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  1. #14
    Join Date
    Sep 2009
    Location
    Arnold mo
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    3,967
    Quote Originally Posted by adamwhatley View Post
    Looking over the duct system & found very little duct leakage! So with with the figure of 130 cfm per drop the Duct system will only be delivering 800-900 cfm for the entire supply. So again the question would be if only supplying 900 cfm max and load showing needed (3) ton at 1200 cfm is the right size but only delivering 2.5 ton max then the system would be oversized for the ductwork?
    Didn't read through the whole thread. Look into "system effect" and how it can reduce the number of cfms your fan is moving. Maybe you can make corrections to the ductwork to increase the air flow.
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  2. #15
    Join Date
    Sep 2011
    Location
    Southern Ontario, Can
    Posts
    26
    As has already been stated to often little to no design was incorporated in the original install. Once upon a time air conditioning here in this area was for the rich and systems were design for heating only and convection had a lot to do with heating upper floors lol.
    Heat gain is heat gain. If you have a load calculation that requires 3 tons of cooling then you require three tons. Options? You will need to reduce the load: awnings/insulation/ grow a shade tree, lol or match the load with equipment making necessary adjustments to duct system design and air handling unit. Only if you want it to run right. My two cents.

  3. #16
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989

    Lightbulb

    Quote Originally Posted by adamwhatley View Post
    Looking over the duct system & found very little duct leakage! So with the figure of 130 cfm per drop the Duct system will only be delivering 800-900 cfm for the entire supply. So again the question would be if only supplying 900 cfm max and load showing needed (3) ton at 1200 cfm is the right size but only delivering 2.5 ton max then the system would be oversized for the ductwork?
    It is possible to make those 6, 7" metal ducts deliver 1100 or 1200-CFM to the rooms.

    1. At 1200-cfm, that is 200-cfm per duct which is a lot
    2. At 1100-cfm, it is 183-cfm per duct, still a lot
    3. For 200-cfm / .267254 the sq.ft Ak for a 7" rd duct is 748-fpm velocity
    4. At 1100-cfm which is 183-cfm each /.267254 Ak is 685-fpm velocity
    5. The diffuser's have to also be sized right, so look-up their Ak & Velocities
    6. The Return Air side must be sized large enough to help SA deliver enough airflow
    7. I would install another 25X20 filter grille & convert the other to one, that will increase the supply air flow; then never use a filter anywhere else.


    Regarding the CFM per duct drop, this is pushing it but with Return Air Work, you might get by...

    If the Supply & return grilles are all properly placed it might function to provide a good air mix.
    Hart & Cooley lists a 6x12, Ak .285, floor diffuser at: 200-cfm; @700-fpm; spread 13 ft; throw 10 ft.
    A manual D should be performed on the duct system with that air handler.
    Last edited by udarrell; 09-14-2011 at 10:41 AM. Reason: Hart&Cooley data...

  4. #17
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,095
    While its possible the air handler m ay only be able to move 900 CFM through that duct system. the actual system capacity m ay still be very close to 3 tons.

    If the current system is still working, measure the static and the temp drop across the coil.
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  5. #18
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,664
    Quote Originally Posted by adamwhatley View Post
    Looking over the duct system & found very little duct leakage! So with with the figure of 130 cfm per drop the Duct system will only be delivering 800-900 cfm for the entire supply. So again the question would be if only supplying 900 cfm max and load showing needed (3) ton at 1200 cfm is the right size but only delivering 2.5 ton max then the system would be oversized for the ductwork?
    so you looked at the duct system and seen no airleaks?? without trying to add or change your verbage there.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  6. #19
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    Load analysis is different from Manual 'S' equipment selection. Have you adjusted the nominal sensible/latent to your local conditions? It could be that at .70 SHR you need 3.0-tons but if you're in an area that's not at 95F OAT or above, you likely can use a lower ratio. That in itself could reduce the nominal size to 2.5-tons. Also, are you in an arid area or a humid area? If humidity is an issue in your area, then 3.0-tons (by Manual 'S') running at 350 CFM/ton is 1050 CFM, which is only 50 CFM above the nominal 1000 CFM for a 2.5-ton system @ 400 CFM/ton.

    But let's not neglect the homeowner's opinion about comfort. He/she said they've been very comfortable at 2.5-tons. What indoor temp are they using? You could be designing for a lower temperature than they maintain. Or could it be that the existing 2.5-ton system is operating at 350 CFM/ton, thus the duct system is only delivering 875 CFM total?

    Or could the refrigerant charge and/or airflow not be correct and that 2.5-ton system is delivering a true capacity of only 22,500 Btu's, 25% reduction?

    These are all things that can impact the actual size specified and the affect on the existing duct system. It's a science, inexact as it may be, like all science but science is always better than just guessing.

  7. #20
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190

    I agree with Skippedover

    In your high RH area you are probably needing 350 CFMs per ton NOT 400,
    So the lower airflow id helping you get more dehumidiofication.
    You'll know airflow is too low when it starts to ice up
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  8. #21
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    As my earlier #10 post indicated, I also agree with skippedover.
    http://hvac-talk.com/vbb/showpost.ph...2&postcount=10

    That duct system should handle 1050-CFM.

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