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Thread: Airflow

  1. #1
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    Airflow

    How much airflow is reduced by using a 4" fresh air intake hood thru the skirt bd. and then connecting to a 6" duct ? My installer want to use a 6" duct all the way but my floor joist are only 8" so there's not enough material left after cutting a 6" hole So I would like him to use a 4" intake hood and then connect to a 6" hard ducting to the return air to the furnace. Is there anything to worry about as far as air flow or static being lost in those first 2"or 3" from the 4" to 6" ?

  2. #2
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    About 50%.
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  3. #3
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    could you use something rectangular and then adapt

  4. #4
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    or you could just frame that area out well and use the 6" he is intending on using

    but i cannot see the job from here
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  5. #5
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    What the difference in the amount of air a 4" duct will draw in vs. 6" duct ? I run my fan 24/7 on low speed all year and my home is just over 1500 sq ft. with me home during the day and a total of 2 adults and one child at night, so would a 4" duct be sufficent based on my size home and number of persons and the fan running 24/7 ?

  6. #6
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    That depends on how much natural infiltration you already have.
    Also if your already light on return or not.(If your light, it will draw more air)
    A 4" might get you 35 CFM, or 55 CFM.
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  7. #7
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    I understand the part on infiltration, as the more infiltration I have the more fresh air I wll in essense I have in total, but I am not sure about how the return air plays a roll in this equation ? How does having more or less return air determine how much fresh air the 4" duct will draw ?

  8. #8
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    It your fresh air is connected to the return drop. And the drops static pressure before you connect the fresh air intake is .4", it will pull more air in then if the drops static was .2".

    At .4" your return system is restrictive, so the fresh air duct would be less retrictive, and bring more air in(path of least resistance).

    At .2", the return isn't very resistive, so less air would come in through the fresh air duct.
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  9. #9
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    I recently installed a 4" fresh air inlet and connect an insulated 6" flex duct. The duct was <8' long. I needed .2" of negative pressure at the return connection to get 60 cfm of fresh air through the inlet. Consider a 13.5" X 3.5" Broan exhaust vent and then reduce to 6". Pin the damper open. Most air handlers do not generate much negative pressure in return when operating on low speed. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  10. #10
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    I went to the store yesterday and picked up one of thos 3-1/4" X 10" exhaust vents for the fresh air, But funny thing was there were no
    3-1/4" X 10" boots there, everything but, they went from 2-1/4" X 12" right up to 4" X 10",12" so I hope they have availible a 3-1/4" X 10" or my installer has one handy to connect this hood. This isn't a odd size for a boot going to 6" rd. is it ?

  11. #11
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    3&#188; x 10 x 6 stack boots are not uncommon.
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