I moved to a house that is over a century old and noticed most of the
upper
floor water lines to be
newer copper but the lower main lines galvanized steel


Won't this lead to rapid deterioration of the lower water lines?


"I would also like to know general guidelines for avoiding galvanic
action on
piping and so forth
I need to have a water heater installed andwas told to use fittings
which
use a
mix of different metals in it's construction...
Why is this, and how does this negate galvanic action by merely being a

mixed metal fitting? "

This is a long and complex subject. If you do a google search for
galvanic and water heater you should find lots of threads. The idea of
the different metal couplings is that the two different metals are
seperated by an insulator, making it a dielectirc union. That way,
there is no current path between the different metals. However, there
is still a path for current through the water itself. And then there
is the issue of ground path bonding. Normally, all metal water pipes
are tied together within a house and tied to ground. Inserting a
dielectric coupling breaks that path. If you jumper over it, then you
defeat the purpose of the dielectric union.

I have heard mixed reports in practice as to whether using these unions
on a water heater makes things better or worse. And I think the answer
depends on a lot of things. Most water heaters don't have them and the
plumbing connections seem to last just fine. Mine doesn't. I replaced
the original gas water heater at about 12 yrs, which is pretty normal.
No evidence of corrosion at the fittings.