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cfm, duct sizing, help!

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  • would someone please explai to me how to size the ducts in a house? i know how to come up with the furnace size, but how do you know how many cfm's each room needs & then what size duct do you use for that room & after all the rooms are calculated, how do you size the main trunk. i know this ? is not up to par with the intelligence i've seen on this site, but i could really use your guys' expertise!!


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  • Ken_40
    Ken_40 Member Posts: 1,320
    Good question,

    wrong forum. We are somewhat absorbed by radiant, hot water and steam here. There is a section way down below for HVAC questions. But I do know the best source for the methods to make the calculations is embraced in a doccument sold inexpensively by ACCA. A thin book called "ACCA Manual 'J' has the secrets you wish answered.

    Perhaps Professor Silberstein down at the bottom of "The Wall" website may hold yet another solution?
  • Plumdog_2
    Plumdog_2 Member Posts: 873
    First things first

    Get (or better yet) learn to do room by room heat loss calcs; then with simple division find the cfm per room. I suggest a "Duct-a-lator" slide rule device from a Forced air Wholesaler. They cost only a few bucks; and can instantly show the relationships between Static Pressure, Duct Size, Developed Length, and Cubic Feet Per Minute. And then change your name from dummy to Inquisitive.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    I agree Plumdog

    I personally like the term "inquisitive" as opposed to the other...

    As was mentioned, the accurate sizing of the equipment as well as the airflow requirements for each room in the structure relies on an accurate heat gain/heat loss calculations. By far, the most comprehensive method of doing this is with ACCA Manual J. Some will argue that this method is too cumbersome, but the results are well worth the effort.

    Estimating the equipment size without performing a heat gain/loss calculation is just an estimate. However, since air conditioning equipment comes in a limited number of capacities, getting the right size "by mistake" is quite possible. Getting the ductwork sizing right "by mistake" is far less likely.

    There are tools, as Plumdog mentioned, such as a duct-u-lator, that will assist you in sizing ducts, but the majority of people use the "default" setting on the slide rule of 0.1. This, once again, is a short cut and, at best, an estimate of the actual system conditions.

    As far as the duct layout and sizing goes, nothing beats ACCA Manual "D". At the college, my students learn Manual J and Manual D side-by-side, hand-in-hand. This ensures that the system and the associated ductowrk are sized properly.

    The duct-u-lator is a great tool to determine equivalent duct sizes such as an X" round duct will move the same air volume as a A" x B" rectangular duct.

    Look into the ACCA Manuals (J and D). Once you get past the initial shock of the size of these products, you will be eager to do it right the first and every time.

    Also, just like everything else in life, there is a learning curve involved here. The more you do it, the better you will be at it.

    Hope this helps.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    And if you are really just curious

    Here is a table of duct sizes I use for residential projects using standard "x 8" deep" ductwork:

    Runouts to Register Boots:

    6" Round (A minimum size for me): up to 100 CFM.

    This also corresponds to a 3.25 x 10 inch stack duct by the way.

    7" Round (really now...) up to 150 CFM

    8" Round up to 200 CFM. Over that, I go rectangular.

    Rectangular sizes:

    8x8 (A minimum size for me): Up to 250 CFM

    10x8: Up to 350 CFM

    12x8: Up to 450 CFM

    14x8: Up to 450 CFM

    16x8: Up to 600 CFM (= 12" round BTW)

    18x8: Up to 700 CFM

    20x8: Up to 800 CFM

    24x8: Up to 900 CFM

    Over a 3:1 aspect ratio, I would use deeper duct or multiple ducts. Too flat and "oil canning" can occur. Not efficient use of sheetmetal either, in my experience.

    Use your judgement. Pressure drops above are reasonable but sound-sensitive spaces may benefit from larger sizes. Stay at the low end of a duct's range if you are in doubt. You will only build the system once.

    Hope this helps.


  • can this manual "j" be purchased @ wholesalers or my local barnes & noble? Thanks for the advice!!


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