How to calculate subcooling
- Run the system in cooling for 10–15 minutes so pressures settle.
- Read liquid (high side) pressure at the liquid line service valve.
- Clamp a pipe probe on the liquid line within about 6 inches of the service valve.
- Find the saturation temperature at that pressure on the P/T chart. The calculator does this for you.
- Subtract the liquid line temperature from the saturation temperature.
An R-410A system reads 340 PSIG on the liquid line. The P/T chart gives 105.0°F saturation. The liquid line is 95°F, so subcooling is 105.0 − 95 = 10.0°F. That's in range for a TXV system (10–18°F).
Typical subcooling ranges
| Metering device | Typical range |
|---|---|
| TXV / EEV | 10–18°F |
| Piston (fixed orifice) | 5–10°F |
Most TXV systems are charged by subcooling. When the condenser data plate lists a target subcooling, use that target.
What high or low subcooling means
Low subcooling: not enough liquid is backing up in the condenser. The most common cause is low charge.
High subcooling: too much liquid is stacking in the condenser. Common causes are overcharge or a restriction in the liquid line or metering device.
Questions
What is normal subcooling for R-410A?
Many TXV systems target about 10°F, and FieldLoom treats 10–18°F as in range. The data plate target on the condenser takes priority when it's listed.
Should I charge by superheat or subcooling?
TXV and EEV systems are charged by subcooling. Piston systems are charged by superheat. Always follow the manufacturer's instructions.
Where do I measure liquid line temperature?
On the liquid line within about 6 inches of the outdoor unit's liquid service valve, with the probe insulated from outside air.