Subcooling on systems with a thermostatic expansion valve (TXV) should be between 10° and 18°. Higher subcooling implies that there is a buildup of refrigerant in the condenser. This is not good because it means that the compressor is working too hard. Lower subcooling means that the compressor is not working hard enough.
On systems without a TXV, subcooling should be as high as possible while still maintaining adequate cooling capacity. Too low of a subcooling value will result in poor heat transfer to the water, which could cause overheating of the chassis or components.
On all system types, subcooling should be adjusted based on how much refrigerant is in the condenser. A full tank of refrigerant will have lower subcooling requirements than an empty one. On many systems, the subcooling value can be adjusted by changing the size of a radiator fan or using a dual-flow radiator.
The default setting for most cases is appropriate for mid-range subcooling values. However, if you want higher subcooling then you can adjust it up. If you want lower subcooling then you can adjust it down. The best way to find out what setting works best with your system is to experiment with different settings and see what results you get.
While superheat shows the amount of refrigerant in the evaporator (high superheat suggests insufficient, low superheat indicates too much), subcooling indicates the amount of refrigerant in the condenser. If the system does not have a TXV, then subcooling can range from 0 to 30 degrees.
Superheat and subcooling are important factors in determining how efficiently your system will run. As heat enters the evaporator, some of it is absorbed by the refrigerant before entering the compressor. The remaining heat causes the refrigerant to change phase from a liquid to a gas, which requires energy. This change in state happens automatically when the refrigerant reaches its boiling point. Systems with high superheat require more frequent recharging than those with lower superheat levels. Also, high superheat levels indicate that you have more refrigerant flow through the evaporator than is necessary for effective cooling. This means you are using more electricity than required to operate the unit.
Subcooling occurs when refrigerant passes through the condenser without changing phase. Refrigerant flows into the condenser from the evaporator at temperatures above the condensing temperature. As it passes though the condenser, it absorbs heat from the surroundings. This excess heat causes the refrigerant to change phase from a gas to a liquid.
Subcooling is typically employed so that when the refrigerant reaches the thermostatic expansion valve, it is completely liquid, allowing the valve to function correctly. If gas enters the expansion valve, a number of undesirable occurrences may occur. The flow might be restricted by the valve's internal design, causing the heat pump to malfunction or fail prematurely. The gas could also enter the valve in large quantities, causing the valve to malfunction or fail entirely.
The gas that enters the thermostatic expansion valve during subfreezing conditions is referred to as "liquid slush." This gas has the same temperature as the heat pump's fluid (water), but it is not in a liquid state. It is actually a mixture of gas and ice particles. Since this mixture has some weight, the force of gravity will cause it to collect at the low point of the circuit-the evaporator in our example-where it can be removed by any further flow toward the compressor.
If the gas that enters the expansion valve during freezing temperatures is not subcooled, then when it reaches the thermostatic valve, it will form ice crystals which could restrict the flow through the valve resulting in reduced efficiency and possible damage to the valve itself. Also, if the ice melts due to heat from within the home or car, gas could enter the system where it would be forced into the walls and floorboards of the home causing damage.
The refrigerant is typically subcooled to between 10 and 20 degrees Celsius at the condenser's output. An incorrect subcooling value might suggest a variety of system issues, such as overcharging, undercharging, liquid line obstruction, or inadequate condenser airflow (or water flow when using water-cooled condensers). A trained technician should inspect your system for these issues.
The normal operating range for the compressor is -20 to 50 degrees Celsius. However, some compressors are designed to operate within a narrower range, while others can withstand temperatures much lower or higher than this range. Consult the owner's manual for specific information on your unit.
Compressor failures can cause overheating and damage to other components if not detected and repaired promptly. If you experience any problems with your unit, have it inspected by a qualified HVAC technician so any underlying issues can be addressed.
"Typically" on TXV systems, the Superheat will vary from 8 to 28 degrees, with a target temperature of 10 to 15 degrees. The Subcool range for TXV systems will be between 8 and 20 degrees Celsius. Target temperatures should be below 5 degrees.
These are only targets that most heat exchangers are designed for. In practice, you will get far better efficiency if you adjust the temperatures up or down depending on how much heat is being transferred at any given time. Heat pumps work best when the water is cold and the air is hot, so usually you would set the superheat high and the subcooler low.
The superheat is the maximum temperature of the fluid, while the subcool is the minimum temperature of the fluid. For example, if the superheat is 40 degrees Celsius and the subcool is 5 degrees Celsius, then the range of possible temperatures of the fluid is 35 to 45 degrees Celsius.
Heat pumps transfer heat through a process called "conduction", which is the movement of thermal energy from a region where there is a higher temperature to a region where there is a lower temperature. Thus, heat flows from areas of high concentration to areas of low concentration. This means that regions of high concentration of heat will have their temperature rise and regions of low concentration will have their temperature fall until they reach an equilibrium value.