How the S-plan and Y-plan layouts route heating and hot water, and how they differ.
On a system boiler with a hot water cylinder, two control layouts come up again and again, and CENWAT expects you to tell them apart: the S-plan and the Y-plan.
The S-plan uses two separate two-port motorised valves, one for the heating circuit and one for the hot water circuit. Each valve is opened by its own control, so the room thermostat drives the heating valve and the cylinder thermostat drives the hot water valve, and the valve’s auxiliary contact provides the firing demand after it opens. Where the boiler controls pump overrun, the pump uses the specified boiler connection.
The Y-plan uses a single three-port mid-position valve instead. That one valve can send the boiler output to heating, to hot water, or to both at once in its mid position, sharing the boiler between the two demands.
Separate S-plan valves let each demand and actuator be checked individually. A Y-plan shares one actuator, so check each combination of heating and hot-water demand against its wiring table. These are control valves: a valve that has been told to close is not a safe isolation for servicing.
Both layouts are designed to give independent timing and temperature control of heating and hot water, and both rely on correct wiring so the boiler interlock works, so the boiler stops firing once the heating and hot water controls are satisfied. Pump overrun may continue after the burner stops, and protective functions must stay available.
For revision, picture the valves: two two-port valves means S-plan; one three-port mid-position valve means Y-plan. For any real wiring, follow the specific manufacturer wiring diagrams for the controls and boiler.