PEX should never see the light of day. Even though it has UV inhibitors, UV exposure can compromise PEX over time. I always transition from PEX to copper when coming out of the wall. There are readily available fittings made just for this purpose, and at times I've even fabricated my own when needed from copper cut-offs. After the drywall is up, the PEX will not be seen. There will just be a copper stub sticking out of the wall. When doing the finish plumbing, the plumber needs to be aware that PEX is on the "other side of the wall" so he doesn't over do it with the torch and deform the PEX. I've never had a problem soldering to a PEX transition fitting, but the plumber should be made aware. Mongo

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.

I need to correct my previous post... The house in question is built on a slab so the house Main Water Shutoff Valve may be inside the house or it may be outside within 3' of the point where the line enters the footer wall. If the house Main Water Shutoff Valve is outside, and if the line to the hose bibb was connected on the house side of the Main water shutoff valve it is okay,, If it was connected to the house supply line before the house main water shutoff valve it is illegal.

Consider all the possibilities for new routes for the piping. The slab is a major obstacle. The first slab built houses in my area were by an inventive builder who used 6" PVC in the slab for supply piping chases with elbows coming to the surface of the slab. He used copper tubing at the time, but PEX would be easy to fish in those chases. Going up or around is about your only choice. For cold water piping, is going around the perimeter of the house in a trench helpful at all?