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In this article, we will define both superheat and total superheat, calculate total superheat, explain how to use total superheat to check the refrigerant charge, and show where the measurement points are taken on an air conditioning system.
Total Superheat Formula:
Actual Vapor Line Temp – Sat Temp = Total Superheat
So what does this mean and what is the difference between Superheat and Total Superheat?
Simply put, superheat is the increase in temperature of the vapor refrigerant. On a split system air conditioner, superheat first occurs in the evaporator coil which is the indoor coil. The superheat method is used to measure the increase in temperature of the vapor refrigerant at the evaporator. The total superheat method is used to measure the increase in temperature of the vapor refrigerant at the evaporator plus any additional temperature change that occurs while the vapor refrigerant travels to the outdoor unit. Below is a picture of refrigerant superheating from 40°F to 54°F inside an evaporator coil.
First, let’s discuss how the refrigerant moves through an air conditioning system. There are four main components to an air conditioning refrigerant circuit. The compressor increases the pressure of the refrigerant and pumps the refrigerant through the system. The metering device is a restriction that reduces the pressure of the refrigerant. The evaporator is the heat absorption part of the system and the condenser is the heat rejection part of the system.
To watch a quick video explaining the basic four part refrigeration circuit, click below!
Now that the basics are covered, let’s get a little deeper into what is happening to the refrigerant in the system. At the inlet of the evaporator coil is a metering device. While the system is running, refrigerant enters the metering device as a high pressure, high temperature liquid. The metering device is a restriction in the tubing that lowers the pressure of the refrigerant. The refrigerant exits the metering device as a low pressure, low temperature liquid and enters the evaporator coil. Because the metering device lowers the pressure and the refrigerant expands, the temperature of the refrigerant lowers too. Due to the availability of space in the evaporator coil, this low temperature refrigerant expands and changes into roughly an 80% liquid 20% flash gas (vapor) mix.
As the low temperature refrigerant travels through the evaporator, it absorbs heat from the building’s indoor air as the air crosses over the evaporator coil. In order to transfer heat from the air to the refrigerant, the indoor fan takes air from within the building and pushes it across the evaporator coil fins. As the air crosses the coil and the refrigerant absorbs heat from the air, the temperature of the air lowers and the air exits the evaporator coil area at roughly 20°F lower than when it entered. (This lowering in temperature is called the Delta T and this change in temp may not always be 20°F but will be dependent on the operating conditions.) This low temperature supply air is then blown into the building.
As mentioned before, the indoor evaporator coil is the heat absorption part of the system while the outdoor condenser coil is the heat rejection part of the air conditioner. Remember that as the refrigerant flows through a running system, it absorbs heat from the air crossing the evaporator, the refrigerant travels to the outdoor unit, and it rejects heat into the air at the condenser. This results in removal of heat from the air within the building. To learn more about the refrigeration cycle check out our video below and also dive into our book “Refrigerant Charging and Service Procedures for Air Conditioning”.
Now that we have a general idea of what is going on in an air conditioning system, let’s focus on the indoor coil and the states of the refrigerant in the evaporator coil. Remember that a state is vapor (otherwise known as steam or gas), liquid, or solid. We find refrigerant in the liquid, vapor, or mixed state of liquid and vapor. When the refrigerant is in a mixed state, it is referred to as “saturated”.
In the indoor evaporator coil of an air conditioner, the refrigerant expands due to the availability of space in this coil. This low temperature refrigerant enters the evaporator coil as roughly an 80% liquid 20% flash gas (vapor) mix. As we mentioned previously, if liquid and vapor both exist, the refrigerant is saturated which means it is presently changing states. While the refrigerant is saturated and moving through the coil, it can absorb heat from the air crossing the coil without increasing in temperature. This is the secret to the refrigerant’s ability to store and transfer heat. Instead of the refrigerant increasing in temperature while absorbing heat, the refrigerant changes from 20% vapor and 80% liquid to a 50% vapor and 50% liquid, all the way to 99% vapor and 1% liquid. The refrigerant can absorb the majority of the heat from the air while the refrigerant is saturated, changing from liquid to the completely vapor state. After the refrigerant changes completely into the vapor state, the refrigerant rises in temperature until it comes out of the evaporator coil as a slightly higher temperature vapor. This increase in temperature of the vapor refrigerant in the evaporator coil is called the Superheat! That is really what we are measuring!
If we know the temperature of the saturated refrigerant in the evaporator, then we know the starting temperature of the vapor before it rises in temperature. If we measure the temperature on the vapor line exiting the evaporator coil, then we know the temperature after the refrigerant has risen in temperature. Subtract the lower temperature, saturated refrigerant measurement from the higher vapor line temperature and you have superheat!
If we measure the temperature on the vapor line at the outdoor unit, this would be after the refrigerant exits the evaporator coil and after the vapor line travels to the outdoor unit. This measurement shows the end temperature of the vapor refrigerant before it enters the outdoor unit. Subtract the lower temperature saturated refrigerant measurement from the higher vapor line temperature at the outdoor unit and you have total superheat! Total superheat is seen in the image below. This shows a actual vapor line temp of 55°F at the outdoor unit and a saturated temp of 40°F.
Ok, now practically speaking, how do we measure superheat and total superheat. Below is an example of an R-410A split system air conditioner. Unfortunately, most air conditioners do not have a pressure port at the outlet of the evaporator in order to measure superheat so we mainly check total superheat. To measure the total superheat, take a pressure measurement on the vapor line where the refrigerant enters the outdoor unit. This is done with a manifold gauge set with the blue, low pressure gauge and hose. There is typically a pressure port on the outdoor unit’s large vapor line service valve. Measure the pressure and convert this pressure to saturated temperature (sat temp) using a chart, gauge face, app, or digital manifold. After you find the sat temp, measure the temperature on the vapor line within 3 inches of the service valve. This will give you the vapor line temp and therefore the actual temperature of the refrigerant running through the line. This line temp will be higher than the sat temp.) Here is an example:
Calculate the total superheat based on the picture:
Actual Vapor Line Temp – Saturated Temp = Total Superheat
55°F - 37°F = 18°F of Total Superheat
I know its hard to picture but the pressure measurement taken at the outdoor unit’s vapor line port is roughly the same pressure found during the saturation of the refrigerant in the evaporator coil. This is even with the temperature of the vapor refrigerant rising before it gets to this vapor service port and the vapor refrigerant traveling some distance before it reaches the outdoor unit service port. (There are some exceptions where there is a pressure or temp change before the vapor reaches the outdoor unit service port such as on systems that have extra-long line set (line set is the vapor and liquid tubes that connect the outdoor unit to the indoor unit.), long vertical line set runs where the outdoor unit is much higher than the indoor unit, and systems with the line set buried in the ground.)
In order to use total superheat to check the charge of a running air conditioner, the unit must be equipped with a piston or capillary tube (fixed orifice) metering device and have a single speed compressor. The unit must also have proper airflow crossing the indoor coil. For every 12,000 BTU/HR of heat removal capacity, the indoor coil must have 350- 425 CFM (cubic feet per minute) of airflow crossing this coil. This means that the air filter must be clean, the ductwork must be sized correctly, and the blower speed is set to the correct airflow speed. An airflow of 400 CFM per 12,000 BTU/HR is a good number to shoot for.
Before checking the refrigerant charge with total superheat, the indoor and outdoor temps must both be above 70°F. This provides a heat load for the system to work with and is the minimum indoor and outdoor temps that you can accurately check the refrigerant charge at. Connect the gauges and purge the air from the hoses prior to starting up the unit. If you want to learn more, check out our book which goes into all the step by step details and procedures. The unit must run for 10-15 minutes before checking the charge with the total superheat method.
Once you calculate the total superheat, this number must be compared to the target superheat. The target superheat is not posted on the outdoor unit rating plate like a target subcooling is. Target superheat is a moving number based on the outdoor dry bulb (DB) temperature and the indoor wet bulb (WB) temperature. DB temp is measured with a standard mercury thermometer or a digital temp reader, outside the building, near the air inlet of the outdoor condenser coil. WB temperature is measured with a mercury thermometer that has a wet sock covering the bulb or with a digital psychrometer in the indoor return air duct. The WB and DB measurements are input onto a target superheat chart, app, calculator, or digital manifold set to calculate the target superheat under the current conditions. Here is an example of the locations for measuring the outdoor DB temp and the indoor WB temp.
To measure the DB temp, you could use a tool such as this one (https://amzn.to/3dNHFkY). To measure the WB temp, you could add this sensor to the DB temp tool (https://amzn.to/346iFB4) and wet the sock with water. You could also use any of these four tools to read the indoor WB temp (https://amzn.to/2UxKxe8, https://amzn.to/2JwYtyQ, https://amzn.to/3bciTcv, http://amzn.to/2nniMVR). Now let’s take the outdoor DB temp of 90°F and the indoor WB temp of 64°F and input them on our target superheat chart.
If the actual total superheat is within plus or minus 2°F of the target superheat, the charge level is accurate. However, you want to be as close to the target superheat as possible.
If the actual total superheat is higher than the target superheat, the unit’s refrigerant level is undercharged. There is a leak that needs to be fixed and refrigerant will need to be added.
If the actual total superheat is lower than the target superheat, the unit’s refrigerant level is overcharged. Some refrigerant will need to be recovered into a recovery bottle.
• Actual Total Superheat +/-2° F of Target Superheat = This is Correct
• Actual Total Superheat > Target Superheat = Add Refrigerant
• Actual Total Superheat < Target Superheat = Recover Refrigerant
We measure the total superheat based on the picture. Then we compare it to the target superheat.
Actual Line Temp – Sat Temp = Total Superheat
55°F - 37°F = 18°F
18°F of Total Superheat > 9°F Target Superheat = Undercharged
Since the actual total superheat measured is higher than the target superheat, we would need to add refrigerant a little at a time until the superheat is the same as the target superheat.
Remember that as the system runs, the WB will lower due to the removal of heat and humidity from within the building and the DB will usually remain close to the initial number. These measurements must continually be taken and input into the target chart, app, calculator, or digital manifold to have the correct target superheat for that moment in time. In other words, if you are spending time charging the unit, the target superheat will typically lower while you are charging the system. Make sure that you are continually checking the target superheat so that you don’t accidentally overcharge the unit. Also, after adding refrigerant, allow some time for the refrigerant to circulate through the system before measuring your new total superheat and comparing it to your current target superheat.
If this was an existing unit that was previously working fine, then there must be a refrigerant leak in the system. Whenever possible, find a fix the leak before adding more refrigerant. If the system is very low on refrigerant, do not just add more refrigerant because it will likely leak out rapidly from the system which will not be good for the service tech, the homeowner, or the environment. I use anti-corrosive bubble leak detector, and primarily the ultrasonic leak detector when searching for refrigerant leaks. Here are the links for those items for searching for refrigerant leaks:
• Ultrasonic Leak Detector - https://amzn.to/2JOLYhX
• Bubble Leak Detector - https://amzn.to/3c0kdzb
• Small Bubble Leak Detector - https://amzn.to/2yI4VRj
If you want to learn how to use total superheat, subcooling, saturated temps, and delta T to troubleshoot a problem with a system, check out our book and self-study workbook available here on our site and on amazon! This book goes from the basics all the way through to troubleshooting complex problems. It is designed for those beginning in the field as well as those who are seasoned in the field. There are great procedures for everyone and its written in laymen’s terms, so it is easy for the average person or tech to understand.
Also make sure to check out our HVAC Quick Reference Cards as well as our Refrigerant Charging Workbook! They are both great resources to give you an advantage out in the field!
Published: 4/16/2020 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/
