# Thread: how to determine undercharge and overcharge

1. Professional Member
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Oct 2003
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hi, i am a student of lincoln tech. i was told by one of my instrutor that undercharge and overcharge. we don't need to use superheat or subcooling. just use temp difference between return air and supply air like for a/c would be 20F difference. is it ture? does this apply to refrigeration system. how? anyone show me example for detail please? like use R-134a for medium-temp refrigerator. if outside temp is 95F. what's the low-side gauge reading would be? what's the high side? is it low-side pressure related to ambient temp?

2. Ask your instructor what the temperature difference would be if the evaporator coil was starting to ice over. (as in coil now about 32° instead of normal temp.)

Assume about 75° return air.

Then ask about the effect of air flow on temperature rise and temperature drop. Think it makes a difference? Use your text book to come up with the answers before you confront him.

3. Regular Guest
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Subcooling and superheat indicate an over or undercharge,period. Temp. drops and rises merely indicate proper air flow with respect to the coil. If for example, the condenser was dirty, you will have a higher than normanl temp. rise. By your instructors theory, you will have too much refrigerant. If your subcooling is between 10F and 20F and there is a 40F temp rise...what do you think is wrong?

4. Professional Member
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Before Lmtd gets wind of this herrassy, let me say this; ask your instructor to show you in a demonstration how his theory actully works.

It's best to NOT go putting down instructors.

However, if this guy isnt a good teacher, you might have good ground to switch schools.

There are enough "old wives tales" already running around rampant in this industry! We dont need our future techs, our new students being TAUGHT bad methods right in our schools!

If he is actually wrong, take him aside as you would someone whom you deeply respect. And confront him gently about the matter. Perhaps there is a misunderstanding.

Do not tell any student about this matter either. It only serves to undermine the position of the instructor. And God knows, they have enough trouble already.

5. Regular Guest
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I'm with R12. Have him show you with a piece of equipment what he is trying to explain. Depending on the type of equipment,(ie: package unit, refrigeration, a/c, cap tube, txv... and on and on) there are differant ways to determine charge. Superheat and subcooling are just one part of the equation, but in many cases they are not the only part. As to the use of TD on the coil... I don't know if that is the definative test for charge.

6. Professional Member
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ask your instructor what actually happens to a system that's overcharged, for instance-if it's overcharged, the refrigerant "backs up" into the condensor where it takes up the space needed for the vapor to give up heat and condense so the result is higher head pressure and more subcooling-also raises suctoin pressure mostly due to higher head creating higher pressure at metering device forcing more freon into low side.

7. Regular Guest
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You'll figure out the right way soon. When I was taught, it was to charge high side to ambient +30.

8. How many high efficiency A/Cs have you seen, where that holds true?

9. none, right?

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