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  1. #1
    Join Date
    Jun 2008
    Posts
    63

    10 degree difference at registers

    Just checked on this hot day and found some registers at 56F and others at 66F. There are 4 ducts, all the same size, coming from the plenum. All the registers on the 2 ducts closest to the coil are all at 56F. All the registers on the 2 ducts further from the coil are all at 66F. The ducts are only a few inches apart. The total distance from the coil to the registers doesn't matter. 2 registers with the shortest run come from the trunk further out and they read 66F.
    What could be causing such a drastic increase in temps from one to another?

    ......_|56|_|66|_
    A/C |...plenum....|
    coil |_.....__....._|
    ........|56|..|66|

  2. #2
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,877
    check the temp at different places in the register

  3. #3
    Join Date
    Jun 2008
    Posts
    63
    Did so. Ran a multi-meter temp probe as far as it would reach and still the same results. Used an IR meter on the face of the grill and got higher readings of course, but the same delta. Only the registers connected to the 2 ducts further out on the plenum read 10F higher...

  4. #4
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    duct leakage?
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  5. #5
    Join Date
    Mar 2012
    Location
    copperas cove
    Posts
    36
    No mixing chamber on plenum, if no duct leakage.

  6. #6
    Join Date
    Apr 2012
    Location
    Paper Street Soap Company
    Posts
    2,304
    Your'e attic temp ? Ducts running through excessively hot attics can pick up heat.

  7. #7
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    Quote Originally Posted by jax1 View Post
    Just checked on this hot day and found some registers at 56F and others at 66F. There are 4 ducts, all the same size, coming from the plenum. All the registers on the 2 ducts closest to the coil are all at 56F.

    All the registers on the 2 ducts furtherest from the coil are all at 66F. The ducts are only a few inches apart. The total distance from the coil to the registers doesn't matter.

    2 registers with the shortest run come from the trunk further out and they read 66F.

    What could be causing such a drastic increase in temps from one to another?

    ......_|56|_|66|_
    A/C |...plenum....|
    coil |_.....__....._|
    ........|56|..|66|
    I would guess that it might be the way the air flows through the furnace, the evaporator &, the plenum design & the takeoff positions.

    Are there any turns before where the air enters the evaporator or, is it an Oil furnace with a large heat exchanger?

    The location positions of the ducts outlets can make a big difference as can the takeoffs type & design.

    Is there a difference in the velocity of the air coming from the registers of the differing temps?

    Are there any air leaks into the plenum area?

    How is the plenum shaped?
    Precisely where are the takeoffs located on the plenum in respect to their temps?

    Is there a transition?

    Can you take some pics of the furnace & plenum situation?
    Last edited by udarrell; 07-08-2012 at 05:35 PM. Reason: Clarity...

  8. #8
    Join Date
    Jun 2008
    Posts
    63
    This is a gas furnace/AC mounted horizontally in the attic. The whole unit is one long box from intake to plenum.
    The air flow is one turn from the return then from the blower fan straight across the gas furnace, AC coils to the plenum, then out the 4 ducts, 2 mounted to the left and 2 to the right sides of the plenum.
    The registers that have the higher temps (66 degrees)are all off the 2 ducts farther out.
    The register with the shortest total run is off the far left duct and it is at 66 degrees as well as all registers along that duct and all the registers off the far right duct(except the last one which is 67)
    The air velocity was not measured, but seems to be the same at all registers.
    I have checked for plenum leaks, but have not found any. The back is very difficult to get to, and I haven't done a touch test there. I did hang a ribbon off a stick and didn't see any movement. Not sure how good a test that would be. If there are no other possibilities, I might have to make an army crawl back there to check.
    I checked for duct leakage and found nothing. Am I correct assuming that the leakage would have to be between the plenum and the first register to affect all the registers off that duct?

  9. #9
    Join Date
    Jun 2008
    Posts
    63
    It's raining today so the attic is cool. Did the army crawl behind the plenum and found no leaks.
    I checked the far left duct entirely and I only found a leak where the register attaches to the ceiling. After sealing, the temps did not change.
    Since the 2 far ducts are directly opposite each other, can a leak in one cause the other's temps to rise?
    Can the leak causing all the registers to read high be anywhere along the run or between the plenum and the first register?
    Measured the register temps again, and with no sun today, the temps haven't changed - 56f off the first 2 ducts, 66f off the far 2 ducts.
    I've attached pics of the left and right side of the unit.

    Thank you for your help.
    Attached Images Attached Images   

  10. #10
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    Quote Originally Posted by jax1 View Post
    It's raining today so the attic is cool. Did the army crawl behind the plenum and found no leaks.
    I checked the far left duct entirely and I only found a leak where the register attaches to the ceiling. After sealing, the temps did not change.
    Since the 2 far ducts are directly opposite each other, can a leak in one cause the other's temps to rise?
    Can the leak causing all the registers to read high be anywhere along the run or between the plenum and the first register?
    Measured the register temps again, and with no sun today, the temps haven't changed - 56f off the first 2 ducts, 66f off the far 2 ducts.
    I've attached pics of the left and right side of the unit.

    Thank you for your help.
    I'm thinking that the location of the takeoffs of each run are getting differing amounts of colder or, warmer air due to their particular location to the airflow stream from the E-Coil.

    Which of the duct runs delivers the coldest air?

  11. #11
    Join Date
    Mar 2012
    Location
    copperas cove
    Posts
    36
    I bet closest to end plenum are the coldest . (No mixing chamber)

  12. #12
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    May have something to do with that poor transition between furnace and coil.

    Next step may be static test. Insert probes above each duct and at end of plenum.

    Could the unit have return restrictions?

  13. #13
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,924

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