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  1. #1
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    Oct 2003
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    I have a 4" PVC fresh air intake. I have run it up to 6' above grade and put a U at the end so that the intake is facing the ground. But the suction is enough that when we have heavy rain the intake sucks in a lot of water.

    Are there any tricks to preventing the intake from taking in water, short of building a roof over it?

  2. #2
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    Nov 2000
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    More then likely it is condensating inside of the pipe, not sucking water in. 4" pipe is a bit large and will condensate quite a bit of moisture from the air.

    In any case, you should have a trapped drain on the combustion air pipe close to the furnace.

    Concentric terminated vents have the issue much less due to the prewarming of the combustion air.
    ...seek, and ye shall find;..
    So always seek the Truth, not just what you want to believe to be true…
    Ecclesiastes 10:2 NIV


  3. #3
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    Jul 2004
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    Are you talking about a fresh air for furnace combustion or a pipe that is tied into your cold air return? I have never had this problem on combustion intakes and if its fresh air for return you have a problem with your duct work if its pulling enough to get water to come in the pipe.

  4. #4
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    Thread Starter
    This is a fresh air intake leading to the cold air return, not furnace/make-up air. It's definitely sucking rain in -- not condensation. I have a FilterVent pulling 75CFM through the pipe, so it's like 1" w.c. of suction. Definitely enough to get rain falling/dripping around the opening.

    Once it crosses to the inside of the house the intake connects to a 6" insulated flex duct. Maybe a trapped drain is the solution? Not sure how I would do that on 6" round duct though.

  5. #5
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    Sorry, I misunderstood.
    ...seek, and ye shall find;..
    So always seek the Truth, not just what you want to believe to be true…
    Ecclesiastes 10:2 NIV


  6. #6
    Join Date
    Sep 2003
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    Put a PVC plug on the intake; That will take care of it!

    If you can find it check out the Rheem condensing furnace direct vent alternative installation diagram. They address that type of problem by installing a T (in your case 4") in such a way that it will trap the water. The side of the T that traps the water has a reducer and then comes off with a hose (about 1/2"). The hose is looped to form a water trap in mid air.

  7. #7
    Join Date
    Nov 2004
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    Originally posted by dbooksta
    This is a fresh air intake leading to the cold air return, not furnace/make-up air.

    It's definitely sucking rain in -- not condensation.

    I have a FilterVent pulling 75CFM through the pipe, so it's like 1" w.c. of suction.

    Definitely enough to get rain falling/dripping around the opening.

    Once it crosses to the inside of the house the intake connects to a 6" insulated flex duct. Maybe a trapped drain is the solution? Not sure how I would do that on 6" round duct though.

    Keep It Simple __ Expand That Sucker

    SLOW the air down DIRECTLY AT the Inlet ...
    IN Other words,
    ______________ expand 4" PVC to a ~ 6" inlet

    Material
    1. 4" by 6" Expander (reducer, if you will),
    2. 6" diameter PVC x 12"

    4" dia., 75 CFM = 860 FPM ... Entrains Water Droplets ! !!
    6" dia., 75 CFM = 380 FPM ... Resolved ?

    Velocity pressure is reduced by > 4 X

    ( or as the more dramatic persons wish to
    state ' by more than 400% ').

    Note: 4" diameter = 12.5 sq. inch,__ .087 sq feet
    ....__6" __________ 28.3 .........__ .196
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  8. #8
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    Oct 2003
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    Thread Starter
    Excellent solutions -- I'l try enlarging the intake and if that doesn't work I'll put a T-trap on it.

    Thanks!

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