[Greenbuilding] Al Pex for Solar Heating System

[Sacie Lambertson]

When we built our house we plumbed it for a back-up solar water heating
system.  Unfortunately this was before I learned that PEX could not be
used.  Now I'm ready to complete the job (our propane bills have gone from
$350/yr to $750/yr) but I'm told that AL PEX should NOT be used either, that
it can't take the extreme heat of the glycol coming out of the solar panel.
I would appreciate hearing some opinions and best advice on this.  Copper is
the only recommended carrier this knowledgeable source told me (though he
had heard of using copper for the first six feet, with an alternative after
that, but didn't think this a good idea either).  I really don't want to use
copper if I can help it.

A follow up question: what is the best insulator for the copper/alternative
line which will have to be buried for 20 ft before it enters the house?  Yes
I know that copper must be contained if it is going to be buried in the
ground.
Thanks