Subcooling and Superheat: The Basics 

 Though HVAC professionals can work on a variety of given tasks from one day to the next, many of these tasks are linked to ensuring that a client’s air conditioning system is working effectively. When an air conditioner starts to offer less support to its user, there are steps that can be taken to reinvigorate the system.

While it is easy to assume that the lack of cool air is linked to low levels of refrigerant, this isn’t always the case. In fact, your system might have too much refrigerant or a few other problems. Checking for subcooling and superheating can help you to make the right diagnosis and fix the problem correctly. In this article, we will break down what you need to know about these two system factors.

What is Subcooling?

Subcooling is a term that is used in reference to heat removal. With subcooling, your focus is on the amount of heat that is being removed from a liquid. For air conditioners, this is specifically in reference to liquid that is below its condensing point.

When heat is removed from the liquid, logic tells us that the liquid will get colder. The act of the liquid getting colder following the removal of heat is subcooling—and it plays a huge role in the temperatures that are supported by the system and the system’s effectiveness. Subcooling can be used to charge systems that rely on a thermostatic expansion valve (TXV).

What is Superheat?

In many ways, superheat is the polar opposite of subcooling. With superheat, we instead focus on the heat being added to a system. In this case, any heat that is added to a material that is past its boiling point. When a liquid moves past its boiling point, it transitions into vapor. So, when we add heat to a vapor, it is referred to as superheat.

Adding heat to vapor might seem strange, but it has very important effects. When heat is added, it will make the vapor or steam that much hotter. The act of taking vapor and adding more heat to it is superheating.

Superheat is an important consideration because it tells us when the boiling point is reached within the system. The superheat readings can show the relative point in the cycle when the liquid begins to boil.

How to Check Superheat

In order to check superheat, you will need the right tools, including a temperature clamp. The temperature clamp will be attached to the suction line on the condenser unit to gather the appropriate temperature. All of the gauges will need to be hooked up on the condenser unit as well.

Your goal is to check both the temperature of the suction line and the reading for the suction pressure. Both factors will need to be referenced, giving you two different temperature readings—one for the suction pressure boiling point and the other for the temperature of the line.

The difference between these two temperatures can tell you if superheat is present. A suction pressure temperature reading of 45ºF and a suction line temperature of 56ºF tell you that there is 11ºF of superheat. This reading, in particular, can also demonstrate that the refrigerant is evaporating correctly and will not be staying in its liquid form.

How to Check Subcoolingt

Checking subcooling is conducted in the same way as checking for superheat. However, you are looking for a different change in temperature. In this case, rather than expecting the line temperature to be higher, you are checking to see if it is cooler than the temperature reading for the liquid pressure temperature.

When the line temperature is colder than the pressure temperature, it means that subcooling is present. A suction pressure temperature reading of 100ºF and a suction line temperature of 95ºF tell you that there is 5ºF of subcooling.

These readings tell us the point at which the vapor turns into liquid, and this is very important. If the vapor does not turn into liquid at the right time, it can negatively influence the system. Systems that use thermostatic expansion valves are dependent upon this liquid to operate correctly.

Determining Appropriate Readings

In order to determine if the system is functioning correctly, you will need to make sure that you are receiving appropriate readings. If your readings fall out of the appropriate range, you may need to take active steps to return the readings to the appropriate range.

Subcooling

Measuring the subcooling for refrigerant occurs between the metering device and the condenser itself. In most cases, the appropriate range for subcooling is 10° to 12°. However, this can vary depending on the system. When in doubt, refer to the manufacturer's details.

Superheat

Measuring superheat for refrigerants is an equally important process. In most cases, the appropriate superheat range is 10° to 15°. Of course, the appropriate range can also vary from one system to the next. Always refer to the manufacturer’s details when you receive a slightly different reading.

Abnormal Readings

It is common for HVAC professionals to check these readings on a regular basis. However, there are times when the readings might come back abnormal. Let’s explore a few of the common abnormal readings.

Negative Subcooling

In most cases, negative subcooling is a reading that only occurs when your tools are not working correctly. Miscalibration can make your tools give a negative reading when it is actually zero subcooling.

Zero Subcooling

With zero subcooling, there is a combination of liquid and vapor within the system. Some systems actively support this type of reading, so be sure to read the manufacturer’s details. In most cases, this is not a good reading.

Low or High Subcooling

When you receive a low or high subcooling reading, it all comes back to the refrigerant. For low readings, there is likely not enough refrigerant in the system. Weak compression, an inappropriate metering device, and undercharge can all cause this outcome.

High subcooling, on the other hand, means that there is too much refrigerant in the system. With these readings, you will want to look for problems with the lines, reevaluate your metering device, and consider that overcharge might be present.

Low or High Superheat

Just like with subcooling, low or high superheat readings mean that the system either has too little or too much refrigerant in most cases. Low superheat means that there is too much in the evaporator. High superheat means that there is not enough in the evaporator. High superheat can be caused by restrictions in the line, significant airflow, or a faulty metering device.

A Quick Recap

Every system has its preferred ranges. When these systems fall outside of their ranges, it is important to take active steps to repair them so that they can be restored to proper functionality. High superheat, low subcooling—or high subcooling, low superheat—can tell us a story about the system and its needs.

Ensuring that a system is fully operational within the appropriate ranges means that you need the right tools to read subcooling and superheat temperatures.