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The various documents (manuals, installation guides, etc) available for gas water heaters refer to the use of what's called a "ground joint union" between the shutoff valve and the appliance tee/drip leg. How is such a thing different from any other union out there, or is this just a fancy name for a plain ol' union? Furthermore, why are ground joint unions called out for gas service?

(BTW: for an example, see Figure 4 on Page 6 of this manual.)

  • Googling "ground joint union vs union" seems to indicate that ground joint unions are designed to for sure carry ground across the union. Whereas a standard union may or may not facilitate a ground connection... New interesting term for me, I'll be watching answers. – Tyson Oct 31 '16 at 1:41
  • @Tyson I think that's the difference between a union, and a dielectric union. – Tester101 Oct 31 '16 at 2:13
  • @Tester101 and the terms become even more confusing.... dielectric vs ground joint union.. how is that different? – Tyson Oct 31 '16 at 2:18
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    @Tyson a dielectric union has an insulating gasket and sleeve, to prevent galvanic reactions. A ground joint union does not use an insulating gasket or sleeve, and so does not do anything to isolate different metals. You can get ground joint dielectric (insulating) unions, like the one in my answer. At any rate ground joint simply refers to the shape of the mating surface of the union, not its ability to work as an electrical ground. – Tester101 Oct 31 '16 at 10:19
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A ground joint union has an angle cut (ground) into the union where the two sections mate. This provides more surface area at the joint, and allows the joint to seat more easily.

ground joint union

This is in comparison to a flat faced union, which does not have an angle ground at the mating surface.

flat faced union

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    So "ground" as in "machined"... – keshlam Oct 31 '16 at 2:53
  • More specifically (if I'm reading correctly) it relies on the ground surfaces to make the seal, rather than an o-ring which can deteriorate over time and leak. – Mark Oct 31 '16 at 10:21

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