manifolds vs Trunk-and-Branch on 100-Year-Old Brooklyn Hydronic System

Hello all,

I’m working on a 100-year-old, two-family hydronic hot-water system (not steam) in Brooklyn. The house is about 1,800 sq ft, with 10′ ceilings on the living floors and a basement just under 8′, which will be finished as living space.

Originally, the system had large cast-iron gravity mains, which were removed years ago to gain basement headroom. About three years ago, during a partial update, the distribution was converted to a ½″ Viega PEX manifolds system with manifolds.

Since that change, I’ve been dealing with:

• Lukewarm radiators
• Uneven heat distribution
• Ongoing balancing issues

The basement is currently open, and before the ceiling is closed, I want to correct the piping properly.

Current system

• 10 cast-iron radiators
• Two pipes at each radiator (½″ supply + ½″ return)
• Single zone / single thermostat
• Boiler: New Yorker CG50DNI-G2 (94,000 BTU)
• Circulator: Grundfos UPS 15-58

Proposed change

Since the radiators are already two-pipe, I’m considering removing the monoflow/diverter tees entirely and repiping as a true two-pipe trunk-and-branch system:

• 1″ supply main
• 1″ return main
• ¾″ branches to each radiator (currently ½″)
• Radiator-level control using TRVs or manual supply valves, with lockshield valves on the returns
• Ability to shut off individual radiators

Both mains would be run along the floor joists.

Questions

From a hydronic design standpoint:

1. Is there any real advantage to keeping a monoflow/diverter-tee system here, given that the radiators are already two-pipe and the mains are being rebuilt?
2. Does a trunk-and-branch layout make more sense for cast-iron radiators on a single-zone system?
3. Any guidance on where to originate the 1″ supply and return mains once the monoflow piping is removed?
4. Any layout or sizing concerns you see with this approach?

I’ll be doing the work myself, but it will be reviewed before final tie-in and before the ceiling is closed.

I appreciate any guidance, I want to do this once and do it right.

Thank you.