Had some work done on hot water system using black iron piping. Transition made to copper with dielectric unions. I see a problem in that a brass valve was placed on the black iron line (basically BI nipple brass valve, short BI nipple, then dielectric union) with the valve piggy backed between 2 Black iron nipples. I asked if the valve should be on the copper side of the union and was told that brass does not react to black iron nearly as bad as copper. I should have no long term problems. Is this the right way or should I have the valve place on the copper side?