How to calculate superheat
- Run the system in cooling for 10–15 minutes so pressures settle.
- Read suction pressure at the suction (vapor) service valve.
- Clamp a pipe probe on the suction 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 saturation temperature from the line temperature.
An R-410A system reads 118 PSIG suction. The P/T chart gives 39.5°F saturation. The suction line is 50°F, so superheat is 50 − 39.5 = 10.5°F. That's in range for a TXV system (8–12°F).
Target superheat ranges
| Metering device | Typical range |
|---|---|
| TXV / EEV | 8–12°F |
| Piston (fixed orifice) | 10–20°F |
For piston systems the exact target depends on indoor wet bulb and outdoor dry bulb. Use the manufacturer's charging chart to set the charge.
What high or low superheat means
High superheat: the evaporator is starved. Common causes are low charge, a restriction, or a TXV that's underfeeding.
Low superheat: the evaporator is flooded. Common causes are overcharge, low indoor airflow (dirty filter or coil), or a TXV that's overfeeding. Zero superheat risks liquid reaching the compressor.
Questions
What is normal superheat for R-410A?
Superheat targets depend on the metering device, not the refrigerant. TXV and EEV systems usually run 8–12°F. Piston systems usually run 10–20°F, with the exact target from the manufacturer's charging chart.
Why is superheat used to charge piston systems?
A piston can't adjust refrigerant flow, so superheat changes directly with charge. A TXV holds superheat steady, so TXV systems are charged by subcooling instead.
Where do I measure suction line temperature?
On the suction line within about 6 inches of the outdoor unit's service valve, with the probe insulated from outside air.