Measuring duct efficiency

If the temperature difference between the plenum and a duct is 8F (ie: Plenum is at 120F duct at register is 112F) can this be translated into duct efficiency?
Not that I can do much about it as all my ducts above a finished basement ceiling just curious.
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No.
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Why?
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Easy enough....what you ask, as you ask, translates to no.
Now, had you said, after a run time of XX minutes, with a duct run of XX feet, with a heat rise of XXX degrees, and a delta of XX with a velocity of XXX FPS can this translate to a duct efficiency...and the answer would have been a possible yes. There is a bit more info that would be needed as well, such as duct construction, age, diameter.......
Otherwise,no.
8 degree loss on a run of say.....30 feet or less isnt enough to worry about. Depending on how long the run time was, your delta with be less or greater...so...as you ask...no.
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That's really what I needed to know.

Thanks, the run is about 30 feet, duct size is 8x14, run time was about 5 mins.
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Not really long enough to heat the duct to plen temp,but not a bad drop. If the duct is not insulated, insulate it to R6 per code, and things get better.

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