Jan 20

A toilet cisternYou use them every day, several times a day in all probability, yet how often do you stop to consider how they work or, more significantly, why they are there?

The photo to the left shows a typical toilet cistern installed (not by us!) in a client's premises. This is a relatively new cistern installed in a very old building; if you look closely you can see the holes in the wall where the older and larger cistern was secured.

From a user's perspective the operation is straightforward; press the button, the toilet flushes and in a little while you can do it again. But ... what is actually going on inside? To find out, let's lift the lid ... literally!

The photo to the right shows the inside of the cistern. We'll go through it one part at a time:

  1. The cistern itself is simply a reservoir to hold the water.
  2. In the centre is the flushing control consisting of two large buttons (white & blue) that activate a half or full flush respectively. They work by lifting the plunger and releasing the water into the flush pipe. Note that the overflow also drops down the flush pipe on newer cisterns (older styles dump it on the floor).
  3. To the right is the filling controls which is a simple float switch that opens when the water drops and closes when the cistern fills up again. The white plug on the left hand side allows the filling controls to be moved over to suit the water pipe.

The components used in a cistern are simple and pretty robust. The most common problem that occurs is the cistern running continuously; this is usually the result of a perished seal in the flushing control but can sometimes be from a maladjusted filling valve that does not cut off before reaching the overflow level.

So that is the how, now to the why. Why do we use cisterns at all?

Myth buster: It isn't to get water pressure. The pressure in a normal water service is typically 5m (50kPa) or more, what you get from a cistern is the height of the flush pipe; about 0.2m (2kPa), so the water pressure in the pipe will easily flush the toilet. So why?

The purpose of a cistern is to provide a physical air break between the water supply, which is usually what we drink, and the sewer, which contains stuff we'd rather not drink. In actual fact a cistern provides 2 air breaks, the break between the pan and the cistern, and the break between the cistern overflow and the filling valve. It is the first one that matters and it is possible and permitted to flush urinals directly from the water main using a device that creates the air break and a solenoid valve and eliminate the cistern entirely.

Now you know!