Replacing cast iron HW radiator distribution with PEX-Al-PEX
I am looking to change out all of the original cast iron pipe on the heat distribution system in a 100+ year old 3-story building that I am renovating in Easton, PA. The distribution system is on two NG boilers with feed pumps (1 for 1st floor; 1 for 2nd/3rd floors). The house was not properly winterized by the previous owners and several pipes and fittings burst. I saw a video on This Old House (you can view here) where they reconfigured a hot water radiator heating system with PEX-Al-PEX off of a manifold. If running the PEX-Al-PEX in this "home run" design, what size tubing would be required with dedicated supply/return to each radiator? I know there are practical requirements for flow and pressure, so I am wondering how small I can go on the ID. Can I get away with 1/2"? 5/8"? Or would I have to go all the way up to 3/4"? I would prefer a smaller diameter over a larger to reduce some of the costs, but not if it means installing an inadequate distribution system.