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.
Lower inlet temperatures downstream must be allowed for when selecting radiator output. If every radiator is sized as though it receives the boiler’s flow temperature, later rooms may be underheated. Correct sizing and flow can compensate: cooler water does not make inadequate room heating inevitable.
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 simplify radiator sizing and balancing, but the pipe arrangement alone does not guarantee adequate flow or comfort. Microbore describes small-diameter pipework; a common two-pipe version supplies individual radiators from flow and return manifolds.
Follow the connections back to the mains. Both arrangements can have two connections at each radiator. In the branched one-pipe layout they rejoin the same continuous main loop; in the two-pipe layout they connect 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.