Being able to measure refrigerant Subcool and Superheat are essential for diagnosing and correcting an air conditioning or refrigeration unit. Unfortunately, there is a lot of misunderstanding as to what exactly Superheat and Subcool actually are and even less understanding on how to measure it. There are so many novice technicians that get ‘stumped’ on a system without even checking Subcool or Superheat temperatures…. or if they did check them they checked in the wrong section. These same people then end up calling for help from experienced technicians, but as they soon find out one of the first questions anybody will ask when diagnosing a unit is what are the Subcool and Superheat temperatures? These two temperatures can tell you so much about a system.
While this article isn’t going into all of the technical details behind each temperature I will do my best to do a high level explanation of Subcool. Of course we also have an explanation of Superheat.
What is Subcool?
To understand Subcool we first have to understand the refrigerant cycle and how it flows through a system. I won’t get into every technical detail here of a air conditioning system. If you want a full rundown check out our ‘Understanding Refrigerants,’ guide. In the case of Subcool we need to follow the cycle at the point where refrigerant enters through the condenser.
When refrigerant enters the systems condenser it is in it’s gaseous, or vapor, state. While in the condenser heat is removed from the gas until that refrigerant reaches it’s saturation condensing temperature. When this point is reached the refrigerant gas turns back into a liquid. Once your refrigerant has turned to liquid while going through the condenser your Subcool can be found. The term Subcool refers to any temperature that is below the saturation condensing point of the refrigerant. Let’s say I have a saturation temperature of eighty degrees and the line temperature is at seventy-seven degrees. We now know that we have a three degree Subcool.
The efficiency of your system depends on how much liquid refrigerant is fed into your evaporator. If your refrigerant is being fed into the evaporator and it is NOT Subcooled then you are feeding refrigerant gas into your evaporator. If you are feeding gas into your evaporator then the system will act as if it is restricted and your cooling capacity and efficiency will plummet dramatically. In most systems the condenser acts like a metering device as to how much liquid refrigerant can be fed into your evaporator. If your system is suffering from poor Subcool then you can determine nearly right away that something is wrong with your condenser.
How to Check Subcool
Determining Subcool is similar to how we found Superheat. Just like with Superheat, Subcool is a calculated value by taking the difference between two temperatures. First you must find the actual temperature of the refrigerant vapor and then you need the saturation or boiling point of that same refrigerant. The saturation point can be found by using the high side manifold on your gauge set. This will allow you to measure the pressure of the condenser. Once you have this pressure you can then convert it to a temperature either using your gauge or a PT conversion table.
Secondly, you need to take your thermostat or thermometer and measure the liquid line temperature for your next reading. The liquid line is located between the condenser and the evaporator. In order to get the most accurate reading you should take the temperature as close to the evaporator as possible and before the metering device. (If your system has a metering device.)
Here is where one of the key differences between Superheat and Subcool come into play. With Subcool the gauge/saturation temperature is going to be higher then your line temperature. (Remember, with Superheat your line temp should always be higher then gauge.) Once you have these two readings you then do the math. For example. Let’s say we have a saturation temperature of eighty-seven degrees and our line temperature is at seventy-nine degrees. We now have a Subcool reading of eight degrees.
Conclusion
As I mentioned in the introduction of this article I did not plan to dive deep into every little thing about Superheat or Subcool. I would prefer to save the really technical stuff for the guys who have already done their homework and have it mapped out quite well already. If you have more questions on these topics please refer to the links below. They provide a wealth of information on the topics and will give you more information then you would ever need.