How subcooling is calculated
Subcooling = saturation temperature at liquid line pressure - liquid line temperature
ASHRAE defines subcooling as the difference between a liquid temperature and the bubble-point temperature at the same pressure. It defines the bubble point as the liquid saturation temperature, where liquid refrigerant first begins to boil. For single compounds such as R-22 and R-32 it is simply the saturation temperature. For blends such as R-454B and R-407C, the calculator uses the bubble column, as Lennox's R-454B instructions specify (saturated liquid temperature is used to calculate liquid subcooling); the dew column would overstate subcooling by the glide.
Where to measure
Take pressure and temperature together on the liquid line at the outdoor unit. Carrier's instructions say to read pressure at the liquid service valve port and clamp the thermometer on the liquid line near the outdoor coil; Trane says to measure liquid line temperature and pressure at the outdoor unit's service valve.
- Run the system in cooling for at least 15 minutes first (Carrier), and let it settle again after every adjustment; Carrier allows 15 minutes and Trane waits 20 minutes between adjustments.
- Clean the line where the clamp goes so it makes good thermal contact (DOE Building America diagnostics guide); Daikin also says to make sure the thermometer makes adequate contact.
- Charge heat pumps in cooling mode. Payne's heat pump instructions check charge after the system runs in cooling mode, and Trane calls subcooling in cooling mode the only recommended charging method above 55°F outdoors. Below 55°F, Trane recommends weighing in the charge and returning in warmer weather to check it by subcooling.
More on probe placement: how to measure superheat and subcooling.
What subcooling tells you
Subcooling is how many degrees the liquid sits below its bubble point. It shows whether the metering device is getting all liquid.
- Low subcooling risks vapor reaching the metering device. The DOE guide says to measure subcooling even on fixed-orifice systems to make sure the expansion device is receiving a solid stream of liquid, and Carrier's R-454B long line guideline requires at least 10°F so no refrigerant flashes before the TXV.
- High subcooling can mean an overcharge, but the DOE guide notes that a liquid line restriction or non-condensable gas also brings high subcooling, along with a high condensing temperature over ambient.
The DOE guide reads the two together: high subcooling with low superheat typically means an overcharge, low subcooling with high superheat an undercharge, and both high or both low point to other problems. Conditions matter on fixed-orifice systems: the guide says subcooling drops as outdoor temperature rises and may fall to zero above 105°F, while about 25°F can be expected at 70°F outdoors.
Charging TXV and EEV systems by subcooling
A TXV or EEV holds evaporator superheat roughly steady, so superheat cannot show the charge. Subcooling can. Carrier's instructions state that units with a cooling-mode TXV "require charging by the subcooling method", and Carrier, Payne and Lennox put the required subcooling on the unit rating plate or charging label. Targets differ by model: Carrier's charging table covers rating plate targets from 6 to 16°F, and one Daikin manual lists 8°F plus or minus 1°F for its TXV systems. Use the number on the unit in front of you.
- Below target: add refrigerant. Carrier: add if the liquid line is warmer than the chart value for the measured pressure.
- Above target: recover refrigerant.
- Tolerance: Carrier allows 3°F; California's Title 24 test gives installers the same 3°F around the manufacturer's target.
After the charge is set, check superheat to confirm the valve is controlling. Title 24 accepts 4 to 25°F on TXV and EXV systems unless the manufacturer specifies a range. Fixed-orifice systems are charged by superheat instead; see the superheat calculator.
Worked example
An R-410A heat pump with a TXV, running in cooling mode. Say the rating plate gives a subcooling target of 10°F; use whatever your unit lists.
- The liquid line gauge at the outdoor unit reads 341.9 psig.
- On the R-410A chart, that pressure is a bubble point of 105°F.
- The clamp on the liquid line near the service valve reads 95°F.
- Subcooling = 105 - 95 = 10°F, on target.
If the line had read 99°F, subcooling would be 6°F, more than 3°F below target. Carrier's procedure is to add refrigerant, adjusting slowly and letting the system run 15 minutes to stabilize. A line at 90°F (15°F of subcooling) would call for recovering some refrigerant.
On a blend the column matters. An R-454B liquid line at 322.8 psig is a 105°F bubble point, and the dew point at that pressure is 2.4°F higher. Subcooling is measured from the 105°F bubble point.