# Explain to me the physics of toilets

Suddenly today, out of nowhere, I found myself mystified by how a toilet works. I feel like I have a pretty good grasp of what’s happening in the tank, in other words, the mechanical part of the toilet. There’s a valve to let water in, and a float-operated shutoff to cut the water when it gets to a certain level. When you push down on the handle, the attached chain lifts the gaskety thing in the bottom of the tank, allowing the water to suddenly drain into the toilet bowl through holes in the rim.

But here’s where things get mysterious to me. At the start, there’s a certain water level in the bowl. The first thing that happens when you flush is that water surges in from the tank, sending the water level temporarily higher. But then the water in the bowl doesn’t just fall back to its original level, it more or less completely empties, before slowly refilling.

So what accounts for the force that causes the bowl to empty, taking everything with it? And what determines the baseline water level in the bowl between uses? The physics of the whole things eludes me (as well it might, since my knowledge of physics would fit comfortably in a thimble).

diagrams here

The whole mechanism is quite ingenious, really. It combines several physical principles in a perfect balance, but which is fairly basic to establish and maintain.

I always wondered this too, and then a previous thread on toilets enlightened me about the magic of the siphon. Somebody also said that in countries other than America, toilets often use a different mechanism like suction rather than siphonationalizing.

Later, I started cleaning my daughter’s guinea pig’s cage by washing it out in the shower and then dumping the dirty water into the toilet. It’s quite fascinating to watch the sudden influx of water cause the toilet to flush, and then see the water level stay super-low without refilling.

So, when a toilet clogs, where does the blockage usually occur? Is it in that part going up behind the bowl before it bends back down?

This is all very helpful, especially the diagram. I had suspected Archimedes was involved, but couldn’t quite account for the actual sucking of the water out of the bowl.

And the light has dawned about how the water level is set. I guess once the siphon is broken, then the amount of refilling is determined by the mechanical bits of the toilet up in the tank.

I cannot top the technical explainations already given. But I cannot resist adding this: If you get a physic you are going to need a toilet. if you get physics you are going to be there for awhile.

I know, bad form, but it had to be said, or I would not feel right all day.

Time to change that bulb.

The water level in the bowl has nothing to do with the mechanism in the tank–that only fills the tank to a preset level. The water level in the bowl is set by how high the top of the s-bend in the siphon is. Once that level is reached, any additional water than flows into the bowl as the tank fills overflows through the siphon and down the drain.

So does the valve that allows water to enter the tank also simultaneously allow water to flow into the bowl? Does it flow into the bowl from the tank or straight from the water pipe as it’s filling?

After the flapper valve closes following a flush, some of the water from the ballcock valve which fills the tank is diverted to the overflow tube through a small length of tubing to fill the bowl.

I had a mysterious leak the other day. The tank in my daughter’s bathroom was somehow overflowing a small amount onto the floor after each flush. After several examinations, I could see no source of the leak.

The toilet where there’s a straw-sized tube that goes into the overflow tube, so that while the tank is filling, the tube shoots extra water into the bowl. After the tank is full, this flow stops. Normally this tube is attached, but somehow it came loose.

When it was loose and pressurized, it had been flying round the inside of the tank, spraying everywhere, including the top of the tank, and under the edge of the lid. But when the flushing was done, it returned to it’s normal location, just from it’s own stiffness, and looked completely normal.

I reattached the little tube - mystery solved.

Is this essential and do you know if this is true for all WCs? I’ve never seen anything like this “small length of tubing” on a toilet in the UK.

I’ll second that. It appears to me that as the cistern empties and the flow reduces the siphon effect in the bowl is broken - insufficient flow to maintain it but enough water left to reseal the u tube.

Traditionally, overflow pipes had to be diverted outside the building, normally over a doorway but in a prominent position at least. The reasoning being that it would annoy the hell out of you and you would remedy the cause pretty quickly and thus not waste water. If it overflowed into the bowl you wouldn’t really care and it would go on forever. Building regulations have now changed and overflow into the bowl is the norm on newer toilets.

Something not mentioned thus far, but pretty important to the siphon, is that air pressure in the sewer pipes remains equalized during the flush. This is one of the reasons why the sewer stack pipe vents out of the roof - the air has somewhere to go when the waste is pushing it down the pipe. (The other reason is to get rid of the stink.) A clogged air vent (damned dead squirrels!) can result in poor flushing and/or other traps in the house getting sucked dry by the vacuum created in the pipes. Better explanation here.

Wow. Just Wow.

I had no idea toilets are so complex.

I don’t think my toilets work on principles like that at all. We seem to just use gravity and U-Bend. Can anyone provide diagrams like the ones above for standard European toilets ?

Wow. Just Wow.

I had no idea toilets are so complex.

I don’t think my toilets work on principles like that at all. We seem to just use gravity and U-Bend. Can anyone provide diagrams like the ones above for standard European toilets ?