indoor conditions, and compressor amps near nameplate RLA. Document readings.
If subcooling and superheat both land in range with the correct airflow, the restriction was the root cause and no further charge adjustment is warranted.