Most people say “use a dielectric union” we here at the Twin Plumbers stopped using those years ago because of the build up that is created in just a few years, especially on the hot side.
The best way to connect the two are to use a brass nipple in between, preferably 6″ long. We have found by installing the correct methods to dissimilar metals, that the water system last a very long time. This also goes for strapping the copper pipe as well. We see a lot where the copper pipes hanging in a sub garage are held by metal hangers, or copper pipes that are installed along a wall metal straps are used to, this also causes electrolysis, affecting other parts of the water system. When this occurs,the owners assume right away that the water system is failing and will all need to be replaced, where one simple low cost method of a installing a pad in between the two metals will stop it all together and save the water system.
So getting down to the brass tacks here, is that brass does not corrode,and should always be used in between dissimilar metals, this is a metal that is also great with gas appliances as well.
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