If you really want the correct answer go to the Wall.
http://forums.invision.net/index.cfm?CFApp=2

IMO, copper has no place in a steam system. The constant expansion (when
the BOILER, not furnace is steaming) and contraction (when the boiler is
cooling off) can cause early failure of the copper. Early in this case can
be 5-10 years.

I do agree with an other poster that 6" pipe isn't easily threaded, but
that's no excuse not to do it right.


"Ray" wrote in message
news:ArOWg.3664$9Y1.3389@trndny03...
Recently we installed in our six-unit apartment building a new furnace.
The
installer was a reputable local company. The old header pipes -- those
6-inch pipes that come directly out of the boiler -- were replaced with
new
copper pipes.

A local guy who is something of a busibody and claims to be an expert on
all
things says this was a mistake. He said the copper pipes will begin
leaking
steam within 2 years and will have to be replaced with steel.

It makes no sense to me that a company with a good reputation would
install
a furnace that would fail in 2 years.

Still, I'm uneasy enough to ask if there's any remote validity to the
busibody's prediction. I guess the question is whether the industry
standard
is copper or steel.

Any comments welcome.