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 Originally Posted by MurDoc04
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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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.
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Originally Posted by cjpwalker
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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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.
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Originally Posted by MurDoc04
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.
Make your expertise uniquely valuable.
Make your influence uniquely far-reaching.
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Originally Posted by jimj
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
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
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Originally Posted by dan sw fl
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
  You would make Hank R proud!
Make your expertise uniquely valuable.
Make your influence uniquely far-reaching.
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Originally Posted by jimj
You would make Hank R proud!
HVAC 101
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
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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.
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Originally Posted by cjpwalker
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
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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?
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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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Originally Posted by cjpwalker
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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