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Thread: Heat Pumps?

  1. #14
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    Quote Originally Posted by MurDoc04 View Post
    I'm being to think my teach is a little out of touch on heat pumps and that irritates me because I'm paying for this school. Thank you fellows for the info.
    A few years back there was a thread or 2 about this in the Pro forums. I believe it was Normchris that posted that heat pumps are more efficient at 450 CFM per ton.
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  2. #15
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    Systems are designed for nominal 400 cfm per ton. The air handler doesn't differentiate between heat and cool modes, and a heat pump is an air conditioner first.

    Yes there are some exceptions - carrier built equipment is designed at 350, and one day every system will have a variable speed air handler and an inverter drive compressor, Skynet will become self-aware, and we will all be checking superheat in the matrix. Until then, 400 cfm per ton.

  3. #16
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    Quote Originally Posted by cjpwalker View Post
    Systems are designed for nominal 400 cfm per ton. The air handler doesn't differentiate between heat and cool modes, and a heat pump is an air conditioner first.

    Yes there are some exceptions - carrier built equipment is designed at 350, and one day every system will have a variable speed air handler and an inverter drive compressor, Skynet will become self-aware, and we will all be checking superheat in the matrix. Until then, 400 cfm per ton.
    Many systems are also commonly sized using .1" FR. Doesn't mean its right, its just commonly done.
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  4. #17
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    Just from this thread I understand more about heat pumps now. I'm still new to this field and I'm slowly learning but I'm getting there.

  5. #18
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    Quote Originally Posted by MurDoc04 View Post
    I know that it is standard 400 CFM per ton for residential AC / Heat... What is standard CFM for heat pumps? I have been told by my instructor that 450 is a rule if thumb but I can not find any hard evidence in that. Need some help please!
    I would order Manual S and H from ACCA's website and do a little reading. I just spent the week with the author of these two books ( along with Manual J and D) and believe me he's not into rule of thumbs!

    And 400cfm per ton on A/C is also BS.
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  6. #19
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    Quote Originally Posted by jimj View Post
    I would order Manual S and H from ACCA's website and do a little reading. I just spent the week with the author of these two books ( along with Manual J and D) and believe me he's not into rule of thumbs!

    And 400cfm per ton on A/C is also BS.
    RIGHT, 400 CFM/ ton +/-10% is a good observation
    _ and B asic S cience.

    Q = 1.08 * CFM * dT
    Q ton = 12,000
    SHR avg 0.75
    Q sensible 9,000 ______ 9,000 ____ 9,000
    dt = ____ 21 ______ 18 ________ 23
    _______ 1.08 ______ 1.08 ______ 1.08
    _ CFM 397 ____ 463 ____ 362
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  7. #20
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    Quote Originally Posted by dan sw fl View Post
    RIGHT, 400 CFM/ ton +/-10% is a good observation
    _ and B asic S cience.

    Q = 1.08 * CFM * dT
    Q ton = 12,000
    SHR avg 0.75
    Q sensible 9,000 ______ 9,000 ____ 9,000
    dt = ____ 21 ______ 18 ________ 23
    _______ 1.08 ______ 1.08 ______ 1.08
    _ CFM 397 ____ 463 ____ 362
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  8. #21
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    Quote Originally Posted by jimj View Post
    You would make Hank R proud!
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    Q= 1.08 * CFM * dT
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  9. #22
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    Ok, so 400cfm per ton is wrong... so walking up to a new system for a startup where do you set the blower? I always shoot for 400 per (nominal) ton based on TESP and fan charts.

  10. #23
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    Quote Originally Posted by cjpwalker View Post
    Ok, so 400cfm per ton is wrong... so walking up to a new system for a startup where do you set the blower? I always shoot for 400 per (nominal) ton based on TESP and fan charts.
    jumj from Arizona has a different xxxx xx thumb, perhaps, IT'S GREEN.!
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  11. #24
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    So we actually learned it as Cooling BTUs = CFM x delta h x 4.5, h=enthalpy. CFM x 1.08 x dt is for heating. I've never learned to use it for cooling with 9000 btu sensible. Where does that number come from?

  12. #25
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    If your going for efficiency only, then 450 CFM for a heat pump. If for comfort in cooling then CFM should be set up to meet the SHR you need in cooling.
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  13. #26
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    Quote Originally Posted by cjpwalker View Post
    So we actually learned it as Cooling BTUs = CFM x delta h x 4.5, h=enthalpy. CFM x 1.08 x dt is for heating. I've never learned to use it for cooling with 9000 btu sensible. Where does that number come from?
    1200X6.4*4.5=34,560 total BTUs.
    1200X20X1.08=25,920 sensible BTUs.
    34560-25920=8,640 latent BTUs.
    25920/34560=.75 SHR.
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