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  1. #1
    Join Date
    Nov 2016
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    Question Furnace cfm and temperature rise

    Hello,
    I am an HVAC designer, and there is a question I hope you can answer.
    When selecting the residential furnace CFM for duct system design, I usually pick the number which is sufficient for air conditioning tons (400 cfm per ton), and always check the temperature rise to make sure it is within the manufacturer's required range.

    The question is: why there are low, medium, medium-high and high blower speeds with corresponding CFM, if usually low, and medium produce too high temperature rise? What's the point of even showing them on the blowers data table?

    For example, now I am looking at a existing system at the customer house. They have 60 years old ducts with a 5-years old grossly oversized single stage furnace. The CFM at medium-high is OK for keeping the temp. rise within 35-65 degree range but is too high for the existing ducts. The table shows lower CFM for low and medium speed, but then the tem. rise becomes 72 degrees which is more than allowed.
    So, what the low and medium speeds can be used for, why they are even there? Is it only for the a/c, and for heating we need higher CFM?

    Sorry if it is a stupid question, I have been not able to find the answer myself.
    Thank you.

  2. #2
    Join Date
    Feb 2015
    Location
    Steubenville Ohio
    Posts
    687
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    TESP. Most homes are not identical and airflow requirements can vary greatly.

  3. #3
    Join Date
    Mar 2013
    Location
    Billington Heights, NY
    Posts
    19,928
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    additionally, those are provided assuming that the house is a perfect system. Since it's not (ever) you can tweak the blower to fit a set of specs within a range of "good".
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  4. #4
    Join Date
    Aug 2002
    Location
    Avon Lake, Ohio
    Posts
    2,884
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    CFM and Delta T are determined by the furnace design not the building. Furnaces have and input and output. We want to establish the output. CFM X Delta T X ACDF = BTU output. CFM is for heat transfer. Too fast or too slow you lose capacity.
    Average airflow for an Induced 80% is 130 cfm per 10,000 btus of input. Condensing equipment the average airflow is 150 dcfm per 10,000 btus of input.

    Airflow is just as critical on furnaces as it is on A/C.
    captain CO

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  6. #5
    Join Date
    Mar 2018
    Location
    Kansas Flatlands
    Posts
    127
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    Some places have cooling loads less than the heating load. Continous fan speed can be desired for constant circulation and filtration without running as much air.

  7. #6
    Join Date
    Jan 2004
    Location
    PA
    Posts
    75,314
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    A heat only system is suppose to have the duct work designed for a TESP of .2"

    That is when you would use your lower speed taps.
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