Use a product called "radsnaps" to protect exposed pex from sunlight. DO NOT put copper anywhere near those steel panel rads or cast rads if you don't want them to corrode prematurely. I don't like copper anywhere in a system hosting steel/iron rads but that's me. Sure, Joe Schmoe will chime in saying he ran copper 50 years ago in his best friends bosses sons place and no problems since or "I'm the best plumber in Maine and only use copper and people beg me to plumb their homes" BS. Don't do it. PEX-AL is fine just check each fitting & make sure the isolation gasket is there. Its there to protect the brass just as much as it protects the steel/iron. TRV's should thread onto the upper knob after removing the cover which doubles as a manual knob, the top opposite side should have a "coin" bleed valve. Closed systems only for steel rads, no pools/hot tubs and pH balance & test your water yearly. Use auto air vents at all high points. Fix leaks to reduce make-up, its best practice anyhow and those steel units will last forever.
Edit: Siegenthaler WAAAY overthinks things. As long as you somewhat balance the lengths of your runs, you're fine - the TRV does the fine tuning. I ran 3 loops with 2, 3 run manis on each for 18 rads total. The run length on each isn`t perfect but its close. All rads spank out the heat until the TRV closes. As long as your pump is sized for the task, you`re set. I use the DHW and bathroom towel warmers as non-TRV`d, persistent loads so I don`t need a DP valve. Anything else, just ask. Rads are still new science in NA, last thing you want to do is take advise from some clown who knocks out a fin tube instal each day. I ran into plenty when installing my setup. They just don`t get it. I also used cast rads from the UK and had to ream out the 15mm TRV`s to 5é8 on a lathe. If you need to use rigid tubing like I did, use stainless. I`ll take pics if you want. Good luck.
