So, how do dual flush toilets work? Before we look at their operation it will be useful to examine the function of a normal toilet.
There are several types of toilets, the siphon toilet fully described in the next section which was developed by Crapper in the 1800’s and the flap valve system. These two were the most popular gravity flushing toilets until the development of the dual flush toilet.
The siphon type used a siphon in the cistern to supply the water to the bowl. Another siphon in the actual bowl outlet was used to drag the contents from the bowl, through the S bend and into the soil pipe.
The flap valve system was relatively simple, the operation of the lever caused a valve in the bottom of the cistern to open, allowing water to flow down into the bowl.
Both types used a ball-cock system to maintain the water levels in the cisterns and a lever on the side of the cistern to operate the flush mechanism.
However, the British Government became concerned about excessive water losses from the cisterns of the nation’s toilets, and opted for the siphon toilet, relaxing the laws governing the water loss through the visible overflow pipe. The overflow was usually due to people jamming the water flap valve slightly ajar, (supposedly to freshen the loo) rather than to bad design fault.
Eventually, an Australian invented the dual flush toilet, a great water saver, and money (if you were metered) was made. This was particularly important to drought prone countries such as Australia, the USA and the Far East.
So, how do dual flush toilets work? Let’s examine the function of a normal siphon toilet in its two parts and compare their different operating criteria.
Cistern
- Ball-Cock & Water Shut Off Valve– this is a hollow ball on the end of an arm which floats on the surface of the water in the tank. It is connected to the water inlet valve and controls the flow of water into the cistern. As the water is used, the ball falls to the bottom opening the inlet valve admitting water to the cistern, then as the water reaches a predetermined level, the ball floats up again and the water shuts off the water inlet valve.
- Tank Siphon – this is an inverted U tube with a circular plunger plate within a larger chamber at one end, the other end being connected to the bowl for flushing. The plunger plate is perforated with slots and a thick flexible rubber gasket covers the slots. The plate and gasket are connected at their center to the flush operating lever on the side of the cistern and, when pushed down the plate is pulled up pushing water up the chamber and into siphon. This causes the siphoning effect which draws the rest of the water into the tank through the plunger plate slot and into the siphon, until the tank has emptied into the bowl. The plate then sinks to the bottom of the chamber.
Bowl
- Bowl Siphon – this is a siphon built into the outlet pipe at the bottom of the bowl just before the S bend. When the toilet is flushed the water enters the bowl through a series of holes around the rim. The water then flows into the siphon which causes a dragging effect. This effect drags the waste away with the water right over the S bend until the seal is broken by air. This maintains the level of water in the bowl and, prevents odors emitting from the down pipe into the bathroom.
As you can see there is no control over the amount of water used to flush the toilet, it is all or none, so a lot of water is wasted.
Now we will look at how dual flush toilets work and, compare their operation to the normal flushing toilets and discover how it is that they save water in flushing.