R-410A Split System With High Superheat and High Subcooling

Superheat: the R-410A saturation temperature at 105 psig is about 33°F, so superheat = 65 − 33 = about 32°F against a 10°F target.

Subcooling: the saturation temperature at 335 psig is about 105°F, so subcooling = 105 − 84 = 21°F against a 10°F target.

High superheat with high subcooling is the signature of a liquid-line restriction between the condenser and the evaporator. Liquid backs up in the condenser (raising subcooling) while the evaporator is starved (raising superheat and lowering suction pressure). The 7°F temperature drop across the filter drier localizes it: more than about 2–3°F across a drier means a pressure drop there. Replace the drier.

Ruled out:

• Undercharge - subcooling would be low or near zero, not 21°F.
• Low evaporator airflow - verified at roughly 1,050 CFM with a clean coil; low airflow drives superheat down (flooding), never to 32°F.
• Dirty condenser - high head and high subcooling could appear, but superheat would stay near target and suction pressure would not be this low.
• Overcharge - would raise subcooling but leave superheat low or normal.

The 6°F supply-to-return split is consistent with a starved evaporator removing far less heat than its rated capacity.

Adding refrigerant does not open the restriction, so the flow through the metering device stays limited. What changes is the liquid side: head pressure and subcooling climb further, the compressor works harder, and the suction pressure barely moves because the evaporator is still starved. The superheat stays high, so the system still will not cool.

Worse, the added charge becomes a genuine overcharge once the drier is replaced. After the repair you would be starting diagnosis from a system carrying several pounds more than the nameplate allows, so your superheat and subcooling verification would be meaningless, and the unit would be at risk of high-pressure trips, liquid floodback, and compressor damage.

Refrigerant also cannot be vented when you correct the charge - it must be recovered under Section 608 and weighed back in.

What to tell the homeowner: the readings show the restriction, the repair is a drier replacement plus a weighed charge, and the previous top-off is why the system has two problems instead of one. Back it up with the numbers on the invoice, not an opinion. That is both the technically correct answer and the one that keeps you out of a callback argument.

1. Recover the existing charge into a recovery cylinder per Section 608; record the amount removed.
2. Cut out and replace the filter drier with the manufacturer-specified part; flow nitrogen while brazing to prevent internal oxidation.
3. Pressure test the new joints with dry nitrogen at the manufacturer's test pressure (typically several hundred psig), and hold to confirm no decay.
4. Evacuate through both high- and low-side ports with the valve cores removed, pulling below 500 microns; isolate and confirm the vacuum does not rise significantly (a standing-vacuum decay test).
5. Weigh in the nameplate charge on a scale, adjusting for line-set length per the manufacturer's table. Never use pressures alone to set the charge.
6. Run at least 15–20 minutes at steady conditions; verify superheat 10°F ± 2 and subcooling 10°F ± 2, supply-to-return split in the 18–22°F range at 1,050 CFM, no frost or temperature drop at the drier, and compressor amps within the nameplate RLA.
7. Document the before-and-after readings, the recovered and charged weights, and the vacuum level.

Acceptance: both superheat and subcooling within the nameplate tolerance at steady state, a normal split, and no measurable temperature drop across the new drier.