How NOT to Check the Refrigerant Charge!
There are many ways that technicians have tried to get around actually checking the charge of air conditioners and heat pumps with superheat and subcooling! I go into great detail explaining methods that you should use and methods that you should NOT use in the book “Refrigerant Charging and Service Procedures for Air Conditioning”. In this article I just want to highlight some of these shortcut methods to give you a taste of what junior techs are being told and may be choosing to do in the field. There are many junior techs being taught the right way but unfortunately there are just far too many that are being taught rules of thumb that are detrimental to the system, bad for the equipment owner, and bad for the tech themselves.
First, I want to discuss the why. Why are some junior and senior techs using shortcuts instead of proven methods to check the charge? Often, it is done with good intentions like trying not to contaminate the system with old oil or water by attaching the gauge set. Another reason may be time. It takes longer to do things right and if the boss or manager has fooled themselves into thinking a service call or pm takes less time than it should, they may place unrealistic goals in front of service techs. This often happens when a service tech takes it upon himself/herself to apply a short-cut and continues to use these short cuts on jobs such as preventative maintenances. Then that service tech leaves the company and a new tech comes into the position of being expected to do the same job just as quickly as the previous tech did. The new tech is pressured into thinking that its ok to take a short cut to get all the pm’s or service calls done for the day.
Has anyone ever heard of a tech doing 37 preventative maintenances in one day and not all of them at the same complex? Well, it happens and its just not good for anybody involved. I need to tell you that I hear this kind of story a lot from eager technicians that want to grow and do things the right way. They understand that they must be efficient with their time but they are frustrated that they are expected to do the job but not allowed to do it the right way based on the time allotted. There will always be a choice for how much time we can allow ourselves to spend on each system but we all need to check ourselves to make sure that we are giving our best to our company and the best to the equipment owner.
So, let’s look at some of the rules of thumb that are out there and explore why they should not be used to check the charge.
Beer Can Cold:
This is probably the most well-known and funny method out there that is known to just be wrong. It includes taking a refrigerated beer straight out of the refrigerator and holding it in one hand while you grab a hold of the suction (vapor) line with your other hand, comparing the two temps. This is after the air conditioning system has been running in ac mode for at least 10 minutes. If the temperature of the beer is close to the temperature on the vapor line, the system is working correctly, and the system’s charge is correct. That is the thought at least. While some systems would run with a vapor temp close to the temperature of a beverage just pulled from the fridge, it just does not give you insight into the system.
Practically speaking, techs in the field are not doing this but some may be trying to target a specific temperature on the vapor line. If a tech is always targeting a vapor temp of 40 or 50 °F, they will certainly have problems with the systems they service. I will be covering the problems after discussing each of these shortcut methods.
Targeting a Certain Vapor Pressure:
This shortcut/method involves adding or recovering refrigerant in an effort to reach a certain vapor pressure. Of course, this just won’t work on systems with a TXV metering device because the TXV will control the superheat across the evaporator. (Make sure to check out our full length articles on Superheat and Subcooling to learn about these charging methods.) This means that even after refrigerant is added, the TXV may keep the vapor pressure steady instead of allowing it to rise like on a system with a fixed orifice.
If you target a specific vapor pressure on a system, you will run into big problems. On a system with a fixed orifice, you could accidentally allow liquid refrigerant into the compressor and on a system with a TXV, you could easily overcharge the system leading to low efficiency and low capacity.
Targeting a Certain Vapor and Liquid Pressure Based on Experience and Indoor and Outdoor Temperature:
This method involves measuring temperatures inside and outside the building and trying to guess what the pressures should be and adjusting the charge of the system based on that guess. At least with this method, the tech is reading pressure on both the high side and the low pressure side but it is just not enough info to make the correct choice. In the end, it is still a guess and that means that you will not always be correct. Systems are usually left undercharged or overcharged using this method. Also, if there is a problem with the system, the tech will not have enough measurements to determine the problem.
Targeting a Specific Delta T Only
Some techs try not to connect their gauge set to a system. This is to avoid system oil contamination and/or to reduce time on the job. They notice a pattern in systems that are working well such as an indoor Delta T of 18 to 21°F. Because this pattern is noticed, they try to reverse engineer it and say, “Hey, if I have 18-21°F, then the system charge and health must be good.” Unfortunately, this may not be the case. Delta T is only one part of the picture. It is a nice part of the picture because it shows that heat transfer is taking place but it is certainly not a full picture. Some systems, especially those with a fixed orifice may not have as high of a Delta T due to the indoor heat load. Systems with low airflow and a Delta T of 20 degrees may be running at low capacity.
Ambient +30 or Ambient +25
Ambient +30 was applied to R-22 10 seer units and Ambient +25 was applied to R-22 13 seer outdoor units. To use this shortcut method, you would measure the outdoor ambient dry bulb temperature and add 30°F to this dry bulb temp. That will be the target saturated temp on the high side. Then you set the high side saturated temp to this target temp. Ambient +25 was used in the same way except 25°F was added to the outdoor ambient dry bulb temperature to find the target high side saturated temperature.
This method does not take into consideration anything on the low pressure side of the system and it may get the tech close to the correct high side saturated temp. Unfortunately, it may not give you an accurate subcooling.
Targeting a specific condensing unit exiting air temp
This is a shortcut used to try to determine what the saturated temperature of the high side of the system is without attaching gauges to the system. This will give you a temperature close to the high side saturated temp but it is only part of the story. Liquid line temp, the vapor sat temp and vapor line temps are also needed. Delta T should also be measured and applied.
Vapor line and liquid line temps only
These line temperature measurements will give you the temp of the refrigerant as it travels through the system but they will not give you a clear picture of the systems charge without reading the saturated temps on both the high and low pressure sides of the system.
Vapor line sweating
The vapor line will only sweat in certain circumstances. Granted, most of the time when an air conditioning system is working properly, the vapor line will sweat, but not in all circumstances. On a fixed orifice system with a high indoor heat load, the vapor line may not sweat. Also, the vapor line may sweat regardless of whether the system is under charged a little or overcharged.
The Cold Foot Method
This is just a silly one I thought I would throw in there. We often hear of some other method that is being using to check the charge and sometimes we just have to laugh because we know how absurd it is! In this case, this a more crazy version of the beer can cold method by using feet to make the temperature comparison between the ice water and the vapor line. Please don't do this, Ha ha!!!
The whole point of this article is to make sure that techs are aware of the many different aspects involved with checking the charge. If a tech is using a short cut to try to check the charge then you certainly know that they have not checked for proper airflow based on the capacity size of the system. If someone is using shortcuts, they cannot confirm compressor safety, system capacity, humidity removal capacity, or the system’s electrical efficiency. If a system has a fixed orifice metering device, compressor safety is a very big concern whereas a system with a TXV will partially protect the compressor by maintaining the superheat across the evaporator. If a system has a TXV, the vapor pressure will not rise when adding refrigerant like on a system with a fixed orifice. This leads to overcharging if the tech is not aware to check subcooling on the high-pressure side of the TXV system.
I want the best for the technician, the best for the equipment owner and the best for the HVACR business owner and I believe that comes from doing things the right way. Regarding the technician, if they are able to check the charge correctly by taking all of the measurements that are needed to determine a systems health, they can also easily troubleshoot a system. If a tech is always using some short cut to complete the job, they may be able to get by for a while but they will run into problems when the system does not work as they anticipate.
Taking all the measurements needed will give the tech a clear insight into how each system works and will allow troubleshooting to become clear and quicker. Imagine if you can see into the inner workings of a system. That is what eventually happens in the mind of a tech that consistently takes all the measurements needed. They can imagine how the system is working at each part within the system based on the measurements taken. Next, speed follows. If they can imagine how each part of the system is functioning based on the measurements, the tech becomes very accurate during the diagnosis of a problem. The end result is an efficient technician that is profitable and valuable. We want troubleshooters out there in the field, not just those who are barely getting by. Educated technicians that care, intelligently problem solve, and are happy to teach others, are worth their weight in gold. Be one!
If you want to learn more about all the fine details on charging methods, 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!
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Published: 5/28/2020 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/