High Superheat, Zero Subcooling on a Fixed-Orifice Split System
A technician is sent to a 3-ton R-410A residential split system with a fixed-orifice (piston) metering device. The filter is clean and the evaporator coil has been washed. The homeowner says the house will not drop below 78°F. Conditions at the time of the call: outdoor 95°F DB; indoor 75°F DB / 63°F WB; return static −0.20 in. w.c. and supply static +0.25 in. w.c.; the manufacturer's blower table shows 1,180 CFM at that total external static. At the outdoor unit the suction pressure is 118 psig with a suction line temperature of 68°F at the service valve, and the liquid pressure is 340 psig with a liquid line temperature of 108°F. The line set is 12 ft with a 3/8 in. liquid line. There is no frost, no oil staining at the braze joints, and the contactor and condenser fan are running normally.
1. Calculate superheat and subcooling and interpret what the combination means on this system.
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Superheat (measured at the outdoor service valve)
- R-410A saturation at 118 psig ≈ 40°F.
- SH = 68°F − 40°F = 28°F.
- Suction-line heat gain means true evaporator-outlet superheat is slightly lower than this, so 28°F is a conservative floor.
Subcooling
- Saturation at 340 psig ≈ 108°F.
- SC = 108°F − 108°F = 0°F.
Target superheat (fixed orifice)
- Target SH = (3 × IWB − 80 − ODB) ÷ 2 = (3 × 63 − 80 − 95) ÷ 2 = (189 − 175) ÷ 2 = 7°F.
Interpretation
- Measured SH is about 21°F above target, and the condenser is producing no liquid seal at all (0°F SC).
- High superheat plus zero subcooling is not a metering-device or airflow signature - it is a system that does not have enough refrigerant to fill the condenser. The diagnosis is undercharge, and the next step is a leak search, not a top-off.
2. Name two other faults that could produce elevated superheat and explain, using the readings given, how each is ruled in or out.
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1. Low evaporator airflow
- Low airflow lowers suction pressure and raises superheat, so it must be considered.
- Ruled out: TESP is 0.45 in. w.c. with 1,180 CFM at the tapped speed (essentially nominal for 3 tons), the coil and filter are clean, and no frost is present.
- Critically, low airflow does not drive subcooling to 0°F - the condenser would still have a normal liquid seal.
2. Liquid line restriction (kinked line, plugged filter-drier, partially closed valve)
- A restriction also starves the evaporator and produces high superheat.
- Ruled out: a restriction causes a pressure/temperature drop across it, which shows up as abnormally high subcooling upstream and a sharp temperature drop across the drier. Here subcooling is 0°F and there is no temperature differential along the liquid line.
Also worth eliminating
- Overcharge or non-condensables: both raise subcooling; excluded by 0°F SC.
- Undersized/wrong piston: would raise superheat while subcooling stayed near normal; it cannot explain a condenser with no liquid seal.
3. Describe the corrective procedure from this point through final verification, including the numbers you would expect to see when the job is right.
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1. Find the leak first
- Recover the remaining charge, pressurize with dry nitrogen plus a trace of refrigerant, and use an electronic detector at braze joints, Schrader ports, the coil headers, and the flare/braze at the air handler.
- A system that lost this much charge has a leak; recharging without repair guarantees a callback.
2. Repair and evacuate
- Braze or replace the failed component, replace the filter-drier, evacuate to 500 microns and hold with the pump isolated.
3. Weigh in the charge
- Use nameplate charge plus the line-set adjustment for length beyond the table's reference length; charge R-410A as a liquid into the liquid line.
- Do not chase superheat by guesswork - weighing in removes line length and ambient from the equation.
4. Verify at stable conditions (indoor ~75/63, outdoor ~95)
- Superheat at the evaporator outlet: 7 ± 3°F.
- Suction pressure should rise from 118 psig toward roughly 130–140 psig, and subcooling should climb from 0°F into the 8–12°F range.
- If superheat lands on target but subcooling stays near zero, suspect a piston/orifice mismatch rather than charge.
- Confirm a 18–20°F indoor temperature split and record all readings on the invoice.