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  1. #27
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    You would want what before the secondary HX?
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  2. #28
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    Quote Originally Posted by dash View Post
    did you view the sketch I posted?

    from the pics in the other thread,a lot could have been done to improve the take-offs from the plenum and on the return as well.

    Increasing the size isn't the only way to reduce the static.

    Adding a 5" filter to the side of the furnace,isn't the best way to do it.

    Wondering if the furnace is large enough to requie,bottom or returns from both sides,might want to check on that.
    I saw your sketch but stage 1 of the duct work has already started. I will consider it for stage 2. Increasing the take-off from 8x9.5 to 8x16 is a lot easier and cheaper than increasing the entire trunk duct.

    What do you mean returns from both sides?? This is the 1st time I've heard of this. Would'nt I have to run an entire new length of return duct and add a return grille somewhere in the house to connect it to? Or are you suggesting to tie it in to the existing panned return duct in the basement (which covers 2 joist bays before branching out to the 5 return grills in the house?)
    Key1

  3. #29
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    Quote Originally Posted by a0128958 View Post
    I'm surprised this is step 3, and instead not an earlier step.

    Doesn't changing the return from 24x10 to 24x16 change the return cfm capacity from about 1050 to about 1600 cfm (assuming 700 fps)?

    And since the unit is 4 ton, would the aproximate max cfm capacity be 1600, assuming 400 cfm/ton?

    Thus, simply to have the return sized to the capacity of the 4 ton unit, shouldn't the return be at least 24x16?

    Thanks and best regards,

    Bill
    I agree the return is too small, but with a static of 1.1 and 0.8 coming from the supply....I took Beenthere's advice and adressed that first.

    Key1

  4. #30
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    Quote Originally Posted by udarrell View Post
    It is not easy to design for 1800-cfm & above on residential systems.
    You should try for the 0.5" ESP, at least at a 350-cfm per/ton you might achieve the 0.5" ESP. ESP is after the filter & before the blower & before the evaporator coil.
    - Darrell
    My dips are currently set for 350 cfm/ton.....and I have 1.1 total SP.

    Great web-site by the way. I have referenced it many times.

    Key1

  5. #31
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    Quote Originally Posted by key1cc View Post
    I saw your sketch but stage 1 of the duct work has already started. I will consider it for stage 2. Increasing the take-off from 8x9.5 to 8x16 is a lot easier and cheaper than increasing the entire trunk duct.

    What do you mean returns from both sides?? This is the 1st time I've heard of this. Would'nt I have to run an entire new length of return duct and add a return grille somewhere in the house to connect it to? Or are you suggesting to tie it in to the existing panned return duct in the basement (which covers 2 joist bays before branching out to the 5 return grills in the house?)
    Key1
    Larger furnaces require this for proper airflow/static.Check the install manual.

  6. #32
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    Don't recall seeing a requirement for both sides for a 4 ton system on any furnace.

    5 tons, as far as I know they all do.
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  7. #33
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    Quote Originally Posted by dash View Post
    Larger furnaces require this for proper airflow/static.Check the install manual.
    I have read my Trane installation manual cover to cover and it mentions nothing about this. In fact the only time the words "return air" comes up is for instructions of how to install the filter rack on the bottom or on the side.

    The manual is written fo all sizes of direct vent 2-stage condensing furnaces with variable speed....and for AC's from 1.5 to 5 ton. It never mentions duct sizing or return sizing or whether there should be multiple returns sites.

    It does however tell you the static pressure range versus CFM and wattage.

    Key1

  8. #34
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    Quote Originally Posted by beenthere View Post
    Don't recall seeing a requirement for both sides for a 4 ton system on any furnace.

    5 tons, as far as I know they all do.
    It was just a question,agree 4 tons is not an issueusually around 1800 cms.How large is the furnace,and what cfms?

    FYI,the media filter would be best installed horizontially in the middle of the return drop,then vanes in the elbow going into the furnace,to lower the static.

    Just think the OP neded a better plan,to lower the static.

  9. #35
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    I think he is going to have the media filter installed horizontally.
    Atleast in another thread that came up.

    He has static issues in both his return and supply. And I think both need addressed. His supply static rises when he lowers his return static. Being that its a VS blower, doesn't that mean he's not moving set CFM.
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  10. #36
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    Yes, I think he's moving less then it's set at.

    We have only seen that a statics higher then then have been mentioned,but I guess that depends on the model.

  11. #37
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    Yorks drop off after .6". So if you have a .4" supply and .5" return, and then decrease the return, you see an increase in the supply.
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  12. #38
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    Apr 2008
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    I have re-measured and have decided to convert the 24x10 return drop to 24x18 or 24x20 (instead of 24x16 as previously planned). The bigger the better right?

    I do plan to have the filter installed horizontally in the Return Dop.

    I'm currently set at 350 CFM/ton which is theoretically 1400 CFMs for the 4 ton AC.

    The furnace is 100K btu in 95K but out.

    Key1

  13. #39
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    Quote Originally Posted by key1cc View Post
    I have re-measured and have decided to convert the 24x10 return drop to 24x18 or 24x20 (instead of 24x16 as previously planned). The bigger the better right?

    I do plan to have the filter installed horizontally in the Return Dop.

    I'm currently set at 350 CFM/ton which is theoretically 1400 CFMs for the 4 ton AC.

    The furnace is 100K btu in 95K but out.

    Key1
    Bigger isn't always better.

    If you increase the duct ,velocity(air speed) drops,if velocity increases again,like the reuduction in duct size to eneter the furnace,this increases the static.That said,this is often unavoidable,but when done just for amusement,it may do more harm then good.


    Looks in the pics like the return drop is tapped into a duct that is wider and shorter.If so ,removing he drop,and adding an elbow with vane to turn the return down,would be much less restrictive.

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