Best Of

if the relief is dripping it is probably an issue with the expansion tank although the relief valve should be replaced every few years anyhow.

if the expansion tank is waterlogged and causing the relief valve to blow off when the system is hot that will bring fresh water in to the system and cause corrosion.

Not as much of an issue with a boiler, but code here for potable is to use a brass transition fitting between copper and iron pipe. Wouldn't hurt in any event.

Water heaters always need dielectric unions.

It was either a bad solder joint or some issue with the water quality in the system. If it has glycol in it, that needs to be maintained or it will become corrosive. that is why @pecmsg asked. Occasionally the potable water that was used to fill the system is very aggressive. If there are small leaks and a lot of fresh water being added to the system, that will cause corrosion. In a closed hot water heating system with reasonable quality fluid connecting copper and steel directly isn't an issue.

i think that is just the shadow of the mc

Experience has proven to me that a brass fitting or valve between copper and black pipe is a great transition method. It causes the least amount of issues.

@bassman92 You can also use a dielectric nipple as a transition fitting.