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Flush toilet

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This is an old revision of this page, as edited by Glogger (talk | contribs) at 16:50, 11 July 2004 (Corrected/revised flushometer entry, tank flush, and added autoflush). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.
Flush toilet

A flush toilet or water closet is a toilet that disposes of the waste products by using water to sweep them away down a drainpipe. The water is also used as a hygenic barrier between the drainpipe and the user.

It is widely held that Alexander Cummings was the true inventor of the modern flush toilet. Some legends misattribute the invention to Thomas Crapper. And yet, the ancient Cretans already had perfectly functional flushing toilets circa 2000 B.C.

The bowl siphon

The bowl of a flush toilet is a porcelain vessel with a built-in siphon, usually visible as a curved pipe protruding from the back. Normally, the bowl contains a small amount of water which is enough to form an air trap inside the siphon pipe, preventing foul air escaping from the sewer. When the toilet is used, liquid flows slowly through the siphon pipe as waste matter is added, but the flow volume is too small to fill the siphon. To flush the toilet, the user activates a flushing mechanism (see below) which pours a large quantity of water quickly into the bowl. This creates a flow large enough to fill the siphon tube, causing the bowl to empty rapidly due to the weight of liquid in the tube. The flow stops when the liquid level in the bowl drops below the first bend of the siphon, allowing air to enter which breaks the column of liquid.

Flushing mechanisms

The bowl siphon described above is triggered by a large flow of water into the bowl, which is provided by the flushing mechanism. This is usually of one of the following designs:

  • Direct flush (flushometer). The user presses a button, which opens a valve allowing mains-pressure water to flow into the bowl. The valve contains a pneumatic mechanism that closes it after a preset time. This system requires no storage tank. This system requires high volume water, thus a 3/4 inch pipe at minumum, and preferably a 1 inch pipe, but the high volume is used only for a short duration. Direct valves are regulated by a device called a "flushometer" that meters out a certain controlled amount of water per flush. Direct flush

makes the most efficient use of water, because it uses the water at full pressure and full volume. The ability of water to perform the work in removing waste matter from the toilet bowl is given by pressure times volume. Typical pressure in an urban commercial building where flushometers are usually used is around 60 pounds per square inch which is enough pressure to raise the water 137 feet above the toilet bowl. Thus, in some sense, the effectiveness of Direct flush is like having the tank 137 feet above the bowl (lots of "flush energy").

  • Tank-type. A storage tank (similar to that in the photograph above) collects between 6 and 9 litres of water. The storage tank is kept full by a float valve or ball cock. An outlet in the bottom of the tank is covered by a buoyant plastic cover (the flush valve) which is held in place by water pressure. To flush the toilet, the user pushes a lever, which lifts the flush valve from the outlet. The valve then floats clear of the outlet, allowing the tank to empty quickly into the bowl. As the water level drops, the floating flush valve descends back to the bottom of the tank and covers the outlet pipe again. This system is common in the USA.

Tank type toilets waste the energy in the water by converting the potential energy of 137 feet (the "head" of pressure in typical city watermains) times acceleration due to gravity, into something like only 18 inches or so above the bowl. Some older style toilets mitigate this waste of energy to some degree by having the tank be as high as possible (up near the bathroom ceiling), but modern tank type toilets waste the energy in the water, and therefore either use more water than necessary for a given flushing job, or for the same amount of water, flush less effectively than Direct flush.

  • Siphon. This system, common in the UK, uses a storage tank similar to that used in the flush valve system above. The user pushes a lever or button, forcing the water into a siphon (not to be confused with the bowl siphon) which empties the entire tank into the bowl. The advantage of a siphon over the flush valve is that is has no sealing washers that can wear out and cause leaks, so it is favoured in places where there is a need to conserve water. Older installations used a high-level cistern, fitted above head height, that was operated by pulling a chain hanging down from a lever attached to the cistern. Modern versions have a low-level cistern with a lever that the user can reach directly, or a 'close-coupled' cistern that is even lower down and integrated with the bowl.
  • Automatic flush. This system is usually implemented together with Direct flush. In retrofit installations a flushometer can be replaced with a self-contained battery operated machine vision system that actuates a solenoid when a user departs.

[[image:Plumbingsurveillancelightvectorpainting.jpg|thumb|200px|right|Retrofit Direct Flush installation in which the flushometer has been replaced with a sensor-operated system that automatically flushes the fixture when a user departs. The system uses machine vision to track a user approaching the fixture, then it waits until the user departs. A solenoid is used to actuate the flush from a 6 volt battery inside the unit that also powers the vision system.]

If the flush mechanism should fail in any of these systems, the bowl siphon can be activated by emptying a large bucket of water into the bowl. A domestic hose pipe will not work, as it cannot supply water fast enough to fill the siphon tube.

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