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  1. #1
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    Evaporator temperature versus return air temperature

    I was wondering what was the rule of thumb when looking at a RTU evaporater temperature (Saturation tempreature) versus the return air tempreature
    is there a basic number for how cold the evaorator should be in regards to the return air tempreature

  2. #2
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    Most A coils are 20* coils if that's what you're asking. One can see the temp of the coil with a gauge set.

  3. #3
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    I am asking about a roof top unit, slab coil. I am not asking what temperature drop across the coil I should be looking for but is there a say for instance certain temperature (saturated Pressure) that I shoul be looking for say at a 73 degree return air temperature

  4. #4
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    Quote Originally Posted by criss View Post
    I was wondering what was the rule of thumb when looking at a RTU evaporater temperature (Saturation tempreature) versus the return air tempreature
    is there a basic number for how cold the evaorator should be in regards to the return air tempreature
    This is called the evaporator TD and generally runs between 25-35° on normally operating systems. Lower SEER systems run higher TD's and higher SEER systems run lower TD's. The exact TD for any particular system will depend upon a number of factors including indoor/outdoor conditions, airlfow, etc., and under some circumstances it can run outside the range that I gave above. Also, as a rule of thumb, for every degree that the indoor temperature drops the coil temperature will drop a degree.

  5. #5
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    73* RAT, 53* air temp off of evap coil, 33* freon inside evap coil. Old school

  6. #6
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    Quote Originally Posted by TechmanTerry View Post
    73* RAT, 53* air temp off of evap coil, 33* freon inside evap coil. Old school
    That isn't old school, that's ancient school.
    Looks like a 6-8 SEER system with that 40° TD. New school would be 73* RAT, 53* air temp off of evap coil, 43* freon inside evap coil.

  7. #7
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    Thank You
    That is what I was looking for. Apprecite the help

  8. #8
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    New school, 73-53-43* ? I thought the split would be the same , like 17 1/2* & 17 1/2*, like 73*-54 1/2*-43*.As in Old school being a 20-20split..

  9. #9
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    Are you saying old school like 1979 the TD was 20 same as the delta T of 20

  10. #10
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    NO!! I'm saying Ancient School, from the 60s,70s,80s,90s pretty much right upto/including the 10SEER stuff.As in post#5 & #8

  11. #11
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    Quote Originally Posted by TechmanTerry View Post
    New school, 73-53-43* ? I thought the split would be the same , like 17 1/2* & 17 1/2*, like 73*-54 1/2*-43*.As in Old school being a 20-20split..
    There's no logical reason for those differences to be numerically equal, as aesthetically pleasing as that might have been.

  12. #12
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    Quote Originally Posted by criss View Post
    Are you saying old school like 1979 the TD was 20 same as the delta T of 20
    He was saying TD was 40°, deltaT 20°. On modern systems (10 SEER and above) a TD of 40° would be considered extremely high and would indicate a heat transfer issue. Exceptions would be systems in dehumidify mode with reduced airflow.

  13. #13
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    so are you saying that I probably was tought that the TD and Delta where both 20 because that is what I was thinking when I wrote to the forum. I just couldnt believe that and was looking for conformation.
    It has been 36 years since I was in school. I normally dont look at this in my every day troubleshooting I was just looking at a invoice from one of my techs and was trying to see if this could be of assistance to reach a conclusion of what may be wrong

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