When can we get this right?
I started in the HVAC business in 1977, and superheat and subcooling measurements and science were taught in my trade school Universal Technical Institute in Phoenix. In practicality, though superheat and subcooling was only understood after leaving school in those of us doing commercial refrigeration. Why is that? Digital tools such as digital charging manifolds, electronic thermometers, and psychrometers were not invented yet. In fact, I was one of the few technicians/students that possessed an electronic calculator at the time! The point of the matter is that I and (in my defense) my classmates seemed like hacks compared to the adequately equipped technicians of today, but some habits die hard.
As time moved along and the world discovered that the HVAC systems of the day were rarely charged correctly, these two measurements moved to the forefront. Since then, trade schools and contractors have tried to teach these two measurements into the technician workforce, which has not been so successful. Once the student left the classroom or lab, they went back to the beer-can-cold method of charging refrigeration system. The beer can is the ultimate test of superheat and subcooling in those days. Why? It seems simpler and easier to understand. Or maybe they were taught beer can cold from the person that broke them in (the old-timer). My mentors taught me the beer can method, and now I was teaching my mentors the modern methods after a decade or so, long after having graduated from the trade school.
Definition of Beer Can Cold
Beer can cold is the technique of wrapping one hand around the suction line and adding refrigerant until the suction line gets as cold as a beer can. NO, it never worked, but that is what was done.
Before we move along, I acknowledge that the old-timers endlessly debate the merits of charging by the beer can cold, but the fact remains that it is wrong. Sorry-not-sorry. Let’s move on.
Electronic Tools
Electronic tools came along in the 20th century and beyond that created the opportunity to measure super heat and subcooling precisely. They were expensive, and techs did not want to buy them. The problem of measuring the correct operating performance of the HVAC system remained due to the expense of the tools, and the idea of calculating the performance metrics remained elusive due to the remaining manual steps. In my experience, it’s easy to spot the analog manifold gauges still hanging in service trucks and the pocket thermometers stuck in technician pockets. Ask the type of refrigerant scale they have/use to experience what blank stares look like.
The industry has wised up and created single package digital charging manifolds and applications that calculate these values entirely without the technician’s manual intervention. If the values were taken and applied to the manufacturer charging instructions and charts, then the two values would be valuable for proving the HVAC system’s performance. To avoid other issues with refrigerant loss when connecting charging manifolds, digital transducers are available.
Charging Charts
The following sounds negative, but it is not (really) meant to be. The charging instructions are almost always located on a large sticker someplace in the HVAC system. For too many reasons to cover, the instructions were ignored (hard to find?), and beer can cold, or other rules of thumbs were still being used by technicians to determine the operating quality of the system. For instance, a rule of thumb that superheat should be “5 to 18 ºF” is a massive margin of error and does not factor ambient and indoor temperature, humidity, and airflow. Declaring correct operating performance using this range is wrong. Remember that the manufacturer always correctly states that “the correct airflow needs to be available before considering any superheat or subcooling values as being correct.” The instruction is paraphrased before anyone starts in the comments, as the exact wording varies between manufacturers and languages.
This rule of thumb is just irresponsible and wrong. Consumers buying expensive new HVAC systems should have assurance and proof that their systems are capable of the correct airflow and refrigeration performance, but they are not.
Inverters and Variable Capacity to the Rescue
Why do the idea of commonly and correctly interpreting superheat and subcooling seem like a lost cause? Inverter systems (aka mini splits and variable capacity) are now in the United States, although they have been widely used outside the country even longer. The only correct method of charging an inverter system is critically weighed in refrigerant charge that considers line set length and installation. The responsibility for operating the inverter system falls on control boards and firmware. Problem over, right? The short answer is no, but we have gotten closer to a solution.
How does one know if the refrigerant charge is correct?
- If it was weighed correctly, the refrigerant charge is correct. If there is a remaining difficulty, it is something else. Period.
- If it was NOT weighed incorrectly, then the refrigerant needs to be recovered and tried again to get it right.
- If there is a leak, the refrigerant charge needs to be recovered, the leak fixed, and the correct refrigerant charge weighed in.
Technician Certification Is Not The Entire Answer
Training, supervision, and verification are the only after-installation manner left on the table to ensure optimal performance. Caution: If the person training, supervising, and verifying have not been trained…more to come on this another time.
Mini Splits
Since the system control board is the only device that knows the currently operating settings such as dehumidifying, low or high heating, or cooling, it makes no sense to “put in a little refrigerant” to see what happens. We only know for sure that we no longer have any idea if the system is operating correctly at the exact moment after “adding a little refrigerant” occurs.
Every manufacturer has the magic dip switch or app setting that can force the system into the full capacity for service. Still, the white elephant in the room is that the unknown result remains unless the technician has been specifically trained on that manufacturer’s specific diagnostic steps. The consumer gets the short end of the stick.
Disclaimer
There are many technicians that have taken the time to excel in the HVAC industry and have been proactively learning their craft and investing in the tools. Kudos!
