Superheat and Subcooling The Ying and Yang of Air Conditioning

Updated: Feb 13

Together, they give you insight into a refrigeration circuit’s performance, and provide clues to problems, but on their own, they’re about useless. Unfortunately, most technicians have a fascination with superheat and don’t give a moment’s thought to superheats cousin, subcooling.

They’re quick to add refrigerant if the superheat is “high”, or start cranking on the TXV’s adjustment if it’s “low”. My first question is, what the heck do you consider high and what exactly is low? Some Techs will tell you 8° is good, others swear by 18°, and some think 10° is the bomb. So, which one is correct? The answer is simple, the one recommended by the manufacturer is correct.

But what if the value isn’t what the manufacturer recommends? As a diagnostic technician, you also need to know what superheat value is wrong, and what’s causing it to be wrong. This is where subcooling comes into play. Say your superheat is high, does that mean the system is low on charge? Answer; check your subcooling. Suppose the superheat is low…is the system overcharged? Answer; check your subcooling.

Commit this to memory; superheat doesn’t tell you jack-$hit without Subcooling, but together, they’ll point you in the right direction. And remember, superheat and subcooling only point you in the right direction, but you’ll probably need other data to identify the root problem.

Stay tuned, we'll did deeper and provide some examples in upcoming articles.