One-pipe, two-pipe, and fully pumped S-plan and Y-plan arrangements.
Under C&G Unit 206, a domestic central heating system moves heat from the boiler to the emitters (radiators) and, usually, to a hot water cylinder. How the pipes are run and how the flow is controlled is what separates the layouts.
One-pipe vs two-pipe
- One-pipe system: the radiators sit on a single loop, and cooled water from each radiator drops back into the same pipe. That means the water feeding each successive radiator is a little cooler, so radiators further down the loop run progressively colder and are harder to balance. It is an older arrangement you should recognise rather than install.
- Two-pipe system: separate flow and return pipes. Each radiator is fed hot water from the flow and returns its cooled water to the return, so every radiator sees roughly the same flow temperature and the system is far easier to balance. This is the standard domestic arrangement (microbore is a two-pipe variant using small-bore pipes from a manifold).
Gravity, semi-pumped and fully pumped
Older systems used gravity circulation to the hot water cylinder and a pump only for the radiators (semi-pumped). Modern systems are fully pumped: the pump drives both the heating and the hot water, and motorised valves decide where that heat goes. Fully pumped is faster to respond, more controllable and required for a condensing boiler to work efficiently.
S-plan vs Y-plan
Both are fully pumped, and both must have a boiler interlock so the boiler and pump only run when there is a genuine demand (via the room and cylinder thermostats and the valve end-switches) rather than cycling for nothing.
- S-plan uses two separate 2-port zone valves — one for heating, one for hot water. Because each zone has its own valve and stat, heating and hot water are fully independent.
- Y-plan uses one 3-port mid-position valve that can send heat to the heating, the hot water, or both at once. It uses fewer parts but shares one valve between the two demands.
*Original revision guidance; confirm against Approved Document L and the standard domestic control schemes.*