Motorised valve controls: S-plan and Y-plan auxiliary switches

In an S-plan system the boiler fires only after a motorised valve has opened and its end switch has made. That is how valve position becomes part of the boiler interlock.

From call for heat to burner firing

Each circuit in an S-plan has its own two-port motorised zone valve. For one zone:

1. The programmer's time period is on and that zone's thermostat calls for heat: the room thermostat for heating, the cylinder thermostat for hot water.
2. In the spring-return valve shown here, a thermostat demand applies live to the motor and drives the port open. At full travel the motor stops; the valve stays open while that live remains. When the demand ends, the spring closes it.
3. When the valve is fully open, its auxiliary (end) switch makes a separate set of contacts, on the grey and orange wires.
4. The end-switch output on the orange lead gives the boiler a 230 V heat demand. In a basic S-plan it can also feed the pump; on a pump-overrun boiler the pump connection follows the boiler wiring diagram.
5. When the thermostat is satisfied it drops its demand, the valve closes and the end switch opens, so that zone no longer calls the boiler.

Why the end switch matters

The end contact changes state when the actuator reaches the valve's open stop. Its switched output lets the boiler receive a normal heat demand only after that travel. When every zone is satisfied the contacts reopen and that demand ends. This confirms valve travel, not water circulation: a stopped pump, blocked circuit or stuck valve mechanism still needs its own check.

The end switch is not a timer, and it is not the boiler's main electrical supply.

Y-plan

A Y-plan uses one three-port mid-position valve to route primary water to the radiators, the cylinder coil or both. In the usual V4073A circuit, a hot-water-only call leaves the valve at its spring-return cylinder position and the cylinder thermostat supplies the boiler demand directly. For heating only, the actuator drives to the heating port and its internal switch provides the demand. Trace the actual wiring diagram for each operating state; the S-plan end-switch sequence does not describe every Y-plan call.

The wiring centre

Use the wiring centre as a map of the system: the programmer and thermostats create demands, and the terminals join their wires to the valve and appliance leads. In an S-plan the valve end switch, reached only after the port opens, permits the call to reach the boiler.

The conventional terminal-block centre described here provides connections. Other control-centre designs can contain relays or electronics, so check the actual wiring diagram. The circuit is supplied through a fused spur; the boiler and its controls handle pump overrun, while valves and other controls determine which circuit receives heat.

Fault reasoning

A boiler keeps firing for heating with the programmer off and every thermostat satisfied:

1. Rule out legitimate functions first. Frost protection can fire the boiler in very cold weather with the programmer off, and combi preheat acts separately.
2. Then investigate why a firing demand remains, such as incorrect interlock wiring or a sticking contact. The symptom does not tell you which wire or part is at fault.
3. Do not confuse a heat-demand signal with the permanent supply the boiler may correctly need for its electronics and pump overrun.

Follow the manufacturer wiring diagrams for the actual controls and boiler.