How an External Cistern Toilet Flush System Works (by Capitan Farloc)

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Introduction

Ever wondered how your toilet flush system actually works — especially the external tank type like the CATIS model? In this article, I’ll walk you through the basics in a simple and straightforward way. With the help of clear illustrations, we’ll explore the key components and how they all work together to flush properly. Understanding how it all functions will come in handy when things go wrong — helping you spot issues and maybe even fix them yourself!

The External Cistern Flush System ("CATIS")

The external cistern flush system, also known as a siphon flush, is commonly (though incorrectly) referred to as "CATIS," after the Italian company that invented and has been manufacturing it for over sixty years.
Its operating principle is quite simple, relying on three main components. If you look inside the cistern from above (as shown in the picture), you can easily identify them:
A) The flush siphon (highlighted in red)
B) The priming valve (highlighted in green)
C) The float valve (highlighted in blue)

The components of an external cassette flush

A) The Flush Siphon

The term "siphon" refers to an inverted U-shaped tube used to transfer liquid from one container to another at a lower level.
Thanks to the siphon effect, once the tube is filled, the liquid in the longer section begins to flow downward due to its weight, pulling along the liquid in the shorter section, which is naturally lighter.
The driving force behind this process is the difference in liquid levels between the two containers. The greater the difference, the stronger the force—resulting in a faster flow through the tube.

External cistern drain - The drain siphon

B) The Priming Valve

The siphon effect described earlier can only start once the tube is completely filled. That’s why, when you press the flush button, it compresses air inside a metal tube connected to a plunger (as shown in the upper part of the diagram). This action triggers the priming valve, opening it.
The priming valve consists of a semi-spherical rubber seal, and when it opens, it allows water to fill the lower section of the tube—activating the siphon effect.

External cassette flush - Opening the trigger valve

The siphon effect

Once you release the flush button, the rubber seal of the priming valve returns to its position. However, since the lower tube is now full of water, the siphon effect continues, pulling water from the cistern and draining it into the toilet bowl.

External cassette flush - Closing the trigger valve

End of flush cycle

The siphon effect continues until equilibrium is reached between the two containers. This can happen when both containers reach the same level or when the water level in the higher container does not drop below the inlet section of the siphon tube.
In our case, where the containers are at entirely different levels, the flush cycle ends when the water level drops below the siphon opening, allowing air to enter and causing the lower tube to empty.

External cassette flush - End of flush cycle

C) The Float Valve

The cistern fills up using a float valve, which consists of a hollow plastic sphere that floats on the water. This sphere is connected to a rod, allowing the valve to automatically shut off once the water reaches the desired level.
This component is commonly called the float valve, but in everyday language, it’s often simply referred to as the float.

The float tap

Operating Principle

The float valve works on a simple mechanism: the float is connected to a rubber seal via a metal rod. By leveraging the principle of a lever, where the pivot point is positioned very close to the seal, the system effectively closes off the inlet of the pipe that allows water to enter.

Float tap - Operating principle

Water remains blocked from entering as long as the level stays at the predetermined height.
When the flushing process begins, the float lowers along with the water level in the cistern. This movement opens the valve, allowing fresh water to flow in.

Float tap - opening the tap

The float valve rubber tube

The float valve is equipped with a rubber tube, which directs water to the lower part of the cistern.
This tube serves two purposes:

Reducing noise during the refill process.
Preventing water from splashing upwards and spilling over the edges of the cistern.

One important detail to keep in mind: when purchasing a new float valve, the rubber tube is often placed on the metal rod rather than its intended position. Before installing the valve, you should:

Unscrew the float from the rod.
Remove the rubber tube.
Attach the tube to the outlet pipe.
Reassemble the float onto the rod.

Skipping this step may cause water to spray uncontrollably inside the cistern during flushing.

Float valve - Function of the rubber hose

I hope, as always, that this article has been useful and interesting.

For now, I’ll say goodbye!
Luciano (Capitan Farloc)

Warning!

The advice and instructions provided on this site come from hobbyists rather than professional technicians, meaning there is no guarantee that they are accurate or represent the best solution.
Additionally, using the tools and techniques described here should be done with careful consideration of one’s own abilities.
These articles assume that the tasks will be performed by individuals with the necessary skills and knowledge to use the required tools safely, without risking harm to themselves, others, or property.
Therefore, the owners of this site cannot be held responsible for any direct or indirect damages resulting from the application of the content found here.
Before undertaking any DIY activity, it is strongly recommended to read the Safety Notice, which provide a basic (though non-exhaustive) list of precautions to follow.

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