Superheat, Subcooling, and the "Topped Off" System
A 3-ton R-410A residential split with a TXV has been running all afternoon. Outdoor 95°F, indoor 80°F DB / 67°F WB. The compressor nameplate RLA is 16 A. Steady-state readings at the outdoor unit: suction pressure 102 psig with a suction line temperature of 68°F at the service valve; liquid pressure 350 psig at the condenser outlet with a liquid line temperature of 90°F there; the liquid line measures 76°F immediately downstream of the filter drier. Compressor current is 11.8 A. Indoor airflow was verified at 1,180 cfm. The homeowner mentions a different contractor 'topped the system off' last week and the house has not cooled since.
1. Using the pressure/temperature relationship for R-410A, calculate superheat and subcooling and state whether each is in or out of the normal range.
Read the worked answer
Suction side
102 psig on R-410A corresponds to a saturation temperature of about 30°F.
Superheat = suction line temp − saturation temp = 68 − 30 = 38°F
For a TXV system at this load, target superheat is roughly 8–12°F at the coil outlet. 38°F is far out of range (starved evaporator).
Liquid side
350 psig on R-410A corresponds to a saturation temperature of about 108°F.
Subcooling = saturation temp − liquid line temp = 108 − 90 = 18°F
Target subcooling is about 8–12°F. 18°F is high.
Two supporting measurements worth noting
- With 95°F ambient, a 108°F condensing temperature (about 13°F above ambient) is not elevated - the condenser is not the problem.
- Compressor current of 11.8 A against 16 A RLA is low, consistent with reduced refrigerant mass flow.
- Airflow of 1,180 cfm is approximately 393 cfm/ton - normal, so the high superheat is not an airflow artifact.
So the pattern is: high superheat + high subcooling + low amp draw + verified airflow. That combination is not normal operation at any load condition.
2. Interpret the combination of readings. What is the most likely fault, and how does it differ from the two obvious alternatives (undercharge and overcharge)?
Read the worked answer
The key measurement
The liquid line is 90°F at the condenser outlet and 76°F just past the filter drier - a 14°F temperature drop across the drier. A filter drier should show essentially no temperature drop (a few degrees at most, and pressure drop under a couple of psi). A large temperature drop means the refrigerant is flashing across a restriction, which is exactly what a plugged or partially blocked drier does.
Why the whole pattern fits a liquid line restriction
- Liquid backs up upstream of the restriction → condenser outlet liquid is cold relative to its saturation temperature → high subcooling (18°F).
- Only vapor and flash gas reach the TXV → the valve cannot feed the coil → high superheat (38°F) and low suction pressure.
- Starved coil means reduced mass flow and low load on the compressor → low amps (11.8 A).
Ruling out the alternatives
- Undercharge: subcooling would be low or zero, not 18°F, and there would be no temperature drop across the drier. Suction superheat would be high in both cases, so superheat alone cannot separate them - subcooling and the drier ΔT do.
- Overcharge: subcooling would be high, but superheat would be low (flooded coil), amps would be high, and the condenser would run hot. Here superheat is 38°F, which is the opposite.
The homeowner's story actually points the technician toward overcharge, because 'topped off' suggests extra refrigerant. The gauges say otherwise. If the system really were overcharged on top of a restriction, subcooling would be even higher and condensing temperature would climb - it did not.
3. Outline the repair and the verification steps that would prove the diagnosis before you leave the job.
Read the worked answer
Diagnosis confirmation before opening the system
- Take a pressure reading on both sides of the filter drier (or the liquid service port vs. a port downstream) to confirm a pressure drop matching the 14°F temperature drop.
- Pump the system down and confirm the suction pressure pulls down and holds - a restriction at the drier may prevent a clean pump-down, which is itself informative.
- Confirm the restriction is at the drier and not at the TXV inlet screen or a kinked line: feel/measure temperature along the liquid line - the cold spot marks the restriction.
Repair
- Recover the refrigerant (do not vent). Record the amount removed and compare it to the nameplate charge plus line-set adjustment - that answers the 'topped off' question at the same time.
- Replace the filter drier. A restriction this severe usually means the drier did its job and captured debris from a dirty install or a compressor burn - cut it open for evidence.
- Braze with nitrogen flowing, then pressure test with dry nitrogen (R-410A systems are typically tested to a few hundred psi, not to the operating pressure of the low side).
- Triple evacuate to below 500 microns and perform a standing vacuum test.
- Weigh in the charge per nameplate plus line-set adjustment.
Verification
- Subcooling 8–12°F and superheat 8–12°F at the same indoor/outdoor conditions used for the diagnosis.
- No temperature drop across the new drier (within a couple of degrees).
- Compressor amps back near nameplate RLA for the conditions.
- Suction pressure and evaporator ΔT consistent with 1,180 cfm.
Document the before/after numbers. Without them the next contractor will repeat the same 'top it off' decision.