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Thread: Dielectric union vs brass adapters

  1. #1
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    Dielectric union vs brass adapters

    I have been seeing brass male propress adapters connected to black unions lately. Is this an acceptable practice now? I was always taught to use a dielectric union in this application. This piping is used for boiler hot water.

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  2. #2
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    i will bite , obviously no black pipe being used for the water. I have only seen dielectric unions on the water side.

    and as far as the gas line i believe you can use dissimilar metals.
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  3. #3
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    Not necessary in a closed loop

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  4. #4
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    Quote Originally Posted by Spitz View Post
    Not necessary in a closed loop

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    Dielectric unions are not necessary on a closed loop system? All of the commercial sites that I am on use them.
    Is this something specified by engineers or required by code?

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  5. #5
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    Lots of different opinions on this. I believe dialectic breaks between copper and steel ARE needed in a closed boiler/chiller system. I have been taught that if using brass fittings between the copper and steel to accomplish this, the brass should be at least 4" long.

    I don't like dielectric unions because they tend to be a weak link that will end up leaking. The brass propress adapter screwed into a black union might not provide a long enough break between the steel and the copper, but it's better than screwing copper right into the steel, and it's probably better than using a dielectric union that has a high possibility of leaking.

    If I were doing it, I would use the black union with a short brass nipple, going to a brass coupling, going to the brass propress male adapter.

    Or I might do the black union with the brass nipple going to a brass propress female adapter, but I try to avoid female adapters if I can help it because they seem to leak often too. The brass propress female adapters are better than normal copper female adapters though. I avoid copper female adapters at all costs whenever possible.

    Another solution would be to incorporate a brass ball valve as the dielectric break. For example, a black threaded union going to a short black nipple going to a threaded brass ball valve with a brass or copper male adapter screwed into it. I think that would be alright too.
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  6. #6
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    Quote Originally Posted by ammoniadog View Post
    Lots of different opinions on this. I believe dialectic breaks between copper and steel ARE needed in a closed boiler/chiller system. I have been taught that if using brass fittings between the copper and steel to accomplish this, the brass should be at least 4" long.

    I don't like dielectric unions because they tend to be a weak link that will end up leaking. The brass propress adapter screwed into a black union might not provide a long enough break between the steel and the copper, but it's better than screwing copper right into the steel, and it's probably better than using a dielectric union that has a high possibility of leaking.

    If I were doing it, I would use the black union with a short brass nipple, going to a brass coupling, going to the brass propress male adapter.

    Or I might do the black union with the brass nipple going to a brass propress female adapter, but I try to avoid female adapters if I can help it because they seem to leak often too. The brass propress female adapters are better than normal copper female adapters though. I avoid copper female adapters at all costs whenever possible.

    Another solution would be to incorporate a brass ball valve as the dielectric break. For example, a black threaded union going to a short black nipple going to a threaded brass ball valve with a brass or copper male adapter screwed into it. I think that would be alright too.
    Thanks for your info.

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  7. #7
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    I was taught to use dielectric nipples with a brass threaded union in heating water situations.

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  8. #8
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    Most all threaded fittings are taped and doped. It’s not designed to be a gasket but still allows some measure of separation from corrosion.

  9. #9
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    Quote Originally Posted by Calgaryhvac View Post
    Most all threaded fittings are taped and doped. It’s not designed to be a gasket but still allows some measure of separation from corrosion.
    Nice try, but not good enough. I do subscribe to your theory a little bit by using anti-sieze instead of teflon tape or pipe dope when installing probe-type low water cut-outs in boilers. My theory is that the anti-sieze is electrically conductive, which in my mind is what you would want with these types of probes.

    When you screw a copper male adapter into a steel fitting (or worse yet a copper female adapter onto a steel pipe), the tape/dope won't do much to prevent the electrolysis from happening if the conditions are right.
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