How one-pipe and two-pipe central heating circuits differ and why two-pipe is the usual modern choice.
Wet central heating can be piped in more than one way, and Level 2 expects you to tell a one-pipe layout from a two-pipe one and know why it matters.
In a one-pipe circuit, a single pipe forms a loop around the building and each radiator takes some hot water from the loop and returns cooler water to the same loop. Because the returned cooler water mixes back into the flow, the water feeding each successive radiator is a little cooler than the last.
The consequence is uneven heat: radiators near the start of a one-pipe loop run hotter, and those near the end run cooler, which makes balancing and sizing harder. One-pipe layouts are found in older systems.
In a two-pipe circuit, there are separate flow and return pipes, and each radiator draws hot water from the flow and sends its cooled water to the return. Every radiator therefore receives water at a similar temperature rather than progressively cooler water.
The result is more even heating across the system, easier balancing, and more predictable radiator sizing, which is why two-pipe is the usual arrangement in modern installations. Microbore is a two-pipe variation using small-bore pipes fed from manifolds.
For revision, follow the water: if cooled water rejoins one shared loop it is one-pipe; if every radiator has its own flow and return it is two-pipe. Confirm design specifics against the current guidance and the system design.