Refrigeration Troubleshooting: Reading Pressures, Subcool, and Superheat
Subcool, superheat, and the two numbers that tell a tech why a refrigeration system is not working.
Table of contents
What the work looks like
Troubleshooting refrigeration starts with connecting a manifold (or a pair of digital probes) to the service ports and reading two numbers: superheat and subcooling. Those two numbers tell you whether the system is properly charged and which component is failing.
Superheat (measured at the suction line near the evaporator exit): how much hotter the refrigerant vapor is than its saturation temperature at that pressure. Target: 8 to 12 F on a fixed-orifice AC, TXV target varies by manufacturer (usually 10 F). Low superheat means too much charge or TXV flooding. High superheat means undercharge or restriction.
Subcooling (measured at the liquid line near the condenser exit): how much cooler the refrigerant liquid is than its saturation temperature at that pressure. Target: 8 to 12 F. Low subcool means undercharge. High subcool means overcharge or condenser airflow problem.
A first service call in July on an AC that is not cooling: high superheat, low subcool, low head pressure, low suction pressure = undercharge. Low superheat, high subcool, high head, high suction = overcharge. High subcool, high head, normal superheat = dirty condenser or bad fan. High superheat, normal subcool, normal head = restriction at the metering device or filter drier.
Job titles: HVAC Service Tech, Refrigeration Tech, Senior HVAC Tech. Diagnosing correctly is what moves a tech from $22/hr installer to $40+/hr service.
Safety and tools
Every service call includes:
- Putting on safety glasses before connecting the manifold.
- Connecting to Schrader service ports. Use ball valves (Yellow Jacket or Accutools) to minimize vented refrigerant each time you disconnect.
- Reading saturation temperature from the manifold's built-in temperature scale for the specific refrigerant (R-410A, R-32, R-454B, R-22 for older legacy systems).
- Taking line-temperature readings with a pipe clamp thermocouple (Fieldpiece ATC1R or Testo clamp probe).
- EPA 608 rules for any recovery or charge adjustment.
Tools: digital manifold (Fieldpiece SMAN, Testo 557s), two clamp-on pipe thermocouples, a clamp ammeter (to read compressor amps and verify not running on start winding), a psychrometer (for wet-bulb and dry-bulb air measurements on the coil), superheat-subcool app or table.
Your first exercise
On an AC running on a hot day, place a thermometer on the suction line 6 inches from the service port and another on the liquid line 6 inches from the service port. Connect a manifold and read suction and liquid pressures. Convert each pressure to saturation temperature using the scale printed on the gauge (R-410A: 130 psi = 45 F; 400 psi = 118 F). Subtract for superheat and subcool. Compare to target.
You just did the whole diagnostic. Every refrigeration tech in the country runs those four numbers first, every call.
Where to go next
Build on Refrigeration Troubleshooting with Refrigeration Systems (Introduction to Refrigeration), EPA 608 (Introduction to EPA 608 Certification), Heat Pumps (Introduction to Heat Pumps), HVAC Fundamentals (Introduction to HVAC), and HVAC Controls and Thermostats. For commercial and supermarket work, add Commercial Refrigeration and Cascade / Transcritical CO2 systems. Safety: Hazardous Materials Handling, Workplace Safety.