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Thread: Ducts in slab?

  1. #1
    Join Date
    Jul 2004
    Location
    Nashville TN.
    Posts
    44
    Does it make sense to blow air thru slab on grade to help cool the house?
    Would I use metal ducts or will pvc work? I have a HVAC contractor designing the geo system, but just wanted to ask this. All the main ducts will be below ceiling in living space. Thanks for the input.
    I live in Nashville TN. 3000 sq. ft.

  2. #2
    Never seen that done before, If you want to stick with typical designs I would say not to.

  3. #3
    Join Date
    Apr 2003
    Posts
    469
    I have in slab ducts
    Your better off planting a few trees

  4. #4
    Join Date
    Dec 2002
    Location
    peoples republic of maryland
    Posts
    954
    I have never seen in slab/underground ducts that don't have water that seeps in. Unless your slab is 8-10" thick the ducts will be buried under the slab. Don't bother unless you like high humidity and mold.
    "The meek shall inherit the earth"
    "he that's walking with wise persons will become wise, but he that is having dealings with the stupid ones will fare badly" Proverbs 13:20
    "Pressure is something people feel when they don't know what their doing". Peyton Manning-superbowl MVP

  5. #5
    Originally posted by md master
    I have never seen in slab/underground ducts that don't have water that seeps in. Unless your slab is 8-10" thick the ducts will be buried under the slab. Don't bother unless you like high humidity and mold.
    Exactly. Ducts under an underground slab is a VERY BAD IDEA. Even the tightest of ducts will gain moisture and from that comes a whole new host of issues.

    Is the contractor thinking about pouring the concrete once the ducts are in place? Another nutty idea that you should avoid. If you want something like that, go with a water system and use PEX in the slab.

  6. #6
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
    Posts
    1,209
    Check out some 70's 2-story buildings with 1st floor slabs. Many of them had ducting in the slab, churches also. When you pull the registers and inspect them you will see water. Bad idea.
    Proud supporter of Springfield Millers and Oregon Ducks.

  7. #7
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    I must not understand

    Why would u put air through slab on grade. Concrete slab has no effect on cooling btuh, so what would cooling the slab do for you ??
    If you are talking the supply & return ducts running through slab. I agree with most everyone. Have had to vacuum out duct system below grade due to water in ducts.
    Owner had complained of gurgling sound when unit on.

  8. #8
    Join Date
    Feb 2005
    Posts
    665
    Its surprising how many diggers we have here in So. Utah, I dont normally bid them, but have done a few.

    I dont like the fact that it increases the overall load of the home, the ground acts as one big heat sink. Try running 120 air through an underground system then check that leaving air temp in the back bedroom, lucky to get 90!

  9. #9
    Join Date
    Feb 2005
    Location
    Alabama
    Posts
    534
    I've actually seen a few here in AL but they were return ducts, not supply. During the change out, the worst of worst happened, a kitten got in the ducts and went exploring. took an hour to get the little sh#$ out.
    "If you can't fix it, don't break it."

  10. #10
    Join Date
    Apr 2004
    Location
    Niantic, Illinois
    Posts
    545
    Originally posted by ac4me
    Does it make sense to blow air thru slab on grade to help cool the house?
    Would I use metal ducts or will pvc work? I have a HVAC contractor designing the geo system, but just wanted to ask this. All the main ducts will be below ceiling in living space. Thanks for the input.
    I live in Nashville TN. 3000 sq. ft.
    It was used all of the time in my area until the water table rose due to city growth causing poor watershed. They used metal. I would rather see something that won't rust away. Many of these did, but as I siad, the local climate changed and the ducts spent a lot of time wet. Most of these slab houses have now been converted to horizontal units in the attic a long with all of the ductwork.

  11. #11
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    use pvc ducts in lots of #69 gravel with insulation under ducts, over gravel -- slab must be elevated --

    but, all depends -- where are you?
    someone can design a workable system & hse

    I would just have crawl, or bsmt --

  12. #12
    I think what he's getting at is this-
    They are designing the system with main trunks that are going to be in his living space taking up head room and he wants to know if it would work just as well to put the mains in the slab via transite heating and save the room by not having a soffit. Am I correct?

  13. #13
    Join Date
    Jan 2005
    Location
    Seattle
    Posts
    191

    Hmm In slab cooling

    I have Auncle in noth Carolina.
    When he built his home about
    5 years ago he did something
    very similar to what you are
    describeing.He did NOT use
    metal ducting.What he did was to
    lay down 10" to 12" of gravel
    (for drainage)than installed
    concrete building blocks.
    the ones with three holes
    in them.They were laid down
    diagonally to the width of
    the floor plan.This was done
    to get the maximum length
    possible.Then the foundation
    was laid doen on top of that.
    a couple of dams were installed
    so that the air flow went down
    one side than back though the
    the other.Keep in mind that
    he built on top of a mountain
    so water saturation was not
    a problem.He also has a chiller
    as a back up but it rarely
    comes on.Keeps a three story
    house at 72 to 74 degrees
    with no problems.If I understand
    what you are trying to do
    is using the ground as a great
    big heat sink.Than if you spend
    alot of time thinking on how
    you can do it in your area.
    than I'm sure you can come
    up with a way.I have to tell
    you that I think that a good
    geo system is the way to go.
    from some of the home owners
    and techs that have posted here
    they are very very economical
    to operate
    41Gasman






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