In this article, we will discuss an HVAC unit’s refrigerant pressure on both the high side and the low-pressure side while the system runs. It is important to know the range of these pressures in order to understand why and how charging and recovery are performed. The first thing to realize is that when a system is off and equalized, the system’s pressures on both the high side and the low-pressure side will match. In the example of an R-410A packaged unit with a surrounding air temperature of 70°F, the pressure on both the high and low-pressure side of the system will be 201 PSIG. If a new R-410A refrigerant bottle had a surrounding air temperature of 70°F, the pressure inside the bottle would be 201 PSIG. Likewise, an R-410A recovery bottle with a surrounding air temperature of 70 °F should have an internal pressure of 201 PSIG. Remember that temperature will affect the pressure of a refrigerant. If the air temperature surrounding the refrigerant increases, the refrigerant will absorb that heat and increase in temperature. This will cause the pressure of the refrigerant to rise. In the example of an R-410A packaged unit with a surrounding air temperature of 75°F, the pressure on both the high and low-pressure side of the system will be 217 PSIG. If a new R-410A refrigerant bottle had a surrounding air temperature of 75°F, the pressure inside the bottle would be 217 PSIG. Likewise, an R-410A recovery bottle with a surrounding air temperature of 75°F should have an internal pressure of 217 PSIG. When an air conditioning system turns on, the large vapor line will lower in pressure while the small liquid line will rise in pressure. We will first examine the low-pressure side of the system, otherwise known as the vapor or suction line. During air conditioning mode, the pressure on the vapor line of an R-410A system will be somewhere between 102 to 145 PSIG. If the system had R-22, the vapor pressure would be between 58 to 85 PSIG but these pressures will be dependent on the wet bulb temperature inside the building and the ambient temperature outside the building. The indoor wet bulb temp shows the heat load on the inside of the building because it takes into consideration both temperature and humidity. The higher the heat load on the inside of the building, the higher the pressure will be on the vapor line. Likewise, the higher the outdoor air temperature is, the less heat the system can reject outside. This also results in a higher vapor pressure. Learn more about indoor wet bulb temp and outdoor dry bulb temp and how they affect charging in our book “ Refrigerant Charging and Service Procedures for Air Conditioning ”. Other major factors that affect the vapor pressure are the type of metering device and the indoor airflow. Where techs get into trouble is when they try to guess these pressures when checking the charge of a system. To learn the correct ways for checking the charge, make sure to read the Subcooling Method article and the Total Superheat Method article! Anyway, getting back to this article, if the air conditioning system had R-410A, we know the pressure on the low side of the system will be between 102 and 145 PSIG regardless of the heat load conditions (except for extreme circumstances). If the outdoor temperature is 70°F, a refrigerant bottle outside would have a pressure of roughly 201 PSIG. If the outdoor temperature is 110°F, a refrigerant bottle outside would have a pressure of roughly 366 PSIG. In any instance, the pressure inside the new refrigerant bottle will be higher than the pressure on the vapor/suction line of a running system. Because of this, the refrigerant from the new bottle will exit the bottle and enter the system as long as the system is running and only if the service valve on the manifold connecting the two is opened. The picture below shows a system running on a 85°F day that had 6 ounces of R-410A added. In the picture, the manifold valve to the blue hose is closed so the blue gauge is measuring the pressure inside the running system. The vapor pressure is 118 PSIG and because it is 85°F outside, the R-410A bottle pressure is 254 PSIG. The pressure in the bottle is much higher than the pressure on the low side of the system so if they are connected, refrigerant will exit the bottle and enter the system. When the system is off and the pressure in the system matches the bottle pressure, the only way the refrigerant would exit the bottle and enter the system is if a bottle warmer https://amzn.to/3fOhZom is used to increase the temperature of the bottle. This will increase the pressure of the bottle to a higher pressure than is in the inside of the system. This would allow for slow charging while the system is off. However, the technician needs to be able to check the charge while adding refrigerant in order to know how much to add unless they are charging by weight per foot of line set. In order to learn more about charging by weight, read this article on the Total Weight Method . The only time a technician would add refrigerant into the liquid line of an air conditioning system is if the system is off, empty, and vacuumed. Technicians use the total weight method to break the vacuum of a system with the correct amount of refrigerant needed based on the added line set length. Refrigerant is added into the liquid line for two reasons. One is because the liquid line is small in interior volume so there is a better chance of weighing the full amount of liquid refrigerant needed into the unit. This is because the liquid line is small and will not allow the refrigerant to vaporize as quickly as the larger vapor line would. Remember that after the refrigerant vaporizes, it will apply pressure inside the system and this pressure will increase to the same pressure as the pressure inside the bottle. This will stop the refrigerant from flowing from the bottle into the system. The other reason liquid refrigerant is added into the liquid line of an off, empty, and vacuumed system is so that when the system starts up, the compressor will not get slugged with liquid refrigerant. If the refrigerant is added into the liquid line, the refrigerant will have to make its way through the metering device before it can enter the vapor line. This allows less saturated refrigerant to be inside the vapor line for the initial start-up. This will keep the vapor compressor safer from having liquid refrigerant enter it. As far as recovering a small amount of refrigerant from a running system, this can be done without a recovery machine by connecting the liquid line of the running system to the recovery bottle. However, this method should not be used for recovering large amounts of refrigerant because mixed in with the high-pressure liquid will be the system’s oil. Remember that the system's oil circulates through the inside of the system with the refrigerant and is carried along by the refrigerant. In cases where a large amount of refrigerant needs to be recovered, make sure to use a recovery machine while the system is off. You can learn more about this setup in our book “Refrigerant Charging and Service Procedures for Air Conditioning” . The liquid line on a running system will have a higher pressure than the pressure inside the recovery bottle as long as the recovery bottle does not have air, nitrogen, or a mix of multiple refrigerants inside. It is very important to check the pressure of a recovery bottle before using it to recover refrigerant from the system. If the recovery bottle has air in it, the pressure may be higher than the pressure on the liquid line of a running system. If the recovery bottle is connected to the liquid line of a running system in an attempt to recover a little bit of refrigerant from the system, it may allow the air and refrigerant mix to exit the bottle and enter the system instead of the refrigerant exiting the system and entering the bottle. It is crucial to check the recovery bottle pressure before use! To learn more about contaminated refrigerant problems, check out our book ! On a running system, the liquid pressure does not have a consistent range like the vapor pressure does. This is because the outdoor temperature swing is much larger than the indoor temp swing. For instance, it may be anywhere from 68 to 80°F inside the building but outside, it may be anywhere from 65 to 110°F. Also, the liquid pressure will be dependent on the SEER rating, fin condition, shading, and outdoor airflow. If a technician is trying to guess what this pressure should be when trying to check the charge, they may be very far off compared to an actual refrigerant charging method. In our book we go over many of the methods that have been used to try to short cut checking the refrigerant charge the correct way. For each of these methods, we lay out what the disadvantages are. To wrap this up, when we charge refrigerant into a running system, we add the new refrigerant into the vapor line slowly and check the charge as we go. If we want to recover a small amount of refrigerant from a running system, we first CHECK THE PRESSURE of the recovery bottle and then we can de-charge (recover) the refrigerant from the system by connecting the liquid line to the recovery tank and we meter the refrigerant into the recovery bottle slowly using our manifold gauge set valve. Always recover slowly with this method because it will occur quickly due to the liquid state of the refrigerant in the liquid line. Don't recover a large amount of refrigerant in this manner because a large amount of oil will be removed from the system. If a large amount of refrigerant needs to be recovered, turn the system off and connect a recovery machine from the system to the recovery bottle. If you want to learn more about all the fine details on charging methods and troubleshooting, check out our book which is available on our website and on amazon . The full outline and sample pages are available here . We have a 1,000 question workbook with an answer key that you can use to apply your knowledge as well. Check out our free quizzes to test your knowledge here ! If you want to learn the full Total Superheat Charging Method, check out this article ! If you want to learn the full Subcooling Charging Method, check out this article ! If you want to learn about Delta T, check out this article ! Tools that we use: www.amazon.com/shop/acservicetech Follow us on Facebook for Quick Tips and Updates here ! Published: 6/24/2020 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/