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  1. #1
    Join Date
    Dec 2010
    Location
    WV
    Posts
    115

    OMG !!!!! REturn filter grille sizing??????

    I can believe what i am hearing !!! Filter return should be sized at 2 sq in per ton?? right? Any one know the pressure drop on a standard filter???? Mfrs. are asking for a static balance of .5 .....oh not to exceed .5....... just found out my installers are using 24x24 RA filter grille for a 4ton ...and sizing the main supply at .1..... i can NOT get them to understand..... any ideas how i can get the proper static out of this mess??????

  2. #2
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,411
    just found out my installers are using 24x24 RA filter grille for a 4ton
    Thats short for a 3 ton!

    Any one know the pressure drop on a standard filter????
    here ya go!
    http://www.acca.org/Files/?id=66
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  3. #3
    Join Date
    Dec 2010
    Location
    WV
    Posts
    115
    thanks for that link.....how do i get them to listen to me when all they say is " we have been sizing duct for 20yrs ".......this will make my 9th season yes ..... i want it to be right....for all sakes

  4. #4
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,665
    Quote Originally Posted by SuperTech2010 View Post
    thanks for that link.....how do i get them to listen to me when all they say is " we have been sizing duct for 20yrs ".......this will make my 9th season yes ..... i want it to be right....for all sakes
    tell they have been doing it wrong for 20 years and now it's time to do it right

  5. #5
    Join Date
    Sep 2006
    Posts
    1,253
    Quote Originally Posted by SuperTech2010 View Post
    thanks for that link.....how do i get them to listen to me when all they say is " we have been sizing duct for 20yrs ".......this will make my 9th season yes ..... i want it to be right....for all sakes
    Start having the service techs take static pressure readings on some of those installs and see how well the installation actually works. Then you will have some real data to stand on. Also, get yourself a copy of Manual D and the other relevant guides and learn it in detail for yourself. Then you will have the first hand knowledge to support what you know to be right.

  6. #6
    Join Date
    Dec 2010
    Location
    WV
    Posts
    115

    do it right or dont do it at all !!!

    i have taken the static and it is thru the roof.......its a brand new house and a very nice crawl space......i DO NOT understand why even my service mgr. will not do anything to help....its like no one cares. its just very frustrating....... and thanks to all for your suggestions

  7. #7
    Join Date
    Sep 2006
    Posts
    1,253
    Quote Originally Posted by SuperTech2010 View Post
    i have taken the static and it is thru the roof.......its a brand new house and a very nice crawl space......i DO NOT understand why even my service mgr. will not do anything to help....its like no one cares. its just very frustrating....... and thanks to all for your suggestions
    So the guys have been designing duct systems for 20 years but still can't get the static AND air flow to within manufacturers specs? And I guess than also means the manufacturers are all wrong with those spec? The systems don't really need all the air flow the manufacturers spec and the homes don't need all the BTUs - right? Owners don't mind charging customers for systems that can't delivered the BTUs the customer paid for. Interesting business standards. Everyone seems to be well entrenched in their ignorance.

    BTW - you probably know this, but the .1 for the main trunk is most likely only the friction rate of a straight 100' of the main trunk and does not factor in any of the other components of the duct system. Thus, in does little to help in the design of the overall duct system.

  8. #8
    Join Date
    Dec 2010
    Location
    WV
    Posts
    115

    ??? on purpose ???

    do ya think this is being done on purpose?

  9. #9
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989

    Arrow

    Quote Originally Posted by SuperTech2010 View Post
    I can believe what i am hearing !!! Filter return should be sized at 2 sq in per ton?? right? Any one know the pressure drop on a standard filter???? Mfrs. are asking for a static balance of .5 .....oh not to exceed .5....... just found out my installers are using 24x24 RA filter grille for a 4ton ...and sizing the main supply at .1..... i can NOT get them to understand..... any ideas how i can get the proper static out of this mess??????
    Wrong filter area sizing has been a major problem since air conditioning was invented.

    You actually will never have too much Return Air filter area.

    Manual D requires a maximum fpm velocity through a new clean filter of 300-fpm.

    A 4Ton requires 1600-CFM / 300 is 5.33-sq. feet of free-open-air filter-area or (Ak).

    Therefore, 5.33 * 144 is 768-sq.ins., of free-open-air-area Ak.

    So, how much of the physical filter area is open-air-area; let's say .70%.
    So, 768 * 1.3 is 998-sq.ins.

    I would use two (2) 20X25 filters; 500sq.ins, each, for a total of 1000-sq.ins.

    The filtering area is the place to provide better indoor air quality & help reduce the static pressure on the blower.

    Yes, it will increase the pressure on the supply air side & with the right SA sizing it will deliver its Rated airflow.

    If manual D requires it we ought to do it.

    Have them check the fpm velocity through that 24X24 filter!

  10. #10
    Join Date
    Dec 2010
    Location
    WV
    Posts
    115
    with no filter its at 300 fpm....with filter is 400fpm........RA at the AH is -.66 and the supply is at .02 b4 the tee.....

  11. #11
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,665
    Quote Originally Posted by SuperTech2010 View Post
    i have taken the static and it is thru the roof.......its a brand new house and a very nice crawl space......i DO NOT understand why even my service mgr. will not do anything to help....its like no one cares. its just very frustrating....... and thanks to all for your suggestions
    it's like that here in va also I found that the duct board and flex in Texas flowed more air than thes sheet metal and flex here in va and they say duct board is crap and hack

  12. #12
    Join Date
    Jan 2007
    Location
    NE OHIO
    Posts
    778
    Quote Originally Posted by SuperTech2010 View Post
    i have taken the static and it is thru the roof.......its a brand new house and a very nice crawl space......i DO NOT understand why even my service mgr. will not do anything to help....its like no one cares. its just very frustrating....... and thanks to all for your suggestions
    join the club of "It'll work"

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989

    Arrow

    Quote Originally Posted by SuperTech2010 View Post
    with no filter its at 300 fpm....with filter is 400fpm........RA at the AH is -.66 and the supply is at .02 b4 the tee.....
    (Supply can't be that low.)
    It is NOT moving 1600-CFM, I doubt it could be at 400-fpm with the filter in place. Unless, what filter are you using!

    The filter rack area 24X24 is 4-sq.ft., or 576-sq.ins.

    The 24X24 is 576-sq.ins; but that is NOT 100% free-air-area (Ak); see Engineering Data below on a 25X20 filter grille.

    It needs 1600-CFM, it is NOT delivering 1400-CFM / 400-fpm = 3.5-sq.ft., a 25X20 filter at 2.24-sq.ft, (Ak), it is @ 625-fpm velocity; over double the initial filter Velocity Manual D calls for.

    Found a 24X24 filter grille Ak is 2.58; you figure it...only flows 774-CFM at 300-fpm Velocity through the filter; Hart & Cooley.
    Last edited by udarrell; 06-17-2011 at 11:18 PM. Reason: at 400-fpm with the filter in place...

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