A request was made by Simpleman for me to find an old post:

Originally posted by simpleman
Is there a way to go back in the archives and bring up
a thread from 6-11-02?

It was a reply from fixitman.It was the best explanation of
sh/sc that I ever herd before.

I sure wish I could bring it up to put back into my archives,I've lost it some how.

Thank you!
Yes, I do have the post, here it is:

Originally posted by fixitman
The superheat and subcooling tell you where your refrigerant is in the system, and how well you're using your coils, assuming clean coils (you can't always assume that!). You are boiling liquid/vapor in one of them to make a gas, and condensing gaseous refrigerant in the other one to make a liquid.

The superheat tells you, relatively, how much of the coil is in the gaseous phase, after all the refrigerant has boiled. Less cooling takes place in that portion of the coil, and the temperature of the gaseous refrigerant rises pretty quickly. So you get a measurable difference between that temperature and the temperature of the boiling refrigerant.

If the superheat is high, you are not maximizing the use of the coil. Maybe the charge is low, maybe the coil is too big, maybe, maybe, maybe. But you have an idea how much of the coil is delivering cooling. If superheat's too low, you're in danger of sending liquid to the compressor.

Subcooling tells you, relatively, how much of the coil you are actively using to make liquid from gas. You'd like to use most of the coil to do actual condensing of the gas into liquid. Once all the gas is condensed and turns into liquid, the temperature begins to drop pretty quickly.

A big liquid temperature drop means that there is a lot of the coil just holding liquid. That leaves less of the coil to condense the gas, and you effectively reduce the size of the coil. That's what techs mean when they say that a lot of liquid is “stacking up” in the condenser. So there's less effective area of coil for heat exchange, the compressor has to work harder, raising the gas to a higher temperature, trying to get it to condense in fewer inches of the fin tubes.

On the other hand, you don't want near-zero subcooling. If the subcooling is near zero, you are sending gas bubbles to the metering device, and that reduces the system's ability to cool.

Using these ideas, you can tell a great deal about the health of the system, and, eventually, ensure that it is correctly charged. Too much refrigerant increases subcooling and makes the compressor work too hard. Not enough refrigerant decreases the subcooling, and increases the superheat, too, because there's not enough refrigerant and the evaporator coil starves.

Other things also affect the subcooling and superheat -- refrigerant flow restrictions, air movement, air temperature, humidity, and other things. Subcooling and superheat don't tell you everything, but they are incredibly useful diagnostic tools. Along with other things you learn about the system, they help give a complete picture of the system's health.