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.
Allow for the cooler water further along when you choose radiator outputs. If every radiator is sized as though it gets the boiler’s flow temperature, the later rooms may not get warm enough. Correct sizing and flow make up for it: cooler water does not have to mean cold rooms.
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.
Similar inlet temperatures make radiator sizing and balancing simpler, but the system still needs enough flow, and balancing, to keep the rooms warm. Microbore describes small-diameter pipework; a common two-pipe version supplies individual radiators from flow and return manifolds.
To tell them apart, follow the connections back to the mains: both layouts can have two connections at each radiator. In the branched one-pipe layout both rejoin the same continuous loop; in the two-pipe layout they go to separate supply and return mains. Closing a branch leaves the one-pipe main open. A simple series circuit with no bypass is different: shutting its only flow path stops circulation through that circuit.