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  1. #1
    Join Date
    Sep 2006
    Posts
    136

    ESP, VS blower, Furnace filter and CFM. Oh My!

    Stupid HO, trying to learn something. But I promise I'm not going to do the work myself!

    My new dual fuel system is going in tomorrow (AS Heritage 16 HP / 80% VS furnace). I have lots of questions about the way ESP, the VS blower, a furnace filter, CFM and watts used work together.

    1)In looking at the product data for the furnace, I see that the installer can choose 3 different settings (Low, medium, High) for airflow settings for 1st and 2nd stage for heat and separate settings for cooling. What drives that choice? Is it the system ESP or the temp. rise or something else?

    2) If I don't mess with the registers or vents, in theory the ESP of my system will always be the same, but as my filter gets dirtier, the ESP will rise, but the VS motor will compensate for the higher ESP. The motor will work harder to maintain the CFM that was set my installer. In doing so, the motor uses more watts just to maintain the same CFM as when the filter was new/clean. Do I understand that correctly?

    3) Since it appears that the VS motor uses the least amount of Watts with a lower ESP, should you not design the system with a 0.1 ESP or try to modify as system to get to the lowest ESP? Can you lower the ESP on an existing system by adding more returns? What is even more baffling to me is that the Watts used is not always linear. Some settings use less power at .9 than at .7 ESP with the same airflow setting. How can that be? Maybe I need to post the chart.

    Thanks for putting up with me!
    Thanks,

    Mark

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Not familiar with A/S ,but in general;

    1.higher cfms is slightly less cost in cooing as the system will produce more sensibkle btus,lower cfms in cooling will provide more comfort by producing more latenet btus,and dehumidify better.

    Furnace,more cfms cooler supply air ,less warmer,both could be in the required cfms range.

    2.Yes correct


    3.True lower static will reduce the cost to operate the fan.Even with new construction,.4 to .5 is about as low asit's practical to go.

    The cooling coil is likely greater than .2 static,then the filter could be .08 or more.

    Adding returns is usually the best/simple way to reduce the static.


    Post the chart please

  3. #3
    Join Date
    Sep 2006
    Posts
    136
    Dash:

    Thanks. Here is the chart. This is a Trane chart, but I'm sure it is very close if not the same for American Standard.

    Thanks,

    Mark

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    I never noticed that on the Carrier charts,but checking them closely I found one model furnace that at one cfm rating ,where .9 is less watts than .7 static.

    Not sure why this would be,hope someone else knows.

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