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  1. #1
    Join Date
    Feb 2014
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    Airflow per room

    I’m looking to learn more about airflow, how many cfm per room in residential. I have been asked to add ductwork to a new addition on this home.
    Here are the specs,
    Original home 2150sq. Ft. Plus new Addition is 470sq. Ft. Above new second garage added.
    There is a 14x8 supply and a 14x8 return that runs left to right on the house. I was thinking of extending both ducts to the new addition and adding round take offs to supply. How many take offs and what size return is my question.

    Also if you know somewhere that I could learn the basics please pass it on to me!

    Thank you!

  2. #2
    Join Date
    Mar 2015
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    Hamersville, Ohio
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    Start with Your Heat Loss & Gain, .... From there, You determine how many BTUs/ Air are required, then the Size of SA & RA.
    Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I will also reject you, that you may be no priest to me. Because you have forgotten your God’s law, I will also forget your children.


    "You've got to Stand for Something or You'll fall for anything" (A. Tippin)


    We are on a "Man-made Highway to Hell", Our so called "Leaders", Political & Religious, are encouraging The Mushroom like Sheeple to go faster.

    (AC-DC Lyrics)

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  4. #3
    Join Date
    Jan 2004
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    The current duct/trunk line is probably not large enough to handle the addition.
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  6. #4
    Join Date
    Feb 2014
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    Thread Starter
    But with my heat loss/ gain I will still only have the 14x8” supply and return. I am not able to change the whole duct, since I would have to tear down all the ceilings in 1st floor. Is there a “rule of thumb” how many vents per sq. Ft.?

  7. #5
    Join Date
    Feb 2011
    Location
    Chicagoland
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    hammerheating, no there is no 'rule of thumb'. It's all about heat loss and heat gain. Your new system will depend a lot on the quality of construction of the new space. Also you may have to tear down the ceiling in some areas of the existing house to increase the duct size. Tell your customers welcome to the wonderful world of remodeling!

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  9. #6
    Join Date
    Feb 2011
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    Chicagoland
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    Out here we call the space over a garage 'the bonus room'. It's often the worst climate controlled space of the house. Always because of the lack of BTU's.

  10. #7
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    Feb 2014
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    Thread Starter
    Cooling load 7,450 BTU
    Heating load 9,420 BTU

  11. #8
    Join Date
    Mar 2015
    Location
    Hamersville, Ohio
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    I'm curious about the performance of the Unit for the home with a 14 x 8 Supply & Return.

    At a .2 Static that's approx. 850 CFM.
    Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I will also reject you, that you may be no priest to me. Because you have forgotten your God’s law, I will also forget your children.


    "You've got to Stand for Something or You'll fall for anything" (A. Tippin)


    We are on a "Man-made Highway to Hell", Our so called "Leaders", Political & Religious, are encouraging The Mushroom like Sheeple to go faster.

    (AC-DC Lyrics)

  12. #9
    Join Date
    Apr 2004
    Location
    MN
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    Sell them a ductless mini-split. Probably cost about the same in the end.
    A Veteran is a person, who at some point in their life, wrote a blank check payable to the United States of America for payment up to and including their life.
    Gene Castagnetti-Director of the National Memorial Cemetery of the Pacific in Hawaii

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  14. #10
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    Feb 2014
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    Thread Starter
    To clarity there is two 14x8 supply’s. One goes south one goes north and will Continue the north one to the new addition.

    I already pushed the ductles split, customer didn’t go for it because of cost. Guarantee comfort but they didn’t go for it. I kept my cost down low on the quote as well.

  15. #11
    Join Date
    Aug 2003
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    The correct way as stated above is to do a heat/loss calculation for each room (manual J or newer computer programs), next perform a manual D or similar duct design calculation. As stated earlier that 14*8 can only carry so much air. I dont have my duculator in front of me but a 14*8 duct sounds like about a 2 ton system, that seems pretty small for a 2150 sq ft house, but not knowing where you are at (heating/cooling loads) I could be wrong. You cant rob peter to pay paul, have you thought about adding a mini split to the addition?

  16. #12
    Join Date
    Jan 2017
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    Pennsylvania
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    Not sure why people think they know things they don't. I would recommend not taking jobs that you are not qualified for. (gathering that from information you provided) I mean who takes a job and then goes online to find out how to do it. Sorry for harshness but if you take your car in for repairs and the guy starts watching utube videos on how to change the oil I am thinking(hoping) your out! Here's my advice RUN! RUN fast tell them that anything you do will never be good enough to control that room. If they still want just more duct work have them sign a waiver and to never call you. Put in electric heat and a booster fan and pray?? Not sure why people think they know things they don't. I would recommend not taking jobs that you are not qualified for. I am surmising this from you asking rule of thumb for registers.

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  18. #13
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    Go back and do a static pressure check of the supply and return. If your only at a TESP of .4", then it might handle the air flow. If your at .8", your out of luck, as far as it working right, and keeping that room at a comfort temp.

    A bonus room/FROG(free room over garage). Has a much different heat loss and gain rate then the rest of the house. So it will never be summer or winter using the current central unit.

    Might want to tell customer that it ill not always be as cool or as warm as the rest of the house, using their current system.

    CYA
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