A visual walk-through of why S-plan and Y-plan differ only in valve control.
Both layouts heat the same things — a hot water cylinder and a radiator circuit — from one boiler and one pump. The diagram makes the point that the only real difference is how the flow is controlled.
On the left, the S-plan uses two separate 2-port zone valves: one valve feeds the cylinder and the other feeds the radiators. Each valve is driven independently, so the cylinder thermostat and the room thermostat can call for heat on their own.
On the right, the Y-plan uses a single 3-port mid-position valve. It shares the pump between the two circuits and can send heat to hot water, to heating, or to both at once.
Use the schematic to lock in the difference: S-plan means two valves working separately; Y-plan means one valve sharing the flow.