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  1. #1
    I've read a lot of posts here about HO's duct sizing probably being too small. So, I was wondering: can ducts be too big? Assume that all the duct sizes are balanced throughout a house so that all ducts are oversized by the same relative amount (i.e. 30%).

    Also, I'm asking more for the functional aspects. I realize installing duct work that is needlessly oversized would be a waste of money. However, I would think that, to a point, the labor would be basically the same but the cost of materials would be higher.

    If there's no functional problems with balanced ducts that are oversized, then my "proposal" is that it would be worth a little more money for materials to reduce the risk of having undersized ducts and all the problems associated with it.

    I know people will say that doing proper calculations will ensure the proper duct size, but these calculations seem inaccurate at times -- even with HVAC-Calc from what I've read in other posts. There also could be certain exceptions that cause a home to need large duct sizing, but it isn't noticed or compensated for.

    Thanks for entertaining my curiousity,

    Josh

  2. #2
    the only time a heat load is inadequete is when the heat load is performed by an unqualified person wheather or not hes been doing it 20 years or 1.

  3. #3
    Join Date
    Feb 2005
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    Dothan, Al
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    Larger ducts reduce the velocity of the air.
    Found a system yesterday where the air was not coming out of several registers very well. Found that the 'hack' had attached unit to large existing trunk lines ( 2 of them ), which lowered the velocity so that one line was not getting to the supply registers.
    Now its going to cost the owner over a $1000.00 to dedo ductwork properly.
    Ducts should be sized as needed for proper volume and velocity for the system installed.

    Hope this helps,
    Richard

  4. #4
    Join Date
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Manual d ,done properly sizes the duct system correctly.

    If you to "oversize" use a lower Friction Rate then determined by Man D.Or design to use the medium speed of the motor,sas recomended by Man. D.

    Or if variable speed ,just use a lower ESP than needed.

    Oversizing the duct,within reason ,minimum .3"WC TESP ,should not cause a problem,though other then being able to add a more restritive filter ,I don't see much value.

    Using .5"WC as maximum design,would offer lower operating costs on VS.


    Oversized ducts,too low of a static ,can usually be fixed by adding dampers ,to increase the static and redirect the air flow,as needed.

    [Edited by dash on 08-05-2005 at 02:34 PM]

  5. #5
    I don't see how big ducts could cause lack of airflow if all the ducts are balanced, despite being oversized. By definition, the air would not favor one duct path over another. The blower would move a certain volume of air -- whether it moves slowly through large ducts, or faster through proper sized ducts -- same volume of air is exiting the supply registers.

    Is having low static pressure harmful? It seems it would require less work from the blower to move the air.

    Where are my assumptions wrong?

    Thanks,

    Josh

  6. #6
    Join Date
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    VS it seems needs a minimm amount of static.A standard PSC motor will deliver too much air flow at low statics.

    Either can be corrected with dampers to increase the static.

    Now in svere cases as "born" may have ,you could have oversized ducts and grilles to the point that the air "falls" out of the grille and doen't get distributed in the space well enough.Haven't seen that in a home ,but I did see it in a commercial remodel.

  7. #7
    Join Date
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    Orange County, NY
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    QUOTE
    "Is having low static pressure harmful? It seems it would require less work from the blower to move the air. "

    This would be a big waste of energy. Fans are designed to operate against a pressure. A fan is considered at its "operating point" when at 40 to 80% up on its curve and to the right of the heal.

    Static pressure causes the air in the duct to flow.
    Velocity pressure is the result of air movement.

    If a Fan where allowed to run with no resistance it would be at its Max CFM with no Static pressure (0 In. Wc)


    Core

  8. #8
    Join Date
    Jun 2005
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    SW Wisconsin
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    Originally posted by bornriding
    Larger ducts reduce the velocity of the air.
    Found a system yesterday where the air was not coming out of several registers very well. Found that the 'hack' had attached unit to large existing trunk lines ( 2 of them ), which lowered the velocity so that one line was not getting to the supply registers.
    Now its going to cost the owner over a $1000.00 to dedo ductwork properly.
    Ducts should be sized as needed for proper volume and velocity for the system installed.
    Hope this helps,
    Richard
    If you take a blower out of the furnace it will overload the motor. Depending on its SP rated design, too high an SP will cause the blower wheel to unload.

    There is a proper duct size, SP, and velocity for main and branch runs depending on he blower design and the application.

    It is bad application design work to overdo oversizing a duct systems. (Adequate velocity is important...)

    http://www.udarrell.com/proper_cfm_b...g_systems.html

  9. #9
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    If you want to oversize any ducts do it on the return. Returns are undersized by many contractors which leads to equipment failure and air flow problems including noise. I oversize all returns for noise purposes only.

  10. #10
    Join Date
    Jul 2005
    Posts
    126
    Originally posted by bornriding
    Larger ducts reduce the velocity of the air.
    Found a system yesterday where the air was not coming out of several registers very well. Found that the 'hack' had attached unit to large existing trunk lines ( 2 of them ), which lowered the velocity so that one line was not getting to the supply registers.
    Now its going to cost the owner over a $1000.00 to dedo ductwork properly.
    Ducts should be sized as needed for proper volume and velocity for the system installed.

    Hope this helps,
    Richard
    Richard, if the existing duct system is otherwise in good condition, this problem could be resolved with transitions to smaller diffusers. This will increase face velocity at the diffuser which is what determines the throw. Balance issues can be corrected by carefully choosing the diffusers and the installation of trunk-line dampers if needed. Smaller diffusers will increase overall static, but not as much as reducing the entire distribution system. But this is not a problem if you adjust the blower to deliver the design cfm based on the new configuration. See comment #1 below.

    Also keep in mind that a net return-side leak from areas outside the thermal envelope could be contributing to the problem (pressurization effect can lead to reduced airflow at the supply diffusers).

    More to the point of this thread: low static and low duct velocity, in of itself, is not a problem. Here are some additional thoughts on the subject:

    1) Generally speaking, blower motors are more efficient at lower esp's, given the same airflow, as long as you're operating above the min recommended esp for the air handler.

    2) If someone designs a low static system and fails to set the blower speed to achieve the design airflow, then the implementation is at fault, not the design.

    3) Manual D provides an 'optimal' solution which minimizes the duct sizes. A smaller diameter distribution system is slightly less expensive and sometimes significantly easier to install. But there's no reason to downsize an already-installed duct system just because it's larger than optimal!

    4) When downsizing diffusers, one must be careful where and how you make the transitions in order to avoid excess noise.

    David

    [Edited by ginahoy on 08-06-2005 at 01:27 PM]

  11. #11
    Join Date
    Aug 2003
    Posts
    125
    There are situations in commercial where ducts are grossly oversized for noise control. TV and recording studios. But then these systems are engineered to operate this way.

  12. #12
    Join Date
    Aug 2005
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    Most of the replies here imply that a duct that is "too big" is bad...

    Then how do you design a ductwork for a two-stage system whose air flow varies by 40% or so? If you size it for the higher stage, won't the system then be "oversized" for the low stage, which most respondents say is bad?

  13. #13
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    Good point.If our "best" system reduces air flow that much with no problems,ducts would have to be severly oversized to have a problem.Now the two speed is most often used with a variable speed fan,so cfm is being maintained.

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