Starving the supply
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  1. #1
    Join Date
    Mar 2005
    Posts
    16

    Question

    Hello,
    You won't believe this one. A 1850 sq ft house on a slab. House is 14 months old and customer cannot get summer temp in house under 79 degrees inside. Unit has been looked at several times and all is well they say. Customer might have found the problem. The dust pattern on his filter is about 8x15. Who ever installed this unit did not cut the hole big enough.

    How much CFM is the unit producing with that small of a hole. How large should the hole have been? Customer has a 3 ton A/C.

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    If you checked the static just above the filter,and the supply side also to get a total ESP,you could read the cfms on the fan chart(read all foot notes)..

    We find this same thing very often here in Florida,i can tell you is often enough capacity loss to prevent the system for cooling the home,and/or raise the electric bill.

    Usually happens on new construction jobs.

  3. #3
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
    Posts
    1,209
    8X15 I get approximately 675 CFM 3 ton = 1200 cfm
    Proud supporter of Springfield Millers and Oregon Ducks.

  4. #4
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Originally posted by millerman
    8X15 I get approximately 675 CFM 3 ton = 1200 cfm

    The above appears to be using a ductolator with a Friction Rate of .1.

    FR is for 100 feet or equivalent feet of duct,in this case the 8X15 is likely an inch or two at the most,so the cfms are likely much higher,but a restriction ,such as this ,certainly needs to be removed.



    It should be as large as the opening in the airhandler,or very close.


  5. #5
    Join Date
    Mar 2004
    Location
    Gone
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    5,340
    Originally posted by millerman
    8X15 I get approximately 675 CFM 3 ton = 1200 cfm
    If you look at your Ductulator again and look at .06 static, you will notice the 8 X 15 hole will only flow 500 CFM. A correctly designed system would have a TESP of .5. If you look at a fan curve chart you will notice as your TESP rises, your air flow decreases. With this being the case, your incorrectly designed system has a larger TESP than .5 and it is probably more likely around 1.0 causing your 8 X 15 hole to flow much less than 500 CFM.

    As your TESP rises, your CFM decreases.

  6. #6
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    We have seen this many times,and it's likely moving 800 cfm or more.

    The ductolator is of little to no value here,test the ESP,and read the cfm on the fan chart if you want to know what it is.We all know it's less than required ,although I'd check the ESP after the hole is enlarged,to insure proper cfm.


    1.0 ESP is off the fan chart for a PSC motor,which this likely is,but a good 3 ton air handler can deliver 1190 at .6 ESP.

    At 500 or 675 cfm cfms.I'd think it would be icing up,not just unable to go below 79.

    [Edited by dash on 05-10-2005 at 04:10 PM]

  7. #7
    Join Date
    Mar 2002
    Location
    Richmond, Virginia
    Posts
    4,264
    What size is the return duct if the hole was only cut to 8x15? Were they just to lazy to cut the hole larger or did they have what they believed to be a good reason to cut it this size? Seems like there is a missing piece to this puzzle.
    There is nothing more frightful than ignorance in action....Mark Twain

  8. #8
    Join Date
    Mar 2005
    Posts
    16
    The return is larger. I think the filter was 21x10.

  9. #9
    Join Date
    May 2005
    Posts
    996

    Lightbulb

    from what little i know about air flow, the return should be about the same size as the supply and at leat the dimentions in the opening of the furnace. i might sugjust purchasing small return grille capable of holding a filter. measure the square inches on the hole in the furnace (size of filter or slightly smaller). then find the square inches on the opening provided. that was like 8 by 15. the difference between the two is what size filtered return grille you need put in. i dont know if this would work but it's just an idea.

  10. #10
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    3 TON UNIT = 20 X 30 Filter = 20 X 30 Hole behind filter

    Larger filter size is OK.
    Smaller filter size OR smaller hole (i.e. 8 x 15)
    immediatly behind the filter will increase air
    speed through filter beyond filters limits
    This will also cause filter grille to 'sing'

    Since the return was not properly installed in the first
    place... I would venture to guess the rest of the duct
    system would need an inpection.
    First place I'd start is place plastic over return grille
    with fan in ON position (system switch OFF)
    See if any air is blowing out of the supply vents.
    This will let you know if there are any return air leaks
    real fast. You will also be able to hear any leaks
    faster when inspecting the duct.

    After that check for proper airflow through system.
    I'd normally check refrigerant charge and total capacity.

    If airflow looks good and charge looks good but capacity is off...
    I'd check for stuck electric heaters

    If all that looks good. I'd check house for probs. causing
    unit not to cool properly. i.e. not enough insulation, air leaks in structure, dryer vent leaking or run under house, attic ventilation, etc.

    This is just quick stuff off the top of my head.
    Its not my 'full' diagnostic procedure.
    Didn't want anyone to shoot me for missing something hehe.



  11. #11
    i'd check out the thermostat.

  12. #12
    Join Date
    Mar 2004
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    Gone
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    I want to see docholiday check the air flow with his garbage bag setup he mentioned on another thread.


  13. #13
    Join Date
    Feb 2005
    Posts
    26
    Is this a tract home with similar houses in the neighborhood?
    If so check out your neighbors see if they are having problems? If not make a comparison...
    Just a thought....

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