High Superheat, Low Subcooling: Undercharge or Restriction?
A 3-ton R-410A split system with a thermostatic expansion valve serves a residence. Outdoor ambient is 85°F and indoor conditions are 75°F dry bulb / 63% relative humidity. After 20 minutes of runtime you record: suction pressure 115 psig, suction line temperature 65°F measured 6 inches from the compressor service valve, liquid pressure 335 psig, liquid line temperature 100°F taken at the outlet of the condenser. The compressor draws 12.4 A against a nameplate RLA of 16.0 A. The condenser coil is clean and the fan is running; the indoor blower is on high speed with a new filter. The homeowner mentions that 'a guy added two pounds last spring and it ran fine until August.'
1. Using the pressure–temperature relationship for R-410A, calculate superheat and subcooling and state your diagnosis, including which alternatives you are ruling out and why.
Read the worked answer
Superheat: R-410A at 115 psig corresponds to a saturated suction temperature of about 38°F. SH = 65°F − 38°F = 27°F.
Subcooling: R-410A at 335 psig corresponds to a saturated condensing temperature of about 104°F. SC = 104°F − 100°F = 4°F.
A TXV system at these conditions should show roughly 10–12°F superheat and 8–12°F subcooling. Both numbers are wrong, and they are wrong in a specific direction: high superheat with low subcooling = the condenser is starved of liquid, i.e., the system is undercharged.
Ruling out alternatives:
- Liquid-line restriction would show the opposite pattern (abnormally high subcooling, because liquid backs up in the condenser) - SC here is only 4°F.
- Low indoor airflow raises superheat but leaves subcooling near normal, and the blower is on high with a new filter.
- Overcharge would give high subcooling.
- Bad TXV (stuck closed) would give high superheat with normal subcooling; the low amp draw (12.4 A vs 16.0 RLA) also fits a starved evaporator.
The 4°F subcooling is the deciding measurement.
2. What does the service history imply about the system, and what is the correct repair path under refrigerant regulations?
Read the worked answer
Two pounds added last spring, then a return to poor performance by August, means a slow, continuing leak - roughly a pound or two per year. Adding refrigerant without finding the leak just resets the clock.
Correct path:
- Leak-search first: electronic detector at braze joints, Schrader ports, flare connections, and the evaporator coil; follow with a nitrogen isolation test at 150–200 psig if the detector is inconclusive (the indoor coil is the most common culprit on older R-410A systems).
- Repair and verify the repair holds under nitrogen, then evacuate to 500 microns or below with a decay test.
- Charge by weight to the nameplate value (or by subcooling per the manufacturer's chart) - not by 'feel' or by topping off to a pressure.
- Regulatory: R-410A is an HFC with no ozone-depletion potential but high GWP. Under EPA Section 608, appliances with a charge of 5 lb or more that leak above the applicable threshold must have the leak repaired, and the repair must be verified, before the system is recharged; refrigerant must be recovered, never vented, and all service records retained.
- Tell the homeowner why the top-off was a stopgap and give them the leak-repair versus replacement comparison.
3. Suppose the same unit instead read 27°F superheat but 18°F subcooling. How does your diagnosis change and what component do you check first?
Read the worked answer
High superheat with high subcooling is the classic signature of a restriction between the condenser outlet and the metering device - the condenser fills with liquid (driving subcooling up) while the evaporator is starved (driving superheat up).
Check in this order:
- Filter drier - measure the temperature difference across it; more than a couple of degrees of drop means it is plugged.
- Liquid line - look for a kink, a crushed line behind the unit, a closed or partially closed service valve, or a soldered joint with excess solder blocking flow.
- TXV inlet screen and the valve itself - a plugged inlet screen or a stuck-closed valve body produces the same high-SH symptom.
A temperature drop across any component in the liquid line localizes the restriction immediately. The repair is to replace the drier or the restricted section, then evacuate and recharge by weight - do not compensate by adding refrigerant, which would overcharge the condenser and make the subcooling even higher.