Compare the separate zone valves of S-plan with the shared mid-position valve of Y-plan.
Both layouts heat the same things — a hot water cylinder and a radiator circuit — from one boiler and one pump. The diagram compares how the valves route the water; the wiring differs too.
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.
How the valves actually fire the boiler
In a standard S-plan, a thermostat calling for heat opens its two-port valve, and the valve’s auxiliary switch then tells the boiler to fire. In a standard Y-plan, the cylinder thermostat can fire the boiler for hot water; for heating only, the mid-position valve’s internal switch does it. A Y-plan valve does not close both outlets when demand ends.
Interlock stops the boiler firing once the heating and hot water controls are satisfied. It does not need every valve shut or every supply cut. Keep the boiler’s pump overrun and protective functions working, and follow its wiring diagram for the controls you use.
Telling them apart on a fault
An S-plan valve that fails to open can leave its own zone cold while another still heats. A valve stuck open or an auxiliary contact stuck closed gives a different symptom, so identify the failure mode before condemning a part. On a Y-plan, check how the shared valve moves, and its electrical demands, for hot water only, heating only and both together. A symptom narrows down what to check; it does not tell you which part has failed.
Use the schematic to lock in the difference: S-plan means two valves working separately; Y-plan means one valve sharing the flow.