The HVAC Subcooling Charging Method, Explained!

In this article, we will define subcooling, calculate subcooling, explain how to use subcooling to check the refrigerant charge, and show where the measurement points are taken on an air conditioning system.

Subcooling Formula:

Saturated Temp – Actual Line Temp = Subcooling So what does this mean?

Simply put, subcooling is the lowering in temperature of the liquid refrigerant in the condenser coil. On a split system air conditioner, the condenser coil is in the outdoor unit. While the system is running, refrigerant exits the vapor compressor and travels through the condenser coil to reject heat into the outdoor air. In order to do this, a condenser fan pulls outdoor air across the condenser coil fins and blows the hotter air out of the top of the unit. The refrigerant traveling through the condenser coil is higher in temperature than the outdoor air so when the outdoor air crosses the condenser coil fins, the air temp rises as the refrigerant rejects its heat into air. The result is hotter air blowing out of the top of the condensing unit than that being sucked into the side of the unit. This hot air is expelled into the outdoor environment.

The outdoor condenser coil is the heat rejection part of an air conditioner while the indoor evaporator coil is the heat absorption part of the air conditioner. Remember that as the refrigerant flows through a running system, the refrigerant absorbs heat from the air moving across the evaporator coil fins. The refrigerant then travels to the outdoor unit where it rejects the heat into the air moving across the condenser coil fins. This results in removal of heat from the air within the building. To learn more about the refrigeration cycle check out this 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 a running air conditioning system, let’s focus on the outdoor unit and the states of the refrigerant in the condenser coil. Remember that a state can be 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 outdoor unit of an air conditioner, the refrigerant exits the compressor as a high temperature vapor refrigerant. This high temperature vapor refrigerant enters the condenser coil and starts to reject heat while lowering in temperature. Once the refrigerant rejects enough heat and lowers in temperature, the refrigerant becomes saturated (See the Picture Below). After the refrigerant becomes saturated, it no longer lowers in temperature, but it continues to reject heat. This is the secret to the refrigerant’s ability to store and transfer heat. Instead of lowering in temperature while rejecting heat, the refrigerant changes from 99% vapor and 1% liquid mix to a 50% vapor and 50% liquid mix, all the way to 99% liquid and 1% vapor mix. The refrigerant is able to reject the majority of its heat while the refrigerant is in this saturated state changing from vapor to the completely liquid state. After the refrigerant changes completely into the liquid state, the refrigerant lowers in temperature until it comes out of the condenser coil as a lower temperature liquid. The lowering in temperature of the liquid refrigerant in the condenser coil is called the Subcooling. That is really what we are measuring!

If we know the temperature of the saturated refrigerant in the condenser, then we know the starting temperature of the liquid before it starts to lower in temperature. If we measure the temperature on the liquid line exiting the condenser coil then we know the end temperature after the refrigerant has lowered in temperature. Subtract the lower temperature measured on the liquid line from the saturated temperature and you have subcooling!

105°F Sat Temp - 93°F Actual Line Temp = 12°F of Subcooling

Ok, now practically speaking, how do we measure subcooling? Below is an example of an

R-410A split system air conditioner. Take a pressure measurement on the liquid line where the refrigerant exits the condenser coil. This is done with a manifold gauge set using the red, high pressure gauge and hose. There is typically a pressure port on the outdoor unit’s small liquid line service valve. Measure this pressure and convert the pressure to saturated temperature (sat temp) using a P/T chart, the gauge face, an app, or a digital manifold. (We have quick charging and troubleshooting cards available that also include a P/T chart here.) On the gauge face in the picture below, the pressure measured is 318 PSIG. The pressure needle intersects at 100°F saturated temperature for R-410A. Therefore, the saturated temperature inside the condenser coil is 100°F. Below is the example:

Calculate the subcooling based on the picture:

Sat Temp - Actual Line Temp= Subcooling

100°F - 95°F = 5°F of Subcooling

I know its hard to imagine but the pressure measurement taken at the liquid line is roughly the same pressure found during the saturation of the refrigerant in the condenser coil even though the temperature of the liquid refrigerant subcools (lowers) before it gets to this liquid service port.

In order to use subcooling to check the charge of a running air conditioner, the unit must be equipped with a TXV (thermostatic expansion valve) as the metering device and have a single speed compressor or a two-speed compressor running in second speed. (If the unit has a fixed orifice metering device, use the Total Superheat Charging Method.) 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 must be 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 in subcooling, 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 the unit up. If you want to learn more about the step by step procedures, check out our book which goes into all the details. Also check out our video on connecting and disconnecting the manifold gauge set. Remember, the unit must run for 5-10 minutes before checking the subcooling.

Once you calculate the subcooling, this number must be compared to the target subcooling provided by the manufacturer. This may be found on the outdoor unit rating plate, on the inside of the outdoor unit shroud, or in the manufacturer’s literature. The manufacturer may provide you with one target subcooling number or several numbers based on the outdoor temp. If the manufacturer provides just one target subcooling number, this number is the average subcooling that the unit should run with. However, the subcooling of a system will be a little higher when the outdoor temp is high and a little lower when the outdoor temp is closer to 70. If a manufacturer provides several target subcooling numbers, these numbers will not vary a great deal. I just want you to keep this in mind. The target subcooling may be posted on the rating plate as "TXV Subcooling" such as in the picture below.

Anyway, if the actual subcooling is within plus or minus 3°F of the target subcooling, the charge level is accurate. However, you want to be as close to the target subcooling as possible.

If the actual subcooling is lower than the target subcooling, 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 subcooling is higher than the target subcooling, the unit’s refrigerant charge level is overcharged. Some refrigerant will need to be recovered into a recovery bottle.

• Actual Subcooling +/-3° F Target Subcooling = Correct Refrigerant Level

• Actual Subcooling < Target Subcooling = Add Refrigerant

• Actual Subcooling > Target Subcooling = Recover Refrigerant

Also even though we need to check the charge of this unit with subcooling, make sure that the vapor gauge shows a sat temp higher than 32°F, otherwise there could be a problem such as low airflow, liquid line restriction, or low refrigerant charge that would allow the indoor evaporator coil to freeze. For now just make sure the vapor gauge reads above 32°F and check out our articles on troubleshooting a frozen evaporator for more info! In the following example, lets determine if the unit is correctly charged, undercharged, or overcharged.

We measure the subcooling based on the picture. Then we compare it to the target subcooling.

Sat Temp - Actual Line Temp = Subcooling

100°F - 95°F = 5°F of Subcooling

5°F of Subcooling < 12°F Target Subcooling = Undercharged

Since the actual subcooling measured is less than the target subcooling, we would need to add refrigerant a little at a time until the subcooling was the same as the target subcooling.

If this was an existing unit that was previously working fine, then there must be a refrigerant leak in the system. Whenever possible, find and 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 subcooling, superheat, saturated temps, and delta T to troubleshoot a problem with a system, check out our book 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 is great practices 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: 4/8/2019 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/