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How does a toilet work?

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Our modern toilets are an extremely clever solution to a problem that has plagued humanity since people started living in houses. There is a lot of sophisticated physics in them to make everything work the way it’s supposed to.


There are three main mechanisms that we want to take a closer look at:


–          The bowl

–          The flushing mechanism

–          The refill mechanism


The bowl

 The first amazing thing that strikes you when you look into the toilet is that there is always some water in the bowl. So it can’t just be a hole in the ground, because then all the water would run out. And if you look very closely, you can also see that the pipe that leaves the toilet leads back into the wall and not down.

Another “specialty” of the toilet can be tried in a small experiment: What happens if you pour a cup of water into the bowl? – Right, nothing at all. Even with 20 cups of water in a row, nothing happens. So the water level does not change. The water can never run over the top. But why?


The pipe that comes out of the back of the toilet first bends upwards. This means that a certain amount of water is always held in a well. Whenever you add water, some water can flow out of the back of the hollow, and so the bowl empties by itself.


But how does the rinsing work now?

You can try this out by pouring a whole bucket of water down the toilet. First the bowl fills and then everything is sucked out, and there is the typical flushing noise. This is due to the fact that enough water was now given in that the entire pipe (i.e. in front of and behind the kink) was filled with water. The water now flows downwards in an uninterrupted flow, creating a kind of suction that takes everything else in the bowl with it. This suction stops when air comes back into the pipe.


The flushing mechanism

Since you don’t want to bring a bucket of water with you every time you go to the toilet, there is a water tank at the back of the toilet (or sometimes in the wall behind it) that automatically flushes the water. Unfortunately, this cannot be done with a simple water connection, because the amount of water that runs out of a normal tap is not large enough to activate the flushing mechanism. The whole pipe has to be filled all at once. So there is the tank that pours a bucket of water into the bowl at the push of a button.


If you now operate the flushing lever, the plug connected to it is pulled upwards so that the water can flow through a connecting pipe into the bowl. With about 3 gallons, the whole thing only takes about 3 seconds, so it is fast enough to start the flushing effect. In most cases, it is also sufficient to let less water flow out of the cistern, then the suction effect does not last as long, which isn’t always necessary anyway. That is why modern toilets have the ability to stop the water so no precious resource is wasted.


The refill mechanism

Now, of course, all the water that was flushed down the pipe has to be refilled. Technically, this is done via the normal water pipe. There is a kind of tap in the cistern that slowly fills the tank again. That sounds easy, but it is by no means. If you look into a cistern, you will see that it looks pretty complicated inside. For example, the water tank must not overflow. The water supply must therefore be automatically stopped after a certain time. And the valve down to the bowl must also be closed again so that water does not continuously run through the toilet. And all of this works completely without electricity and computers – only with smart mechanics:


After the flush has been activated, the plug slowly moves back down and closes the tube to the bowl. If necessary, you can sometimes do this by hand.

At the same time as the pipe goes down, the faucet is also opened, which lets new water run into the tank. Since the plug is placed a little higher (like on a mountain), the new water cannot run out immediately.

The faucet is connected to a light plastic part, the float. It floats on the surface of the water in the tank and when a certain height is reached, it automatically closes the tap again. The tank is now full and the toilet ready to be flushed again.


Are you interested to learn how other things in your house work that you never thought about?

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