Hi Scott. The plastic stuff is Polybutene-I (PB) and it's great to work with, here is a URL
www.dux.co.nz/cms_display.php?sn=35&st=1&pg=756 Our regulations insist that we use copper two meters either side of the hot water cylinder, but otherwise we can use this. While I'm at it, I tend to improve the flow and presure with better design and less corners.