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  1. #1
    Join Date
    Sep 2011
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    Heat Pump Efficiency VS AHU CFM

    Just curious:
    I have a 2 stage Amana 18SEER Heat Pump with a nominal rating of 3T. When in heating mode the communicating thermostat claims the unit is moving 825 CFM on low heat (24,000 BTU).

    Is this a actual 825CFM as measured by the AHU or is it an assumed value based on a typical pressure and fan speed?

    The thermostat has a trim adjustment to add/subtact up to 10% of the airflow so the AHU could, in theory, provide between 740 and 900 CFM.

    What is the need for this trim adjustment? When would a tech need to make an adjustment in airflow? A assume it could be used to adjust register discharge tempeartures?

    How would the efficiency of the heating performance be affected if the air flow was reduced or increased?

    Sorry for all the questions, just trying to understand my new system.

    Tim

  2. #2
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    Jan 2004
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    Lancaster PA
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    Less=lower efficiency and higher air temps out of the registers. More=higher efficiency and lower temps out of the registers.

    CFM read out is based on feed back from the blower motor.
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  3. #3
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    Sep 2011
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    Thanks beenthere.

    Why would the OEM include this trim adjustment, what benefit does it provide to the homeowner or the installer?

    Interesting about the CFM measurement. Are you saying that the motor circuit looks at the blower RPM and power draw and uses this data to calculate the CFM moving through the AHU giving me an actual true CFM reading? WOW, quite sophisticated.

    For an outside air temp of 35*, If the fan is slowed down 10%, the discharge temp will rise 10% or about 2.5*, correct. Following this logic, will this decrease the efficiency of the unit by 10% and reduce the COP from 3.5 to 3.15?

    Increasing the air temp 2.5* does not seem like a lot of difference to me, but I guess this temperature increase combined with lower air velocity from the vents, would make things feel warmer.

  4. #4
    Join Date
    Jun 2010
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    West Kentucky
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    Not exactly

    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

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    Last edited by beenthere; 12-07-2012 at 04:14 AM. Reason: Non Pro * Member

  5. #5
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    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

    You can find the rules for posting and qualifications here.

    Further infractions may result in loss of posting privileges.
    Last edited by beenthere; 12-07-2012 at 04:15 AM. Reason: Non Pro * Member

  6. #6
    Join Date
    Jan 2004
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    Lancaster PA
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    Quote Originally Posted by pendetim View Post
    Thanks beenthere.

    Why would the OEM include this trim adjustment, what benefit does it provide to the homeowner or the installer?

    In cooling mode allows the installer to set the blower so the system's capacity matches the latent and sensible load/needs of the house better. I heating it provides either better comfort, or higher efficiency.

    Interesting about the CFM measurement. Are you saying that the motor circuit looks at the blower RPM and power draw and uses this data to calculate the CFM moving through the AHU giving me an actual true CFM reading? WOW, quite sophisticated.

    VS ECM motors need to be working at X torque to move X amount of air. The module will speed up or slow down the motor to be at that X torque. The motor(motor module)communicates the amount of work it is doing to the board. This is converted to a CFM. If the static the motor is working against is too high for it to move the set CFM the older boards would still flash that it was moving 1200 CFM, even when it was only moving 1155CFM. it was a rounding algorithm. The communicating stats can display actual CFM since they are digital.

    For an outside air temp of 35*, If the fan is slowed down 10%, the discharge temp will rise 10% or about 2.5*, correct. Following this logic, will this decrease the efficiency of the unit by 10% and reduce the COP from 3.5 to 3.15?

    Increasing the air temp 2.5* does not seem like a lot of difference to me, but I guess this temperature increase combined with lower air velocity from the vents, would make things feel warmer.
    102 degree air feels a lot warmer then 100 degree air. And when its moving slower it feels much better then the 100 degree air. Yes, COp is slightly lowered.
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  7. #7
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    Thanks for your patience educating me. There is some pretty sophisticated controls stuff going on in these units which is needed to get the high efficiencies everyone is demanding

    In our place we keep the ambient at 69* so the Delta T of 23 @35* OAT, give us a register temp of 91-92*. Adding 2* to this and reducing the velocity a bit probably would be a huge noticeable change in the environment. Looking at the "comfort calculator" linked to tedkidd's signature really drives that point home. I " intuitively felt" the results of the slider application on a daily basis, but seeing it quantified is very satisfying (at least for an engineer like my self ) and enlightening.

    Tim

  8. #8
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    gourdhead, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

    You can find the rules for posting and qualifications here.

    Your post has been deleted.
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  9. #9
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    Sometimes a good compromise, if you have a outdoor temp sensor, is to run lower airflow below about 40F to maintain higher temps.

  10. #10
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    Sep 2011
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    Nj and Delaware
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    Thanks for the tip motoguy128

    Can this fan speed trim be automated based on OAT through the Goodman CTK03 T/S? I have an outdoor sensor that is used to lockout the resistance heat above 30* .
    I saw settings for fan trim for the Heat Pump and separate trim adjustments for the Aux heat but did not see any rule that could be used to set fan speed based on OAT. Now if this thing had an AB MicroLogix controller attached....... I could really screw it up!.

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    I took a look at hte intsallation manual. Lots of settings, and yes you can change airlfow all the time, but I'm not seeing the ability to lower it based on outdoor temperature.

    I agree, is you have a PLC on these things, you could do all sorts of intersting controls if you have a good imagination.

  12. #12
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    Was the heat pump on stage 1 or 2 when you got the 825CFM reading? CFM will be lower on stage 1, 825 on low speed is great.

  13. #13
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    It was on low stage. I am thinking ahead to when the OAT gets down in the 20* range and the nominal register discharge temp drops to 85-87* range.

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