In the context of the present invention the term dual flush is taken to mean a cistern that can dispense two different volumes of water with the volumes being dispensed being determined by the user by the method of operation. The two different volumes are referred to as a short flush volume and a long flush volume.

The internationally recommended short flush and long flush volumes are nominally 3 litres and 6 litres respectively.

Siphonic discharge cisterns work on the principle that once discharge of water has been commenced by operation of a liftable plunger, water continues to flow until the siphon is interrupted. This usually occurs when the water level falls to such a level that air is allowed to enter the system.

Various designs of dual flush cisterns have been proposed in the past.

Examples are disclosed in GB 2037842, GB 2041421, GB 2054687, GB 2063950 and GB 2091773. These utilise various complicated valves and valve actuating mechanisms.

The aim of the present invention is to provide an alternative construction which is simpler and more economical to manufacture, being an adaption of the currently used one-piece inverted bell siphon.

Accordingly the present invention provides a dual flush siphon discharge unit of the type comprising an inverted bell containing a liftable plunger, and a discharge passage branching from the bell, first and second siphon breaking passageways communicating with the interior of the bell or discharge passage on an outlet side of the plunger at different levels, and wherein when the plunger is actuated and held in a raised position, the first passageway is covered and the second passageway determines the flush duration.

In a preferred construction the plunger carries an extension or tongue that will cover said first passageway in the raised position of the plunger. Conveniently it is made of plastics. Preferably the extension is formed integrally with the plunger.

Alternatively it may be a separate piece connected to the plunger. Preferably the extension/tongue has a shape that corresponds to the shape of the wall in which the first passageway is located so as to cover the passageway sealingly. The flush duration determined by the second passageway is a long flush. The first passageway is disposed above the second passageway. The first passageway is preferably disposed in an outlet passage branching from the top of the inverted bell.

To further aid sealing of the first passageway in the raised position of the plunger it is preferred that the tongue is urged into contact over the passageway by resilient means. More preferably the resilient means comprises a plastics element.

More preferably still the resilient element takes the form of a longitudinally split cylinder which is fitted into the outlet passageway of the bell and which acts on the tongue. Preferably the resilient element is held in place by bonding at its upper end.

The tongue fits between the wall of the outlet passageway and the resilient element.

More preferably the resilient cylindrical element has a hole in it. Preferably the hole is substantially aligned with the first passageway. Preferably the end of the resilient cylindrical closest to the plunger is bevelled to form a guide for the tongue.

The above described construction has the advantage that it requires only one or two additional parts, all of which can be made of plastics, are economical to produce and require no maintenance.

The present invention will be described further by way of example only, with reference to the accompany drawings; in which:- Figure 1 is a cross sectional view of a siphonic discharge unit embodying the invention, Figure 2 is a perspective view of a plunger and tongue according to the invention; Figure 3 is a perspective view of lower end of a resilient element used in the invention; and Figure 4 is a cross section on A-A of Figure 1.

Referring to the drawings, a flush siphonic discharge unit comprises an inverted bell (1) containing a liftable plunger (3) having an actuating rod (5) extending through a hole in the bell and connected to an operating lever (not shown) by a linkage (not shown) causing the plunger to be raised on operation of the lever.

Raising of the plunger is against the resilient force of a spring (7). The bell communicates with an inverted U-shaped discharge passage (9) leading to an outlet (11). In use the siphonic discharge unit is accommodated in a toilet cistern shown generally as (11). A limb (13) of the discharge passage is generally circular in construction in the illustrated embodiment and opens into the top of the bell (1) and has a hole (first siphon breaking passageway) (15) in the wall thereof. The limb (13) also incorporates a resilient cylindrical element (17) described further hereinafter with reference to Figure 3. The plunger carries an upwardly extending tongue (19). The tongue is positioned to lie in close proximity to the wall of the bell and the discharge passage limb (13) extending therefrom.

Referring now to Figure 3, this shows in more detail the construction of the resilient cylindrical element (17) which has a longitudinal slit (18) and has a cross section (ie: in its relaxed state) which is greater than that of the limb (13) so as to exert resilient force on the tongue (19) when the tongue is inserted between the resilient element and the wall. Furthermore, the resilient cylindrical element has a bevelled lower edge (21) to help guide the tongue into position between it and the wall of the outlet passageway. The cylindrical element (17) has hole (23) which is disposed in alignment with the hole (15) in the outlet passageway.

When the plunger is raised the tongue covers the holes (15) and (23). The action of the resilient element (17) causes the holes (15) and (23) to be sealed, especially the hole (15) thereby preventing the admission of air and thereby keeping the siphonic action intact. Thus, when held in this position water continues to be drawn under siphonic action out of the cistern and down the outlet passage (11) until the water level falls to the level of a hole (second siphon breaking passageway) (21) in the bell whereby the siphonic action is broken. The hole (21) is disposed at a lower level than the hole (15). Thus a long flush duration is achieved.

On the other hand if the lever operating the plunger is released quickly after operation, the plunger will move downwardly under the action of the spring (7) and water will only continue to flow until the water level in the cistern falls below the hole (15) in the limb (13) of the outlet passageway and the hole (23) in the resilient element. Thus a short flush duration is achieved.

The tongue and the cylindrical element are made of plastics. A membrane carried by the plunger acts in the manner of a one-way valve allowing water to be lifted by the plunger and yet thereafter allowing the plunger to fall back under the spring action and allowing water to be drawn through the plunger if necessary.

By means of the present invention a dual flushing action is achieved in a simple manner.