How does a toilet work?

A white standard toilet, set in a monochrome bathroom with dark units

When it comes to fittings and fixtures in our homes, the humble loo is probably the most taken for granted. It’s just there and does its job – what more do we need?

‘How does a toilet work?’ is admittedly not a question I’ve asked myself particularly often. I mean, realistically, how much time do you spend thinking about your loo? I don’t lose a lot of sleep over it.

It’s just… there, for a lot of us.

But maybe that’s precisely why we should take a bit of a closer look. Yes, we’re going there. And we’re looking a bit towards the future, too.

A white Tomas Crapper toilet with a black low-level cistern
Traditional low-level toilet set, £1473 (excluding seat), Thomas Crapper.

How does a toilet work?

Seems easy, right? Water rushes in from the top, flushing the contents of the bowl away. Simple as that.

But, as it turns out, there’s more to it… and yes, it involves physics. But don’t fret, I’m not going to crack out the maths.

There are two main designs: siphonic (which is more common in the US) – and wash down loos. Both have their pros and cons and work in pretty different ways.

What does siphonic mean?

The answer is rather straightforward: because gravity plays a key part in making your toilet work.

A close-coupled back-to-wall WC against a brown herringbone-pattern tiled wall
Burlington Riviera close coupled back-to-wall toilet with soft-close seat, £432.99, Drench.

At the centre of it all sits the U-shaped pipe at the bottom. The actual name for this is siphon, which comes from Ancient Greek and literally means pipe or tube. It helps water flow upwards without any extra pressure – all because of gravity. (We’ll come to why later.)

If your WC is the base level, the top bend of the siphon is above this. The waste pipe or sewer lies lower than your loo.

And that’s where physics come into play. Water can’t flow upwards. right? Well, if you have the right system it totally can. When you flush, water streams into the bowl which ‘starts’ the siphon. As water is expelled at speed, it flows over the high point of the siphon and down into the sewer, which causes gravitational pull.

This, in turn, is what makes your toilet work. The contents of the bowl are literally pulled into the sewer this way.

Clever, right?

A white wall-hung smart toilet set against a dark grey wall
Sensia Arena shower toilet, price on application, Grohe.

So how does a washdown toilet work differently?

This is the more common model in Europe – and they’re more compact, too. Unlike siphonic WCs, washdown models typically come with a dual flush that releases two different quantities of water depending on the button you press.

For comparison, siphonic loos need more water than the stronger flush, plus fitting them with a dual-flush is not advised.

But back to the question at hand.

A washdown toilet works by using the weight and gravitational pull of the water to empty the bowl. Sounds similar to what happened before, right? It’s a little simpler, though. Unlike with a siphonic look, you won’t see the water rise – it just goes straight into the trapway. There’s no going uphill first, it’s all straight down into the drain.

How can I spot the difference?

Apart from the shape, the easiest way is to tell by looking at the water level. If it rises as you flush, only to quickly lower, you have a siphonic toilet. If it stays pretty much the same, you have a washdown model.

How will our toilets work in the future?

While the general principle is unlikely to change, we’ll probably see a lot of new features crop up. In fact, many of these are already out there.

I am, of course, talking about shower toilets. Some look like they belong in space, but they’re also equipped with a host of clever features – not only washing and drying (no more need for loo roll!), but also auto-opening and specialist self-cleaning functions.

A white wall-hung shower toilet against a cream wall
Toto Neorest AC washlet set, £12,504, CP Hart.

Some even feature a night light, so you don’t have to switch on the big light in the wee hours. Warmed seats are a popular feature, too.

And with the Internet of Things progressing more and more, some companies – such as Grohe – even allow you to save your preferred settings in an app. This pairs with every one of their smart toilets, so whether you’re at home in London or on the other side of the world in Osaka or Sydney, you’ll always get the same wash.

In short: our toilets will work a lot smarter.

Featured image: / BongkarnThanyakij

So, did you enjoy learning about how toilets work? Then read about how your dishwasher works.

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