HVAC Technician how-to
How to Check Refrigerant Charge Using Subcooling
On systems with a TXV, subcooling is the standard way to verify charge. Use the manufacturer’s target.
On systems with a TXV, subcooling is the standard way to verify charge. Use the manufacturer’s target. In short: run; connect; read, then complete the remaining steps below.
Step by step
Run the system in cooling mode for at least 15 minutes to stabilize.
Connect gauges or probes to the liquid line service valve.
Read the liquid line pressure and convert it to saturation temperature for the refrigerant.
Measure the actual liquid line temperature with a clamp thermometer near the service valve.
Subtract: saturation temperature minus actual temperature equals subcooling.
Compare with the manufacturer’s target subcooling on the unit’s data plate.
Adjust charge only as needed, following EPA rules, and recheck.
Safety reminders for HVAC technicians
- Disconnect power and lock out before servicing electrical components
- Wear gloves and eye protection when handling refrigerants: they can cause frostbite
- Never vent refrigerants; recover them as required by EPA rules
- Use fall protection on roofs and follow ladder safety
Key terms
BTU · Ton (cooling) · SEER2 · Refrigeration cycle · Compressor · Condenser: see the full HVAC technician glossary.