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  1. #1
    Join Date
    Jul 2011
    Location
    Utah
    Posts
    30

    Duct size, Capacity

    I'd like to get others opinion. I run into alot of homes where it seems that the A/C is slightly to big for the ductwork that is installed. Lots of times the the a/c is working and pressures are too low.And... the superheat never lines up good. Can't get enough superheat. Air temp at coil and in return are too low, but does not cool house. Stays around 78-80 and runs all day. I do see this alot.

    General question.... What is the min of square inches of duct(square) would be acceptable for the volume of 2 ton a/c with 2 ton drive blower??? 2-14x8 supply or 1- 28x8????? I'm only concerned with supply duct size for this question. System as example has 8-six inch round pipe to a 1000 sq. ft home

    Min square duct needed for 3 ton a/c with 3 ton drive blower???? 2-16x8 supply, or 32x8 min??? having 12 six inch round pipe runs to 1300 sq ft home

    Just looking for opinions ....

  2. #2
    Join Date
    May 2011
    Location
    Southern Indiana
    Posts
    97
    2- ton is 18x8 one way. 3-ton is 8x25 one way.

  3. #3
    Join Date
    May 2011
    Location
    Southern Indiana
    Posts
    97
    Main trunk should be reduced at certain intervals along the way to create proper backflow for even cfm to all supply outlets. this is laymans terms but, there is alot more to it.

  4. #4
    Join Date
    May 2009
    Posts
    611
    Get a ductulator. It's a little wheel chart that you match up CFM(cubic feet per minute) to static pressure. Many systems are tested at .2" of Water Column.

    You can also use a computer program to design it.

    Super techs can design it by experience, it can come out many times just as good as the $1000 computer program.

    The branches need to add up to the main trunk. The larger branches can make for a smaller main trunk. The idea is to deliver the proper airflow to every room and have an overall total external static pressure that is within the blower performance chart.

    Diagnosing systems you also need to get a magnehelic guage and read static pressure. This tells you whether or not to increase the blower speed and whether or not they need duct upgrades (which is true on nearly every call in Florida).

    On a piston system bad ductwork will make it challenging if not impossible to charge properly and can reduce the capacity of the unit significantly.

  5. #5
    Join Date
    Jul 2011
    Location
    Utah
    Posts
    30
    Thank you for info!!!

  6. #6
    Join Date
    Jul 2011
    Location
    Utah
    Posts
    30
    Thank you. I appreciate your opinion

  7. #7
    Join Date
    Jul 2011
    Location
    Utah
    Posts
    30
    agreed, and thank you also. I see it alot that with a piston that i never get the ideal superheat of 10-12. Lots of systems seem to get 1 to 0 superheat, running at lets say 44 degree. It always concerns me cause theoretically im not picking up any heat in the evaporator.

  8. #8
    Join Date
    Jul 2011
    Posts
    354
    i'd say get a ductulator at a supply house. use .08 static as a guide,for the supply side,and while your at it, get a ductulator for flex duct, as far as slightly bigger ac than the ducts, what's your definition of slightly? on most residential systems, there's a little give in the size of the ducts, it gets down to expierience, in putting in many systems and seeing how they work out over the years, you can design a system on paper, but the rubber meets the road when the system is installed out in the field.

  9. #9
    Join Date
    May 2009
    Posts
    611
    1 to 0 degree superheat means that liquid is possibly reaching the compressor through the suction side and could really kill it.

    Airflow is generally very misunderstood and misapplied in this trade. The more restricted the supply and return, the higher the fan speed should be turned up. This can produce noise at the registers and can even pull moisture out of the pan or trap in some cases.

    Better to design and install the duct work right from the beginning.

  10. #10
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,033

    Lightbulb

    Quote Originally Posted by jeremyjp View Post
    I'd like to get others opinion. I run into alot of homes where it seems that the A/C is slightly to big for the ductwork that is installed. Lots of times the the a/c is working and pressures are too low.And... the superheat never lines up good. Can't get enough superheat. Air temp at coil and in return are too low, but does not cool house. Stays around 78-80 and runs all day. I do see this a lot. (It is Too Common.)

    General question.... What is the min of square inches of duct(square) would be acceptable for the volume of 2 ton a/c with 2 ton drive blower??? 2-14x8 supply or 1- 28x8????? I'm only concerned with supply duct size for this question. System as example has 8-six inch round pipe to a 1000 sq. ft home

    Min square duct needed for 3 ton a/c with 3 ton drive blower???? 2-16x8 supply, or 32x8 min??? having 12 six inch round pipe runs to 1300 sq ft home

    Just looking for opinions ....
    I only run some numbers on the 3-Ton using (12) six-inch Flex duct on the Supply branch runs. A lot of airflow mistakes happen using flex-duct!

    With one 90-ell each, delivers around 88.33-cfm each run, at 0.09" Friction rate & 450-fpm velocity. The 12- runs would deliver 1060 total CFM, only good for 1050-CFM, without figuring the diffusers. (Use a Manual D!)

    On a 1300-sf home, I'd use 350-CFM per/ton *3 of cooling or, 1150-CFM; though NOT for original duct design!

    Now the big ticket is the Return Air Duct sizing & filter area sizing.
    To get to 300-fpm velocity through a clean filter a 3-Ton system needs around 1000-sq.ins. of physical filter area.

    That is with the higher (Ak) Free-air airflow rated filters. Some filters are very restrictive!
    Here is a link on filter selection:
    http://bit.ly/napnDY

    It appears, if the Return Air is optimal, the supply-side might deliver enough airflow; the only way to be sure is to have a Manual D run!

    http://www.wrightsoft.com/products/right-d.aspx

    However, an ACCA Manual D is the only best way, because you need to know all the data that will affect whether the blower can deliver what is needed through the coil & to each room.

    Manual D Check-List:
    http://media.iccsafe.org/geo/docs/AC...rification.pdf

  11. #11
    Join Date
    Jul 2011
    Posts
    354
    real world. my brother bought a house last year in the N.Y. tri-state area. built in 1971.2400 sf colonial. it has a 24yo carrier cond. with 4 ton a/h. it has a 16" flex return. cools the house fine he's happy with the system. it's what i call a oldie but a goodie!
    ideally on a new system i'd use a bigger return, but the old rule was 14" flex for 3 ton,16" flex for 4 ton,18" flex for 5 ton. there are a lot of systems that have worked like that for years! now is that ok? who knows! but i've been on calls where johnny homeowner had a 5 ton unit with a 18" return, that collapsed and his buddy redid it with a 12" flex with the obvious problems arising! but i approach systems today where if the h/o been there for years with no complaints why open up a hornets nest regarding his duct system. now if it's a new install and from the get go teh h/o is not happy that's a differant story. then i look hard at all aspects of the install. finally, imo, usually a customer who complains the most had the cheapest bid on the job! and got what he paid for. in other words he wants a $$$$$ job done for $$$$nothing.

  12. #12
    Join Date
    Sep 2007
    Location
    Boise, ID
    Posts
    4,398
    The proper method is to know the CFM required for each room. Then look at what your system can deliver CFM wise at a given external static pressure. Then size your ducts to within those boundaries. Note you need to include return air, pressure drop though filter, wet coil, dampers, diffusers and grilles...that's just a start.. Sizing ducts to rule of thumb stuff like 0.8" per 100' does not ensure that you will get the desired CFM because of elbows, length of runs, type of transitions used, and other external factors all affect final results.
    If a nation expects to be ignorant and free, in a state of civilization, it expects what will never be. (Thomas Jefferson 1816)

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,033
    Quote Originally Posted by thes View Post
    real world. my brother bought a house last year in the N.Y. tri-state area. built in 1971.2400 sf colonial. it has a 24yo carrier cond. with 4 ton a/h. it has a 16" flex return. cools the house fine he's happy with the system. it's what i call a oldie but a goodie!

    ideally on a new system i'd use a bigger return, but the old rule was 14" flex for 3 ton,16" flex for 4 ton,18" flex for 5 ton. there are a lot of systems that have worked like that for years! now is that ok? who knows!

    But i've been on calls where johnny homeowner had a 5 ton unit with a 18" return, that collapsed and his buddy redid it with a 12" flex with the obvious problems arising! but i approach systems today where if the h/o been there for years with no complaints why open up a hornets nest regarding his duct system. now if it's a new install and from the get go the h/o is not happy that's a different story. then i look hard at all aspects of the install. finally, imo, usually a customer who complains the most had the cheapest bid on the job! and got what he paid for. in other words he wants a $$$$$ job done for $$$$nothing.
    Well, let's say that 4-Ton was operating at 350-cfm per/ton of cooling or 1400-CFM.

    A 16" flex Return on a 20' run with one 90-ell will have a friction rate of 0.06", but the velocity will be 1030-fpm, when we want a max of 800-fpm or less on a main Return Run. However, it would work rather well that way...

    Return branch runs should be a max of 600-fpm velocity.

    Comparative Tests concerning Pressure Drops of HVAC Filters:
    http://bit.ly/pDzdRh

    At 300-fpm, the 4" deep Merv 7 has the same pressure drop at .10 of an inch, as the 1" fiber glass filter; all filter areas were 24X24.

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