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  1. #1
    Join Date
    Jun 2019
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    Question Help with cold air return sizing?

    Hi...my 1928 home has a Bryant Plus 90i furnace in the basement, and the cold air return is upstairs in the floor of the hallway. Whenever they installed things, someone altered the wood interior of the return in the subfloor so that they could insert a 14x25x1 air filter in it.

    My question is, could I possibly reduce that 14x25 opening a bit...to maybe 12x24? I believe the airflow for my furnace (model 042080) is 690 CFM for low speed, and 1345 for high, and the square footage of my home is 1589.

    I simply didn't know how to determine how big the return opening needs to be (for all I know, it's possible that even the current 14x25 is insufficient/undersized). If anyone can help I'd really appreciate it.

    Thank you so much-
    Robert

  2. #2
    Join Date
    Nov 2006
    Location
    Athens, Ohio
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    How did you arrive at your air flow figures? Many contractors don’t know or measure air flow, so I’m curious how a homeowner would know.
    To answer your question would require the following:
    1. Measure static pressure on the system.
    2. Measure air flow through the return grille.
    Your furnace should have 1200 CFM for 80,000 BTU. It seems big for a house your size.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
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  3. #3
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    Jun 2019
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    Thread Starter
    Thank you for your note...in response to your questions:

    - I found the airflow figures in the owner's manual specs.
    - I really have no way to measure the static pressure or specific airflow; I'm just a homeowner and don't have the tools to perform those tests.

    To note:
    - If it's relevant, according to the manual the furnace puts out 49,000 BTUs in low mode, and 75,000 in high.

    - Also, while 1589 is the correct/legal square footage, I remembered there's also around 250 s/f in the basement that was finished and they installed two registers down there...so the furnace is actually heating closer to 1850 s/f total.

  4. #4
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    Static pressure is very often on the high end of the blower chart in the manual. This is because the duct system is almost always too small and the equipment is too big. There's no way to know without measuring static pressure. A contractor can do it in a few minutes.

    We can make an educated guess how much air your system is moving if you provide the size of the supply and return trunks and the brand and model # of your furnace. But without seeing the configuration of the return in question even our guess could be way off.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  5. #5
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    Jun 2019
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    Oakland, California
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    Thread Starter
    Thank you again.

    I did find in the manual a spec pertaining to static pressure, but this may not be what you were referring to: Certified External Static Pressure (In. wc): 0.15

    If not, I'll see if I can have someone come out and measure things.

    Thanks again

  6. #6
    Join Date
    Jan 2004
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    PA
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    14x25 is actually under sized for 1345 CFM.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  7. #7
    Join Date
    Nov 2006
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    Quote Originally Posted by RinOakland View Post
    Thank you again.

    I did find in the manual a spec pertaining to static pressure, but this may not be what you were referring to: Certified External Static Pressure (In. wc): 0.15

    If not, I'll see if I can have someone come out and measure things.

    Thanks again
    Most blowers are rated for .5" WC. Some cheap models are only rated for .2 or .15. Evidently they're designed for very short ducts and a cheap air filter.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  8. #8
    Join Date
    Aug 2002
    Location
    Southold, NY
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    Quote Originally Posted by RinOakland View Post
    Thank you again.

    I did find in the manual a spec pertaining to static pressure, but this may not be what you were referring to: Certified External Static Pressure (In. wc): 0.15

    If not, I'll see if I can have someone come out and measure things.

    Thanks again
    That’s what the blower is designed for, not what it’s actually producing!

  9. #9
    Join Date
    Dec 2010
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    The Quad-Cities area (midwest).
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    Quote Originally Posted by RinOakland View Post
    Thank you for your note...in response to your questions:

    - I found the airflow figures in the owner's manual specs.
    - I really have no way to measure the static pressure or specific airflow; I'm just a homeowner and don't have the tools to perform those tests.

    To note:
    - If it's relevant, according to the manual the furnace puts out 49,000 BTUs in low mode, and 75,000 in high.

    - Also, while 1589 is the correct/legal square footage, I remembered there's also around 250 s/f in the basement that was finished and they installed two registers down there...so the furnace is actually heating closer to 1850 s/f total.
    Without a Manual J, the Btus required is only a guess. That said, I would guess the home only requires about 45,000 Btus. Maybe less.

    How cold does it ever get in Oakland? The furnace is oversized for sure! Regarding the return air situation, put another RA in the basement. The system is short on RA now.

    Your blower is probably ramping (up) to compensate.

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