Pipe Hangers

The true minimum length of any Hanger is governed by the Hardware selection.
This would include:
Element A - the top attachment element (there are many choices)
Element B - the intermediate element (normally plain or fully threaded rod)
Element C - the lower pipe attachment element (there are many choices)

If the Element C is an under pipe roller support then the pipe will allow required movement during the temperature swings. (example: ardware&biw=1024&bih=609&tbm=isch&tbo=u&source=univ&sa=X&ved=0CFUQsARqFQoTCLfg2Oe678cCFYgdHgodl2UAxA#imgrc=2kIeYDHpwx431M%3A

Go here for other examples:
www.google.com/search?q=pipe+hanger+hard...rc=qRlTkbPDY7Mr-M%3A