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Thread: To prevent copper from touching ductwork?

  1. #1
    DIY Senior Member ToolsRMe's Avatar
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    Default To prevent copper from touching ductwork?

    Somewhere recently on this forum someone wrote that copper should not touch ductwork because of the chemical reaction between the two.

    Is duct tape adequate protection to prevent the reaction?

  2. #2
    Master Plumber Dunbar Plumbing's Avatar
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    No. Use foam pipe insulation or something thick to wrap around the pipe.

    Tape can eventually wear through if vibration is possible.

    Most times when I have to repair water lines that have been in contact with panning or ductwork......it is never in a good spot and always hard to rework the piping.


    Those leaks usually take years to surface as well.....they slow leak a long time before you even see the damage come through the ceiling/walls.
    Read what the end of this sentence means.

  3. #3
    Plumber Cass's Avatar
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    I would use a peice of cardboard / plastic between them.

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    Moderator & Master Plumber hj's Avatar
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    Default pipe

    There is no "reaction" between them since the copper should not be wet. The problem would be movement due to expansion and contraction rather than a reaction or vibration. Anything that separates them, such as pipe wrap tape, will be beneficial, but the ideal thing would be to not have them touching in the first place, even if it is only by the smallest gap.

  5. #5
    Master Plumber Dunbar Plumbing's Avatar
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    Quote Originally Posted by hj
    There is no "reaction" between them since the copper should not be wet. The problem would be movement due to expansion and contraction rather than a reaction or vibration. Anything that separates them, such as pipe wrap tape, will be beneficial, but the ideal thing would be to not have them touching in the first place, even if it is only by the smallest gap.
    hj,

    Condensation. Cold incoming water against a warm duct pipe

    Hot water against a cold duct work. Either way it causes the piping to sweat.

    Next time I find one of these I'll take a picture of both....and the drywall.


    What really sucks is that I know for a fact that when I was in new construction I did this sort of thing all of the time to get water lines in an already crowded joist space and didn't put protection between the duct work and pipes. Most cold air returns the piping pierces perpendicular and usually where the holes are touching the pipe cause the leaks.

    I feel bad now. I wish I would of known now back then. Those people have future plumbing expenses coming.
    Read what the end of this sentence means.

  6. #6
    Plumber Winslow's Avatar
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    you will get electrolysis if copper comes into contact with a dissimilar metal (other than brass) whether there is movement or not or expansion and contraction or not. The water running through the copper pipe will create an electrical current and the pipe will deteriate starting at the point of contact. By the time you realize there is a leak you can have a large portion of paper thin pipe. Milwrap tape, or anything to cause separation (however slight) will prevent electrolysis. using pipe insulation will work well in areas where the copper and dissimilar metals run close together for long runs.

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default reaction

    The water has to touch both metals in order to become the electrolyte. Water inside the pipe will only cause a reaction between dissimilar metals making up the piping system.

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    Moderator & Master Plumber hj's Avatar
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    Default Condensation

    Cold water, warm ducts, no problem unless there is moisture in the air, and there should be no moisture inside the walls. Cold water pipe, below the air's dew point, plus moisture will create condensation even without a duct.

  9. #9
    Plumber Winslow's Avatar
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    Quote Originally Posted by hj
    The water has to touch both metals in order to become the electrolyte. Water inside the pipe will only cause a reaction between dissimilar metals making up the piping system.

    Iv'e done many a repair on looped copper lines that deteriated because they came into contact with nails, rebar, wire mesh, (used when they pour a slab), cast iron pipe, to name just a few. None of these things had water in them besides the looped copper (except the CI drain line) and they all deteriated due to a chemical reaction caused by the copper touching dissimilar metal. It usually takes a long time (although there are exceptions). It happens quite notably when the hot water line contacts thre cold water line (both copper pipe).

  10. #10
    Plumber Cass's Avatar
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    The reason it takes a long time is slabs trap moisture from the earth and create an atmosphere that will cause the contact points to slowly allow electrolysis to do it's damage even when the area seems dry.

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