On Sat, 13 Mar 2010 21:35:41 GMT, (Doug Miller) wrote:

In article , " wrote:
On Sat, 13 Mar 2010 13:24:33 GMT, (Doug Miller) wrote:

In article ,
" wrote:
On Sat, 13 Mar 2010 02:33:59 GMT, (Doug Miller) wrote:

In article ,
" wrote:
On Fri, 12 Mar 2010 11:03:15 GMT, (Doug Miller) wrote:

In article ,
" wrote:
On Fri, 12 Mar 2010 03:34:44 GMT, (Doug Miller)
wrote:

In article ,
" wrote:
On Thu, 11 Mar 2010 23:27:52 GMT, (Doug Miller)
wrote:

In article , "chaniarts"
wrote:
Doug Miller wrote:
In article
,
harry wrote:

The only benifit of cast iron is it doesn't rust much.

As compared to copper or aluminum, which ... don't rust at all,
actually.

huh? rust is the metal changing into an oxide. both copper and
aluminium
rust, forming copper oxide (green) and aluminim oxide (white). they
just
don't rust orange like iron does.

Wrong. Rust is, by definition, hydrated iron oxide. Copper and
aluminum
*oxidize*, but only iron can rust.

Oxides of copper and aluminum are not porous, either (oxidation stops
quickly). Rust is.

Oxidation of iron stops pretty quickly, too, as long as it stays dry.

We are talking about hydronic and steam heat systems.

Yes, and there -- in closed systems, at least -- oxidation stops pretty
quickly, too, as soon as the oxygen is used up.

Except that the systems aren't sealed. The oxygen is never "used up".

Most are, in fact, closed systems.

Wrong. Most hydronic systems have automatic fill valves, mostly because they
need them.

Keeping them open all the time is the way things used to be done. Accepted
best practice now is to keep them closed except when it's actually necessary
to add water to the system -- which is fairly rare, in a well-maintained
system. If you need to keep adding water to a hydronic system, you have a leak
somewhere.

The leak would show. Nope. Fill valves need to be left open to insure the
proper pressure on the system.

That simply isn't true. Once the system is filled and pressurized, it will
remain at that pressure when the fill valve is closed -- unless there's a
leak.

Automatic bleeders.