What is correct way for radiator plumbing/connection in multi-story flat
The pipes should look like this:



There should be an unrestricted flow around the system.

Each radiator should be connected to both the flow (red) pipe and return (blue) pipe. The turnable value (or better still thermostatic valve) should be on the input to each radiator. There should be a simple on/off valve on the outlet which is normally set to fully open.