Rules of Duct Design, Why?

  I went many years as a Service Tech and never had a clue as to these rules.  As a matter of fact I went years as a Service Manager, and quite a while as an Instructor, without seeing these all in one place.  I had this problem with learning the hard way - who knew you could pick up a book (or read a blog!) and learn this so easy?  If you haven't been able to tell, I am all for giving this type of information to those that need it; so please share with your service managers, techs, sales, and even the greenest of tin knockers.
 
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11 comments:

  1. This article has been so helpful! I have been so busy lately and I have been trying to find someone to share about duct design and duct cleaning process! This article has relieved me of a lot of stress, thank you!

    Air duct cleaning services

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  2. Pretty good article, You have clearly defined about the best air duct design.

    Vaughan Duct Cleaning

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  3. This rule is unclear, could someone restate in more detailed manor. Thanks.
    • Never take-off a reduction or increase the mains any closer than the diameter of the branch duct

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  4. Thanks for reading my Blog 'Anonymous'! I agree that the wording is poor, sorry for the confusion out of Manual D! Basically, this means you do not want to cut in a take-off, say 7" branch duct, within 7" of a transition in the trunk...hope this helps!

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  5. Oops, and certainly not 'on' the transition!

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  6. Hi, may I know the reason why the trunk size should be reduced after 24'? is it based on ASHRAE standard or based on your experience? based on your article, this method is useful for residential duct, is it also useful for ducting system for bigger buildings? thanx

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  7. Good Morning Omar! Thanks for reading my blog. The trunk duct should be reduced in residential applications at least every 24' (usually earlier based on take-offs and remaining velocity) to keep the velocity at the minimum. This is part of ACCA's Manual D, Residential Duct Design. For commercial applications (Low Pressure, Low Velocity) you must refer to ACCA Manual Q.

    Hope this helps, good luck!

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  8. I have a system with 10" round trunk and I will have eight 6" takeoffs. How long should the trunk run be? the take offs will be 4 on each side. should I stagger them on each side and how far apart?

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  9. Great article, glad to see people addressing duct layout, as it is just as big of an issue as equipment sizing imho. However, I was under the impression the newest Manual D acknowledged that the 24' rule is a sort of fallacy, even though previous Manual D version uses it. The "velocity/reduction" method (don't quote me on the name) is the preferred trunk sizing technique. By this method you would reduce trunk size before velocity reduced to less than half of starting velocity.

    I don't have my Manual D with me, but it is listed in one of the appendices.

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  10. Quite informative post, i am not aware from the rules for the air duct design. I appriciate it cause of this you can easily clean your duct. These methods are useful for both commercial as well as personal buildings.

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  11. Something that could really help with the fresh feeling in the house is a good design of the air duct. The right design would also help to provide the customer with a thorough air duct cleaning service.

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