(The content of this post is intended for consideration by trained service personnel only)
Superheat and subcooling are the terms used to describe two of a heat pump system’s operating characteristics. We, in the service business, generally rely on these numbers to evaluate system performance as well diagnose system problems. The values essentially provide us with information about what’s going on inside the evaporator and condenser coils. And depending on the metering device used in the system, one or the other number is the value used to determine optimum system charge.
If you wanted to define the words non-mathematically, superheat is the increase in temperature of the refrigerant vapor in the evaporator before it exits the coil, and subcooling is the decrease in temperature of the refrigerant liquid in the condenser before it exits the coil. The diagram below offers a visual illustration.
The two numbers are actually calculated temperature values, using simple arithmetic with saturated temperatures and tubing temperatures.
Normal operation always results in some percentage of the evaporator coil filled 100% with vapor and some percentage of the condenser coil filled 100% with liquid. Since the vapor starts out at the same saturated suction temperature, the vapor will take in heat or warm up, before it exits the evaporator coil. Likewise, the liquid starts out at the same saturated condensing temperature, so it will give up heat or cool down before exiting the condenser coil.
So to calculate the superheat, subtract the saturated suction temperature from the suction line temperature. Which in the diagram is 50 – 40 = 10 F superheat.
To calculate subcooling, subtract the liquid line temperature from the saturated condensing temperature: 110 – 100 = 10 F subcooling.
You can get more explanation of heat pump charging details in the refrigerant system video for rent: