Not to exceed 200 degrees. The fittings well suited for hot pipes is the more rigid stainless. In a closed space with possible chemical reactions, steel is not a good choice, copper is good but not as strong. This is the advice given to me by a plumber who I asked this same question last year.

Thanks for the tips so far. As far as fittings go, I want to go with the most reliable and the easiest. I'm already saving money by not going with all copper plumbing, so I feel I can splurge on the fittings. A little more background on my project: I'm building a full dormer on my existing brick ranch. All plumbing is currently copper. Upstairs will have 1 master bath and a 3/4 bath. Altogther there will be: 2 toliets 3 sink faucets 2 showers 1 whirlpool tub Given that I have to run alot of this perpindicular with my floor joists, PEX will make installation (hopefully) easier. I'm open to ideas about the best fittings to use for my application. Thanks, Shade

Bad analogy, but think of a sponge. When you look at a sponge, you can't see through it. To a certain extent it looks solid. But you know that the sponge is porous, that things can pass through it. Some things are too large to pass through the porous structure of the sponge. Some things are small enough to pass through the sponge. Same with PEX. PEX looks solid. For all intents and purposes, it IS a solid. It holds liquid water. But things can still pass through it. Oxygen is one of those things. Water has dissolved gases in it, oxygen being one of them. In something like a closed heating system with a cast iron boiler, oxygen can be a bad thing. Oxygen plus cast iron? Not a good combination. Iron plus oxygen? Rust. You don't want to rust out your cast iron boiler. In new installation, dissolved gasses in the system will eventually work their way out of solution and be purged from the system, especially if the water is heated. If you have PEX in this system, say with radiant floor heat? If your PEX does not have the O2 barrier, then atmospheric oxygen can pass through the wall of the PEX and enter your closed water system. The circulating water will then carry the oxygen to your cast iron boiler, potentially causing damage to your cast iron boiler. The oxygen barrier in the PEX prevents additional oxygen from getting into your closed system, so once the initial dissolved gasses in the water come out of solution and are purged after system start-up, you have "dead water" in your heating system. "Dead water" will not rust your cast iron boiler as there is no free oxygen in the water to react with the iron in the boiler. Hope that makes sense.

Pex has a slightly smaller diameter than copper, which is why there is this requirement. The valves are designed so that at maximum pressure, all of the water will flow to the tub spout. If pex is used, then it's possible that there will be slightly more pressure than the pex will be able to handle and that extra pressure could result in the extra water getting diverted up to the shower head. So the shower head may drip while you are filling the tub. That's the ramifications of your installation. It may not be an issue in your particular case since your specific pressure may not cause this effect, but it's in the instructions because it could possibly happen.