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  1. #1
    Join Date
    Apr 2005
    Posts
    333
    Here is a problem I ran into today. I probably do NOT have enough info.

    1400 sq.ft. Ranch (one story) with plan to add 800 sq. ft.
    100k btu input variable speed/ecm motor and 3 tons of cooling. Fan speed on Highest setting. Furnace located in garage at end of home. Plenume is 7 feet from top of "N" coil to top of Supply trunk. SUpply trunk runs 45 ft. First 20 ft is 24x8 then reduces to 18x8 for final 25 ft. There are currently 11 supplies off the top of the insulated Supply trunk. All eleven supplies are 8" round flex. Each flex run averages 10 ft in lentgh to a 8" ceiling diffuser. Each ceiling diffuser has a damper with handle. There are only 2 return air grills in the house. One is 8x30". It is in a "smoking" room. The other is a 20x25" located 10 ft from the furnace; in a hallway. The returns are also in the ceiling.

    The system operates properly BUT has poor air flow/velocity. The supply trunk's final two ducts run to a "three season room". 3 sides of glass/outside walls. These two supplie's registers are are the inside wall. NO returns in that area of house--- idea to "draft" return back thru house/hall about 65 feet!

    The 4" Media filter is new/clean.

    The T'stat is located in a hallway with most doors closed.

    Checked Fan speed/dip switch settings.

    AIR VELOCITY IS POOR.

    ??? Would it help if the ELEVEN flex supply ducts were 6" instead of 8"???????
    meanwhile, back at the ranch.....

  2. #2
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    certainly sounds as if the ductwork is adequate, I agree the 8" runs are a bit on the large size. Might want to use a duct calculator to size them. 100k furnace on a 1400 sq. ft. home is awfully large. Where are you located?
    "Go big or Go Home"

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,789
    Check your esp.

    See if your getting enough return.

    Then do a room load, and see how many cfm you relly need to each room, then you should be able to tell which rooms you can change to smaller supplies.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Apr 2005
    Posts
    333
    Black Adder-

    Love the name. I've got a buddy I call "Slack Bladder" from the TV show. Whenever we get together, I tell him I have a "devious and cunning plan".

    Home is in upstate NY; lake front. But again, the system was sold "oversized" with the intention of the HO adding about 800 sq. ft to it.

    I do NOT have a method of measuring STATIC PRESSURE. The unit specifies not to exceed .5" wc.

    *** 95% of all supply ducts I have seen for ten years have been 6" round.
    meanwhile, back at the ranch.....

  5. #5
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    I agree, most units are either running 5" OR 6" supplies, I like to size them for the load required otherwise you end up with a freezing bathroom and a cooking hot bedroom. You should be able to adjust the airflow to each outlet if they have put dampers in the lines. If not it might be easier to put in manual dampers and adjust them than changing the piping especially if there are 8" outlets in the ceiling or floor.
    "Go big or Go Home"

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Depending on return duct sizes,and media filter size,the static is probably too high.

    Get a magnahelic,not that much ,and test it ASAP.

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