Cast Iron Radiator system repiping?

MTC

I haven't been around here in years - guess that's a good thing. I asked input about 6 years ago on a hydronic system in a property I had just bought, expecting to need to replace the boiler right away. Boiler has been going strong all this time with a few things (new gauge, expansion tank, etc).

I had repaired a minor leak years back in the piping, and have had one valve stem leak a little since then. Yesterday I discovered another leak in the transition from the old 2" main to the newer copper 1 1/4" lines off the boiler. Probably some dissimmilar metals corrosion... It managed to soak about 30 feet of pipe insulation before dripping in a very inconvenient place... anyway...

This building is an up/down duplex with single heating system. I was there because 2nd floor was complaining that its too hot in their unit. They lived in the unit last winter and said it was about 69-70, but now is getting about 72-74 degrees and they'd prefer cooler (just glad they told me rather than opening a window). I haven't changed anything in the last year, but checked for basic things - thermostat settings, open windows in first apartment making it run longer, air in those radiators making it run longer, etc, and don't see anything that I can see would cause the unbalance.

So given that I have had a few leaks now in the old piping, and that I have less than ideal balancing (though historically its been pretty decent), and have an old oversized boiler that will need replacing before long, a couple questions...

Should I repipe the whole basement? All mains are accessible in basement, risers to 2nd floor would be difficult to replace, but rest is fairly easy. If so, should I go with a pex manifold system with balancing valves? One step further, should I split the mains and put in 2 manifolds and feed each unit separately, giving the option of splitting the heating when the boiler goes?

House is about 1800 sq ft, has 4 CI radiators in smaller 2nd floor apartment, 1CI rad in common hallway (thinking I'd put this one with 2nd floor as mostly it heats the stairs going up to their door), 4 CI rads in first apartment, 1 baseboard fin rad in first bathroom, and a pair of baseboard fin rads in series in the basement under first floor bedroom/bath to keep water pipes from freezing. Figured I could put those on first floor as thats what they primarily serve. There is also a shared laundry room in basement, currently heated only by some uninsulated piping in the basement (3/4 lines to some rads, all larger mains are insulated). I did a heat load with SlantFin's app and got about 60k after the insulation etc that has been done (currently has ancient 100k output boiler). There is lots more info about the system in this old post if anyone is interested: https://forum.heatinghelp.com/discussion/146936/preparing-ahead-of-time-for-a-boiler-replacement#latest

So would it make sense to split the 1 1/4" copper pipe off the boiler to 2) 1" manifolds - a 5 port for 2nd floor/hall and a 6 port for first floor and the basement? Seems like most people recommend 1/2" pex or PAP for these, will that be enough for some of my giant radiators? Here's a picture of the largest one, on the first floor... Its about 23" tall, 66" long, and not sure on depth off hand, maybe 8"?



Any thoughts on if this system would even be worth bothering to split the units? My bills aren't that crazy, and the added cost for multiple manifolds, 2 boilers/accessories/etc, and getting second gas meter in the property might not really ever be worth it? Would also have to put in second water heater, repipe gas ranges, etc to complete the split...

Is the manifold system the way to go with a system like this? Should I just try to repair the pipes more and leave well enough alone? Just worried about ongoing leaking issues...

Thanks in advance, especially for enduring the book length post.