The initial material cost differences between copper and plastics are significant. A article comparing costs for different plumbing pipes can be found here: http//:www.southce.org/ajwhelton. A US government funded study to investigate what chemicals leach out of American plastic plumbing pipe is also underway. Go to that same web page and type in 'plastic pipe'. There has been quite a bit of work done in Europe but American materials have not been equally publicly scrutinized. Some of the study data has already been released and testing included actual buildings and testing of new pipes purchased at local building supply stores. Here is a link that might be useful: Informational research website

I didn't want to bog down my post with too much information, but to clarify, I added a 40 Imp. gal Giant brand (made in Quebec) electric water heater in series to the 75 US gal Marathon water heater I spec'd for our house, completed last year. The 105 gal Marathon I initially calculated would be enough for us is not available in Canada. The 75 US gal Marathon is mostly adequate, but the Mrs. wanted a greater reserve capacity, so I am experimenting with this set up. We have 43F incoming water temps in winter. In a few years I want to finish the mechanical room, and then I will buy a second Marathon tank. We have very hard water, 36 (thirty six) grains, with a 70,000 grain softener. I want Marathon tanks due to their polybutylene/fibreglass construction, so no need for a rapidly consumed anode rod, nothing to rust. I know this steel tank won't last long with my heavily softened water, so I just bought a standard tank to see if this combo makes the Mrs. happy. So yes, I placed 3/4 PEX onto a PEX/FIP adapter directly onto the hot water nipple of the Marathon tank. The 18 inch copper stub you speak of is needed for gas water heaters, because of the hot flue right next to the pipes. My Marathon set up from the plumber had 2 BrassCraft flexible copper lines from the nipples to 3/4 PEX that runs throughout the house. We have a recirc line, 3/4 PEX not 1/2 for some reason piped to a Grundfos recirc pump that is plumbed into the drain port of the Marathon tank as per Marathon's recommendations. I removed one flexible copper line to connect the hot Marathon port to the cold Giant port. I thought of installing copper stubs with unions on the Marathon tank, but I had left over PEX so I used that instead. So my number 3 question is can I reuse the flexible copper water heater connector if I choose, or is the crushed copper ring next to the rubber washer a one time only item? If so, are these copper rings available separately? Thanks.

If the copper is 8-10" long just put a wet rag or towel on half of the pipe plus the PEX fitting and solder away. If the rag is not dripping on the floor it is not wet enough, use a pan to catch the drips.

Cash Acme, based in Alabama, offers SharkBite Push-Fittings that would be ideal for your application. SharkBite fittings can be connected to cPVC, PEX, and Copper in any combination with no tools. It is certified for underground and behind-the-wall applications. You can pressure test immediately with no wait time. Also it is certified up to 200F and 200psi. Here is a link that might be useful: SharkBite Product Info