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  1. #1

    HVAC Calc Question on return size.

    I just did an audit with the HVAC Calc. It gives the same size of duct work for feeders and returns. Is this correct?

    I thought that the returns need to be twice the size as the feeders. A 14" diameter feeder needs a 20" diameter return.

  2. #2
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    Quote Originally Posted by GoGreen101 View Post
    I just did an audit with the HVAC Calc. It gives the same size of duct work for feeders and returns. Is this correct?

    I thought that the returns need to be twice the size as the feeders. A 14" diameter feeder needs a 20" diameter return.
    Not true.
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  3. #3
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    Your return air ducts carry about 20% more air (air expands with temperature increase) during cooling barring the amount of any OA to your unit. You should size the returns accordingly but not double the size of supply. Thank you very much
    "I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
    "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."
    Barry Goldwater

  4. #4

    Help me out here...

    Quote Originally Posted by jimj View Post
    Not true.
    Could you clarify that please! Are both statements false? or is one true and the other false?

  5. #5
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    You can size the return and supply ducts the same size but you will have more friction loss in the returns which is no problem provided that your air flow calculations allow for the total friction loss to be equal to our less than the unit's specification requirements for the rated CFM. 400 CFM per ton is good for most units. But never lower than 300 CFM per ton cooling.
    "I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
    "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."
    Barry Goldwater

  6. #6
    Quote Originally Posted by glennac View Post
    Your return air ducts carry about 20% more air (air expands with temperature increase) during cooling barring the amount of any OA to your unit. You should size the returns accordingly but not double the size of supply. Thank you very much
    Thanks Glen. My thought are that larger returns would be less taxing on the blower. Granted the air from the returns will expand when heated and shrink when cooled. I guess 20% would take this into consideration.

    Would it be better if the air through the returns were moving at a slower speed than the supply?

    Thank you

  7. #7
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    Quote Originally Posted by GoGreen101 View Post
    Could you clarify that please! Are both statements false? or is one true and the other false?

    Maybe this will help you.

    http://efficientcomfort.net/jsp/ResDuct_Web.jsp
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  8. #8
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    Quote Originally Posted by GoGreen101 View Post
    Thanks Glen. My thought are that larger returns would be less taxing on the blower. Granted the air from the returns will expand when heated and shrink when cooled. I guess 20% would take this into consideration.

    Would it be better if the air through the returns were moving at a slower speed than the supply?

    Thank you
    It is standard practice on large commercial systems to use lower air velocity in returns than supply but not necessary. The main thing is that the returns are not smaller and the total friction losses are added up for the correct amount of CFM flowing in the supply and the correct CFM flowing in the returns.

    Of course on a simple duct layout for residential unit you can be conservative in the sizing of your ducts (low design velocities) and if using flex duct use metal elbows for 90 bends and sizing your flex larger than a ductulator calls for with metal round or rectangular duct.

    Basically a 8" flex will handle the same CFM as a 6" round metal duct with about the same friction loss. I believe that this covers the whole subject. Thank you very much
    "I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
    "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."
    Barry Goldwater

  9. #9
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    Quote Originally Posted by glennac View Post
    It is standard practice on large commercial systems to use lower air velocity in returns than supply but not necessary. The main thing is that the returns are not smaller and the total friction losses are added up for the correct amount of CFM flowing in the supply and the correct CFM flowing in the returns.
    And as Glenn knows that is exactly where most designs fail!
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  10. #10
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    Quote Originally Posted by jimj View Post
    And as Glenn knows that is exactly where most designs fail!
    Agreed and also installing the flex in a sloppy manner thus greatly increasing the friction losses and choking air flow. Thank you very much
    "I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
    "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."
    Barry Goldwater

  11. #11
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    20%? Show your math. Why consider only cooling, BTW?

    The correct answer: It depends. Is there a return for each supply? Central supply? Generally bigger is better for either supplies or returns to reduce pumping losses. Not much available space for duct work? You'll have to compromise.

    The points about watching overall head and sloppy installation are good ones.

  12. #12
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    Quote Originally Posted by Piedmont user View Post
    20%? Show your math. Why consider only cooling, BTW?

    The correct answer: It depends. Is there a return for each supply? Central supply? Generally bigger is better for either supplies or returns to reduce pumping losses. Not much available space for duct work? You'll have to compromise.

    The points about watching overall head and sloppy installation are good ones.
    20% has been the common assumption since I have been in the business for over 35 years when I get the time and near some books and tables I'll look up the volume difference between air at the same pressure one at 52F and the other at 72F and I'll let you know the figure or you can do it yourself.

    Cooling is the main concern here. Air flow is critical in cooling but not near as critical in heating unless you end up not having enough across the heat exchanger.

    Your "correct" answer was already alluded to in my comments on sizing the return and supply duct systems as I stated before the main consideration is that the overall friction losses are not more than the design maximums listed for the unit fan curve to provide the necessary air flow.

    Nothing new or different from you comment. So I don't see any disagreement here other than your concern about the 20% and heating air flow which I believe I have just answered. Thank you very much
    "I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
    "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."
    Barry Goldwater

  13. #13
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    Quote Originally Posted by glennac View Post
    20% has been the common assumption since I have been in the business for over 35 years when I get the time and near some books and tables I'll look up the volume difference between air at the same pressure one at 52F and the other at 72F and I'll let you know the figure or you can do it yourself.
    Answer to your specific question: 4%
    (To get a 20% increase in volume, you'd have to heat that 52°F air to 154°F)

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