110 year-old house, 13 solid water radiators, 2" and larger black pipe, head-high in the basement. I'm going to replace my old boiler with a high-efficiency condensing model; tear out all the horizontal black pipe and run 1/2" pex for the radiators, which I've been told is sufficient for the system. But if I'm going to zone 2 or 3 radiators with a single feed, what size will I need to be coming from the manifold? 3/4"?
Thanks in advance, they'll be plenty more questions a'coming!