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Lifting the lid: The Cistern
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Standard British Cistern | Bell Syphon Cistern | Dual Flush Cisterns | Automatic Flushing Cisterns | Valve Type Cisterns | Pressure Chamber Cisterns


It is easy to assume that a lavatory cistern is just a box with a stopper at the bottom, and that pulling the chain or tweaking the lever simply pulls out the plug and lets out the water. Indeed, this design is used outside Britain, but in the UK it is more usual to find syphonic cisterns. This page looks at the workings of the two most common types of syphonic cistern and their variations, as well as cisterns used outside Britain.

1: The standard system.

The usual design of cistern was developed in the 1880s, and is often attributed to Thomas Crapper. Crapper certainly used and refined this design in his company's cisterns, but it is not known who actually invented the device.

As shown in Figure 1, the principal parts of the cistern visible from the outside are the cistern itself (A), the flush lever (B) and the flush pipe (C) although this latter is sometimes concealed in close-coupled WCs.

Inside the cistern are the syphon (D) and the syphon bell or dome (E) which are joined to the flush pipe. The dome is open at its base. Inside the dome is the perforated plunger plate (F) which is loosely covered by a flexible plate (G). The plunger plate is connected via a rod (H) and a hook (J) to the flush lever.

Before the cistern is activated, water is held in the cistern because it cannot rise up and over the syphon pipe, which extends at least 1.5 inches above the highest water line.

When the flush lever is pulled down outside the cistern, the flush lever rotates and lifts the plunger and plunger plates (Figure 2). The reaction of the static water above the plunger plates forces the flexible plate tightly over the holes in the perforated plate. With the water above the plunger now unable to pass through the plates as the plunger rises, it is forced up through the dome and into the syphon tube. Once over the top of the syphon tube, the water begins to fall under gravity down towards the flush pipe.

As water falls down the syphon tube, the pressure is reduced in the tube. Once the pressure in the tube falls below the air pressure above the water in the cistern, the water begins to be pushed by air pressure into the dome from the cistern. This causes the flexible plate to be pushed away from the perforated plate, allowing free passage for the water through the dome and syphon tube down the flush pipe (Figure 3).

Water continues to flow through the syphon until the water in the cistern reaches the level of the base of the syphon dome. At this point air rushes in to the syphon, breaking the flow of water and rapidly equalising the pressures in the syphon. The water ceases to flow.

Water flows into the cistern through a float-valve (or ballcock), omitted for clarity in figures 1 to 7 and 9 to 10, which consists of a float 4 to 6 inches in diameter attached to a metal lever nearly as long as the cistern. This is described in detail on a separate page.

2: The bell syphon cistern.

The bell syphon cistern is restricted to old, high-level lavatories. The distinctive feature of these cisterns is the 'T' shape with a bulge at the base. Bell type cisterns are exclusively made of cast iron due to their need for robustness. Plastic cisterns with the shape of bell syphon devices have ordinary syphonic mechanisms, but were designed to form an easy replacement to the unreliable bell type.

The bell syphon cistern, show in Figure 4, consists as before of cistern (A), flush lever (B) and flush pipe (C). However, in these cisterns the flush pipe is joined to a standpipe (D) inside the cistern, rather than a syphon tube. Over the standpipe sits a cast-iron bell-shaped dome (E), which gives the mechanism its name. This is hooked on to the flush lever.

As before, water is prevented from leaving the cistern by the outlet pipe projecting well above the water line.

When the chain is pulled (Figure 5), the flush lever rotates around a pivot on the cistern cover. The bell is raised by the hook on the lever. Water in the cistern moves into the lower compartment of the cistern as the thick bell is pulled away from the bottom. Some water is pulled up inside the bell by surface tension.

When the chain is released (Figure 6), the bell falls rapidly to the bottom of the tank. This causes water in the lower compartment to be displaced by the bell, and to be forced up inside the bell and over the top of the standpipe. This starts the syphon, which discharges the water in the cistern as described in (1) above, until air enters the bell, flushing ceases and the cistern refills.

Bell syphon cisterns, also known as 'pull-and-clank' cisterns due to the noise made by the falling bell, are notoriously temperamental. Corrosion of the iron leads to a build-up of rust in the lower compartment, which may lead to the gap beneath the bell being so reduced that water filling the cistern outpaces the flow of water out of the cistern, leading to permanent running. The one advantage of these cisterns was that they are easy to clean- the whole bell assembly can be removed by hand. However, the difficulties with these cisterns are such that these devices have almost entirely disappeared.

Variations: Dual-flush cisterns

Some lavatories offer the cunning, water-saving feature of dual-flush cisterns, offering full or half flushes. Pulling the chain or lever and then releasing it delivers a half flush, emptying half the cistern into the pan. Holding the lever down delivers the full contents of the cistern.

These cisterns are usually of the ordinary syphonic type, although some dual-flush WCs introduced after 2001 operate on a different system. The usual type works by allowing air into the syphon tube when the water in the cistern falls to the half-way point, breaking the syphonic action and stopping the flush (Figure 7B). To prevent this occurring when a full flush is required, the hole designed to allow air into the syphon is blocked by the plunger plate when the flush lever is held down (Figure 7A).

This type of lavatory isn't always marked as being 'Dual-flush'. So if ever a lavatory seems to flush rather too quickly and doesn't clear the pan properly, try holding down the handle when you flush- it may just do the trick!

Variations: Automatic (Urinal) Cisterns

Gents' urinals in Britain almost always have automatic cisterns, which flush at regular intervals. These cisterns work in the same way as ordinary syphonic cisterns, except that the syphon dome and plunger assembly are omitted, and the top of the syphon tube is below the top water line.

As water fills the cistern, either from a simple tap or via a flow control device which lets water in whenever it detects that the urinals are in use, the water level slowly approaches the top lip of the syphon tube. Once the top of the tube is reached, water pours over the top of the syphon lip and down the flushpipe. The small projection in the pipe (a) causes the falling water to pull air down the outlet, causing the air pressure in the pipe to drop. This starts the syphon and the cistern flushes.

Foreign Alternatives: Valve type cisterns

Outside Britain most lavatories have cisterns which operate on a simple valve system, an example of which is shown in Figure 9. The flush lever, A, (or knob) is connected by a chain, B, to a stopper, C. The stopper is held by the pressure of the water in the cistern against an opening in the flush valve apparatus, D, which also includes a vertical tube to form the overflow. Thus the opening is sealed, and water is retained in the tank.

When the flush lever is activated (Figure 10) the stopper is pulled from the opening, and water begins to flow down the flush pipe. The stopper is made of a light-weight material and floats, so water continues to flow until the cistern is empty and the stopper settles back against the aperture. The cistern refills.

Why did Britain evolve such a complex mechanism when this pattern seems so simple? Water economy: these valves can leak and can be kept permanently open, allowing precious gallons to be wasted.

Foreign Alternatives: Pressure Chamber Cisterns

A cunning device which offers a mains-pressure flush with only a small connexion to the mains is the pressure chamber cistern. The precise mechanism is complex, but Figure 11 shows a representation of the device and serves to indicate its function.

The cistern consists of a strong, closed tank (the pressure chamber, A) which is sealed except for a narrow inlet (B), fitted with a one-way valve (C), and the large-bore outlet controlled by a flush valve (D) operated by a flush lever (E). Water never totally fills the tank, the upper portion containing air (F). There is no ballcock in this type of cistern; entry of water is controlled by pressure alone.

As water flows into the tank through the valve, the air in the tank cannot escape, so is compressed. This pressure rises until the pressure in the tank matches that in the mains. At this point water ceases to flow into the tank. Should the mains pressure fall, water is prevented from flowing back out of the cistern by the one-way valve. Just as a finger placed over a dripping mains tap will eventually feel the full force of the mains build up behind it, so the tank will develop full mains pressure with a tiny connexion to the mains, eliminating the need for a large connexion that would be expensive and disruptive to water supply to other appliances.

When the flush lever is activated (Figure 12) water is forced out of the tank at mains pressure by the compressed air. This provides a powerful flush until either the lever is released, and water flows into the tank again, or the tank empties. The tank cannot refill until the flush valve is emptied, but will continue to let the small flow from the mains to pass straight through the tank.

These tanks may be fitted inside a conventional cistern to provide a 'water saving' flush or fitted as a distinct unit, particularly popular in France.

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