Superheat & Subcooling Reference
Target superheat and subcooling values for proper HVAC system charging.
What is Superheat?
Superheat is the temperature of a vapor above its saturation (boiling) temperature at a given pressure. It confirms that all liquid refrigerant has evaporated before reaching the compressor.
Formula: Superheat = Actual suction line temperature − Saturation temperature at suction pressure
Target range: Typically 8–15°F (4–8°C) for most systems. Exact target depends on the system type and manufacturer specs.
Too high superheat = low efficiency, compressor overheating
What is Subcooling?
Subcooling is the temperature of a liquid below its saturation (condensing) temperature at a given pressure. It confirms that all vapor has condensed before reaching the expansion device.
Formula: Subcooling = Saturation temperature at discharge pressure − Actual liquid line temperature
Target range: Typically 10–18°F (6–10°C) for TXV systems. Fixed-orifice systems rely on superheat primarily.
Target Values by System Type
| System Type | Superheat (°F) | Subcooling (°F) | Notes |
|---|---|---|---|
| TXV / EEV system | 8–12 | 10–18 | Charge by subcooling |
| Fixed orifice / cap tube | 10–15 | — | Charge by superheat |
| Walk-in cooler | 8–12 | 10–15 | Medium temp |
| Walk-in freezer | 6–10 | 10–15 | Low temp |
| Heat pump (heating) | 5–10 | 5–10 | Varies by mode |
How to Measure
1. Measure suction pressure at the service port → look up saturation temperature on the PT chart for your refrigerant.
2. Measure actual suction line temperature with a pipe clamp thermometer on the suction line near the outdoor unit.
3. Superheat = Actual temp − Saturation temp. Example: suction pressure is 130.6 psig (R-410A), which is 45°F saturation. Actual line temp is 55°F. Superheat = 55 − 45 = 10°F. ✓
4. For subcooling: measure discharge (liquid line) pressure, look up saturation temp, measure actual liquid line temp. Subcooling = Saturation temp − Actual temp.
Blends with temperature glide: use dew for superheat, bubble for subcooling
Zeotropic blends such as R-407C, R-448A, R-449A, R-407A, R-454C and R-422D boil over a range of temperatures at a single pressure. Look up the vapor (dew) saturation temperature at suction pressure when calculating superheat, and the liquid (bubble) saturation temperature at liquid-line pressure when calculating subcooling. Using the wrong column on R-407C can skew superheat by roughly 10°F. The PT charts for these blends list both values.
Frequently Asked Questions
How do I calculate superheat?
Superheat = Actual suction line temperature minus saturation temperature at suction pressure. Step 1: Read suction pressure at the service port. Step 2: Look up the saturation temperature on the PT chart for your refrigerant. Step 3: Measure actual suction line temperature with a pipe clamp thermometer. Step 4: Subtract saturation temp from actual temp.
What should superheat be for R-410A?
For R-410A systems with a TXV (thermostatic expansion valve), target superheat is typically 8-12°F. For fixed-orifice systems, target is 10-15°F. Always check the manufacturer's specifications as targets can vary by equipment.