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 these guys 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 Superheat.
What is SuperHeat?
Superheat lets you know if the correct amount of refrigerant is being fed into the evaporator. If your Superheat temperature is too high then not enough refrigerant is being fed in. This can result in poor system performance and loss of energy efficiency. However, if you find that the Superheat temperature is too low then you know that you have a surplus of refrigerant being fed into your evaporator. This result can be a sign that you are getting liquid refrigerant into your compressor. This isn’t a good thing! The liquid refrigerant inside a compressor can mix in with the oil at the bottom of the shell. This can result in poor lubrication to your compressor and may result in premature failure. Compressor failures are not cheap to fix.
To understand Superheat 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 click here to be taken to our ‘Understanding Refrigerants,’ guide. In the case of Superheat we need to follow the cycle at the point where refrigerant enters through the evaporator. At the point of entry into the evaporator the refrigerant is a liquid. While it is in the evaporator and heat is added the liquid slowly begins to turn into a vapor once it reaches it’s boiling point. (Also known as saturation temperature.)
Once the refrigerant has boiled to a vapor then any temperature above and beyond the boiling point is known as the Superheat. In other words, Superheat is any temperature of a gas that is above the boiling point for that liquid. The reason that Superheat is so important to measure is that it can give you a direct indicator as to what is wrong with the system.
Checking & Calculating Superheat
Checking and finding Superheat is relatively simple. Superheat 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 temperature that you measure on the refrigerant SHOULD be higher then what your boiling point/saturation point is on the refrigerant. If it is not, then you have no Superheat. Superheat can be determined by subtracting the boiling point/saturation point of the refrigerant from the actual temperature of the refrigerant vapor. As an example, if we had forty-five degrees boiling point and your actual refrigerant temperature is at sixty-seven degrees then you have a Superheat of twenty-three degrees.
To get your saturation or boiling point temperature you will need to use the low side on your gauge set to measure the pressure of the evaporator. Once you have this pressure you can then convert it to a temperature either using your gauge or a PT conversion table.
In order to get the most accurate reading on your refrigerant vapor it is best to take the temperature on the suction line as close to the Condenser as possible. Once you have your temperatures you do the math and presto you now have your Superheat.
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.