Superheat and subcooling: how to measure and interpret them, including glide
Step by step: measure superheat and subcooling correctly, use the dew point and bubble point for blends with glide (R407C, R448A, R449A), and interpret the values when troubleshooting.
Key points
- Superheat = measured suction line temperature minus the saturation temperature at the suction pressure.
- Subcooling = the saturation temperature at the condensing pressure minus the measured liquid line temperature.
- For blends with glide, superheat is calculated from the dew point and subcooling from the bubble point [1][2][3].
- Measure pressure and temperature at the same point and at the same time, with calibrated instruments, and let the system run steadily first.
- The values are used to set the expansion valve, check the charge and find faults, but always compare them with the manufacturer's guide values.
What the values mean
In the evaporator the refrigerant boils at the saturation temperature, which is set by the pressure. When all the liquid has boiled off, the gas continues to warm up. This extra warming is the superheat. It shows that the evaporator is fed correctly: enough for the surface to be used, but not so much that liquid reaches the compressor. In the condenser the gas becomes liquid at the saturation temperature for the condensing pressure. The cooling the liquid receives after that is the subcooling. It shows that there is solid liquid before the expansion valve and gives more refrigerating effect per kilogram of refrigerant.
How to measure superheat
- Let the system run steadily for at least 10–15 minutes at normal load.
- Measure the suction pressure as close to the evaporator outlet as possible, or at the compressor if you want the compressor superheat.
- Read the saturation temperature for the pressure in a pressure-temperature chart or in the table of the digital manifold. Check that the right refrigerant is selected and whether the chart shows gauge or absolute pressure.
- Measure the pipe temperature at the same point with a clamp probe with good contact and insulation from the surroundings. For a thermostatic expansion valve: measure at the bulb.
- Superheat = measured pipe temperature − saturation temperature.
Example with R134a: the suction pressure is 1.9 bar gauge, which corresponds to a saturation temperature of about 0 °C. The pipe at the bulb is 6 °C. The superheat is 6 K.
How to measure subcooling
- Measure the condensing pressure at the condenser outlet or at the liquid line service valve.
- Read the saturation temperature for the pressure.
- Measure the liquid line temperature at the same point, or before the expansion valve if you want to know whether the liquid is solid there.
- Subcooling = saturation temperature − measured liquid temperature.
Example with R134a: the condensing pressure is 9.2 bar gauge, which corresponds to about 40 °C. The liquid line is 35 °C. The subcooling is 5 K.
Blends with glide
Zeotropic blends such as R407C, R448A and R449A do not boil and condense at a single temperature. At a given pressure the liquid starts to boil at the bubble point and has boiled off at the dew point, which is higher. The difference is called glide and is several kelvin, about 6 K for R407C [1][3]. To get the right values:
- Superheat is calculated from the dew point at the suction pressure. Gas that is warmer than the dew point is superheated.
- Subcooling is calculated from the bubble point at the condensing pressure. Liquid that is colder than the bubble point is subcooled.
- If the chart has two columns, liquid and vapour, use the vapour column for superheat and the liquid column for subcooling [1][3].
Interpreting the values when troubleshooting
| Superheat | Subcooling | Likely cause |
|---|---|---|
| High | Low | Too little refrigerant, for example after a leak |
| High | Normal or high | The expansion valve feeds too little (wrongly set, blocked, lost bulb charge, undersized), or a restriction in the liquid line, for example a blocked filter drier |
| Low | High | Too much refrigerant, or the valve feeds too much |
| Low | Normal | The valve feeds too much, or the bulb is badly placed or poorly insulated. Low load on the evaporator, for example an iced surface or a stopped fan |
| Normal | Low with bubbles in the sight glass | Slightly too little refrigerant, or a pressure drop in the liquid line |
The table shows common patterns, not certain diagnoses. Always check the whole picture: pressures, temperatures, air and water flows, clean heat exchangers and whether the system has a liquid receiver. In a system with a receiver, the subcooling after the condenser is often small even when the charge is correct. There the level in the receiver is a better indicator.
Guide values
Typical superheat values at the evaporator outlet are 4–8 K with a thermostatic expansion valve. With an electronic valve it is often lower. Too little superheat risks liquid slugging in the compressor. Too much gives lower capacity and a high discharge temperature. The manufacturer's guide values for the unit, valve and compressor always take precedence over general rules of thumb.