Are dielectric unions required on the boiler/domestic connections?
It does not say they are required in the installation manual. My installing contractor did not use any, but he is not a contractor on whom I would rely.
The storage (inner) tank is stainless steel.
The outer tank, to which the boiler is connected is a more ordinary steel.
The piping coming out of the boiler is one-inch shiny steel (not stainless), and the contractor used 1 1/4 inch black colored (not the black pipe used for gas) steel pipe up to near the closely spaced Ts, and 1 1/4 inch copper for the rest of the near boiler piping and smaller sized copper for the rest of the secondary loops. Also 1-inch copper to the indirect. I have a Plus 40 tank too.It has survived 4 years use and is not leaking. That is not long enough to prove anything, of course, but that is all the data I have.
Here is a link to the installation manual; I cannot find it on the W-M site anymore.