Arkansas HVACR NewsMagazine January 2025

HVACR NewsMagazine January 2025

Tech News

What Should My Subcooling Be?

This is the article you read BEFORE you call and ask a senior tech what your subcooling should be. It's also the one you send to a junior tech when they call and ask you: So, what is subcooling? (Or subcool, as many call it.)

(changing from vapor to liquid). Because the water is at 202°F instead of 212°F, we know it is liquid, and we can also say it is subcooled by 10°F. This 10°F of subcooling proves that

not only is it fully liquid but that it has given up more sensible heat energy — enough to drop 10° below the boiling temperature at that pressure. With refrigerant, we measure the subcooling between the condenser and the metering device, and it gives us a lot of information. It not only tells us whether or not the line is full of liquid; it also gives us indications of refrigerant charge and condenser efficiency when viewed in conjunction with the condensing temperature (high side saturation temperature). Now, be careful. As with all measurements, it is only as accurate as your tools. It must be taken using liquid line pressure and temperature (the line between the condenser and metering device), NOT discharge line pressure and temperature (the line between the compressor and the condenser coil), AND you must have a good connection to the port. I can't tell you how many times green techs have called me with “crazy” readings only to

Subcooling is a measurement of the temperature DECREASE of a liquid below its saturation (mixed liquid/vapor) temperature at a given pressure. For example, water boils at 212° Fahrenheit at sea level (atmospheric pressure of 14.7 PSIA). If water is 212°F and at atmospheric pressure at sea level, you can be sure it is at saturation, which means it is either in the process of boiling or condensing. If you measure that same water, and it is at 202°F, you can be sure that it is fully liquid and that it is no longer in the process of either boiling (changing from liquid to vapor) or condensing