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  1. #1
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    Airflow for AHRI Ratings

    I always thought AHRI rating conditions were 400 cfm per ton, 67 wb and 80 db entering the coil. Carrier's product data for several 4 ton combinations with variable speed fan coils list the same ratings at 1400 cfm for a 4 ton system (350 cfm/ton), or as low as 1300 cfm for the new 25VNA. What do these systems need to be set up for in order to achieve the AHRI ratings?

    Similar story with Trane, for instance: https://www.comfortsite.com/Resource...s/4385456.html. 1490 cfm is what's needed to get the AHRI ratings. Presumably, if the installer set it up for 1600 cfm, you'd get more capacity and efficiency.

    Do manufacturers test these combinations at particular airflows (data you'd need to get from the manufacturer) and provide the ratings to AHRI? I'm trying to understand how the process works.

  2. #2
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    Because you are living in the land of regulations

    You of all people should understand the Gov't's rloe in setting standards even if they have nothing to do with reality!
    So the DOE says the 80degree dry and 67 degree wet for test point A so that everyone has the same test parameters and everyone optimizes their coil selections so that following (Uncle) Sam's rules they appear to have the most efficient equipment and the real answer would be to design for the conditions that most would "see" during operation.

    No real relationship that would allow one to, for instance" really model and therefore predict operating cost
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  3. #3
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    Thanks for your reply. I'm particularly asking about rated airflows.

    Are all ratings listed in the AHRI database assuming 400 cfm/ton? It would seem the answer is no, since Carrier's default for variable speed fan coils is 350 cfm/ton. And the Trane ratings I linked to above have a 4 ton at 1490 cfm, with the same ratings in the AHRI directory for that combination. So there really is no standard airflow by which all systems are rated. It's whatever the manufacturer tests the combination at. This is what I wanted to confirm is true.

  4. #4
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    Quote Originally Posted by RyanHughes View Post
    It's whatever the manufacturer tests the combination at.


    Of course this is the answer.

    With the DB/WB conditions set by the government, they can alter their system configuration to get the performance that they want.

    Airflow is just another parameter for the engineers to play with.
    Last edited by jpsmith1cm; 08-28-2011 at 08:48 PM. Reason: typo

  5. #5
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    Quote Originally Posted by jpsmith1cm View Post


    Of course this is the answer.

    With the DB/WB conditions set by the government, they can alter their system configuration to get the performance that they want.

    Airflow is just another parameter for the engineers to play with.
    Thanks. So AHRI doesn't require a certain airflow per ton for these ratings. That answers my question as to why you might see a manufacturer rate a 4 ton system at 1400 cfm versus 1600 cfm.

    One follow-up question: If Trane or any other manufacturer rates their equipment combination at 1490 cfm (or whatever value) to get the AHRI ratings listed, is there any advantage (or disadvantage) to setting the system up to run at 1600 cfm (which is nominal airflow for a 4 ton system)? Installers may not know what exact airflow Trane intended one particular equipment combination to run at to deliver expected ratings, so if a 4 ton system was intended to run at 1400 cfm to deliver AHRI ratings, it may very likely be configured to run at 1600 cfm once installed (since most go by 400 cfm/ton if in an average climate).

    I appreciate your responses.

  6. #6
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    If the system meets the AHRI rating at 350 CFM per ton. Then setting it to 400 CFM per ton could make it miss the rating Since the blower motor would use more electric to move that air.
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  7. #7
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    Jun 2010
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    The real answer may or may not be more compicated

    In commercial work, the designer will mark the Inside Return Air and the Outside Air point on a PSYC chart , connect the 2 points and extend the line to the saturation temp ( dewpoint ) to establish the ADP apparatus dew point that will in turn tell the designer what the optimal airflow(and discharge temp) will match the design conditions SO,
    if the unit was intelligent with a variable speed fan, that airflow and ADP could be adjusted on the fly.
    Is the unit smart enough to do this? I don't know what programming they use or some approximation that would allow for the handling of varying latent loads/ RH conditions but that is the theory anyway
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  8. #8
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    You a catching onto what most never get. Depending on the equipment selection based on your Manual J load calculation for sensible heat and laten heat load wil allow you to pick the correct size outdoor unit with the propper indoor coil according to Manual S. Not all system combinations will give you the same sensible heat ratio in the same tonage.Thus diffrent coil selections. Once the propper equipment selection is made and installed the airflow should be adjusted to match the sensible/latent load of the structure

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