Thoughts on series piped panel radiators with diverter valves

In series you get temperature drop, so take that into account when sizing the end of the line radiator. Generally 3 radiators in series, but it depends on the size/ btu load.

With a fixed speed circulator, As I see it when one radiator turns off flow, the pressure drop from that radiator is gone. But additional flow is pushed through the H bypass so pressure drop goes up. It may be a wash.

A delta P circ, depending on which P mode, constant pressure for example, may behave differently. Let me think it through so more in regards to the pump type.

A 400-square-foot apartment is likely getting heat from a larger boiler that serves the rest of the building. I’m guessing that cast iron radiators are typical throughout the other apartments. How each apartment receives heat from the main system will determine the best way to reconnect your new radiators.

Is this a condo, or is it a rental property where a management company (or individual) owns the entire building? That may also determine what you are allowed to do when connecting to a common heating source.

Assuming you have the proper permission, how does your apartment receive heat from the central boiler?

• Individual supply and return piping to each radiator, with those pipes hidden from view. This was most common from the 1940s through the 1960s.
• A single supply and a single return from the main boiler system to your apartment, with each radiator branching off that supply and return (like Diagram B in @hot_rod ’s diagram above). This might be what you have, based on your statement: “The current piping is in series, into the first one, then on to the next one, etc.”
• A true series loop: a single supply goes to the first radiator, then from its return to the second radiator, then from the second to the next, and so on until it reaches the single return the the main boiler system. This is generally a poor design because the water loses significant temperature at each step. The last radiator in the loop would receive very little heat. This type of layout is better suited for copper tube with aluminum fin baseboard systems. This scenario sounds like what you described, but as mentioned, it’s unlikely to work properly with cast iron radiators.

Depending on the pressure drop between the supply and the return from the main boiler system that feeds your apartment, you may need to add a small circulator pump to your new radiator system zone.