dielectric union useage

I bought a couple of dielectric unions to connect some glavanized pipe to copper in my heating system. Are these also good for connecting black pipe to copper? This got me thinking that when I installed my water heater I should have used one to connect the heat trap (I think that's what the part is called) to the copper pipe? I just used a threaded female copper connection, but I guess that puts two dissimilar metals in touch with each other.
Ian
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I had a plumber install a new Hot Water heater some years back and he gave me the big speal about how these dielectric unions would extend the life of the hot water tank for several years. I went for the story and told him to put them in. Later talking to a plumber (friend of the family) I asked him about the dielectric unions on the hot water tank and he said I just wasted my money. The hot water heater didn't last more than 8 years. I installed the last one and didn't use the dielectric unions.
That's all I know about the things. Des

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