How it works
Superheat is how far the refrigerant vapor coming out of the evaporator has warmed above its boiling (saturation) point. It tells you whether the evaporator is being fed the right amount of refrigerant. Too little superheat means flooding; too much means starving.
Subcooling is how far the liquid leaving the condenser has cooled below its condensing point. It tells you the liquid is solid liquid on its way to the metering device, and it is the main charging check for TXV systems. You read saturation temperature from the gauge pressure on a PT chart for that refrigerant, and measure the line temperature with a clamp thermometer.
Worked example
A fixed-orifice system with a 55°F suction line at a 42°F saturation, a 96°F liquid line at a 108°F saturation, 67°F indoor wet bulb, and 95°F outdoors:
- Superheat = 55 − 42 = 13°F
- Subcooling = 108 − 96 = 12°F
- Target superheat (fixed orifice) = (3 × 67 − 80 − 95) ÷ 2 = 13°F
| Input | Value |
|---|---|
| Suction line temperature (at the compressor or coil outlet) | 55 °F |
| Evaporator saturation temperature | 42 °F |
| Liquid line temperature | 96 °F |
| Condenser saturation temperature | 108 °F |
| Indoor wet-bulb temperature (return air) | 67 °F |
| Outdoor dry-bulb temperature | 95 °F |
| Target subcooling from the nameplate (TXV systems) | 10 °F |
| Result | Value |
|---|---|
| Measured superheat | 13 °F |
| Measured subcooling | 12 °F |
| Target superheat (fixed-orifice systems only) | 13 °F |
| Superheat above (+) or below (−) target | 0 °F |
| Subcooling above (+) or below (−) nameplate target | 2 °F |
Assumptions and limits
- You supply the saturation temperatures from the correct PT chart for your refrigerant and gauge readings. This tool does not convert pressure to temperature.
- The target superheat formula is the common rule for fixed-orifice (piston) systems. Some manufacturers publish their own charts; use theirs if you have it.
- Measurements must be taken with the system running steadily, with good clamp-on contact and insulated sensors, and with proper airflow across the coils.
- This is a diagnostic aid, not a charging procedure. Refrigerant work requires certification and follows the manufacturer's instructions.
Common questions
What is normal superheat?
For fixed-orifice systems it varies with conditions: use the target formula or the manufacturer's chart. For TXV systems it is usually about 8–12°F at the evaporator outlet.
What is normal subcooling?
Most TXV residential systems specify about 10 to 15°F. The nameplate or installation manual gives the exact target.
What does high superheat with low subcooling mean?
That combination usually points to an undercharge: the evaporator is starved and not much liquid is left at the condenser outlet. Low superheat with high subcooling points to an overcharge or a restriction problem. Diagnose with the full set of readings.
Why check both?
Each is a different part of the circuit. Superheat shows the evaporator feed, subcooling shows the liquid supply, and together they tell overcharge, undercharge, and restrictions apart.
Sources
- Total superheat charging method for fixed-orifice systems: target superheat = (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2, published in manufacturers' charging instructions and in trade references such as ACHR News, "Charging Methods for Metering Devices".
- Definitions of superheat and subcooling: standard refrigeration cycle practice (ASHRAE Handbook, Refrigeration).
- Read them: Charging methods for metering devices (ACHR News)
Updated 2026-09-30