How Toilets Work

There are several interrelated components that make a toilet do what it does.

If you take off the tank cover and peer inside, you will see all of the parts. The three main systems that work together are:

Let's look at each of these parts separately until the secrets of the toilet are revealed.

Let's say that you somehow disconnected the tank, and all you had in your bathroom was the bowl. You would still have a toilet. Even though it has no moving parts, the bowl solves all of the problems a toilet needs to solve. The crucial mechanism that is molded into the bowl is called the bowl siphon, shown here:

You can understand how the siphon works by trying two experiments with your toilet. First, take a cup of water and pour it into the bowl. You will find that approximately nothing happens. What's even more interesting is that you can pour 25 cups (6 L) of water into a toilet, one at a time, and still, nothing will happen. That is, no matter how many cups of water you pour in, the level of the water in the bowl never rises. You can see in the figure why this is the case. When you pour the cup of water in, the water level in the bowl rises, but the extra water immediately spills over the edge of the siphon tube and drains away.

Now, take a bucket of water -- approximately 2 gallons (7.6 L) -- and pour it into the bowl. You will find that pouring in this amount of water causes the bowl to flush. That is, almost all of the water is sucked out of the bowl, and the bowl makes the recognizable "flush" sound and all of the water goes down the pipe. What's happened is this: You've poured enough water into the bowl fast enough to fill the siphon tube. And once the tube was filled, the rest was automatic. The siphon sucked the water out of the bowl and down the sewer pipe. As soon as the bowl emptied, air entered the siphon tube, producing that distinctive gurgling sound and stopping the siphoning process.

You can see that, even if someone were to cut off the water to your bathroom, you could still flush the toilet. All you need is a bucket containing a couple of gallons of water.

The purpose of the tank is to act like the bucket of water described in the previous section. You have to get enough water into the bowl fast enough to activate the siphon. If you tried to do that using a normal house water pipe, water would not come in fast enough -- the siphon would never start. So the tank acts as a capacitor. It holds several gallons of water, which it takes perhaps 30 to 60 seconds to accumulate. When you flush, all of the water in the tank is dumped into the bowl in about three seconds -- the equivalent of pouring in a bucket of water.

There is a chain attached to the handle on the side of the tank. When you push on the handle, it pulls the chain, which is connected to the flush valve. The chain lifts the flush valve, which then floats out of the way, revealing a 2- to 3-inch (5.08- to 7.62-cm) diameter drain hole. Uncovering this hole allows the water to enter the bowl. In most toilets, the bowl has been molded so that the water enters the rim, and some of it drains out through holes in the rim. A good portion of the water flows down to a larger hole at the bottom of the bowl. This hole is known as the siphon jet. It releases most of the water directly into the siphon tube. Because all of the water in the bowl enters the tank in about three seconds, it is enough to fill and activate the siphon effect, and all of the water and waste in the bowl is sucked out.

So the bowl will flush as long as we dump enough water into it to activate the siphon. And the purpose of the tank and the flush valve is to hold and then dump about 2 gallons (7.6 L) of water very quickly into the bowl. Once the tank has emptied, the flush valve resituates itself in the bottom of the tank, covering the drain hole so the tank can be refilled. It is the job of the refill mechanism to fill the tank back up with enough water to start the whole process again.

The refill mechanism has a valve that turns the water on and off. The valve turns the water on when the filler float (or ball float) falls. The float falls when the water level in the tank drops. The filler valve (or refill valve) sends water in two directions.

Some of the water goes down the refill tube and starts refilling the tank. The rest goes through the bowl refill tube, and down the overflow tube into the bowl. This refills the bowl slowly. As the water level in the tank rises, so does the float. Eventually the float rises far enough to turn the valve off. What would happen if the float were to become detached, or the filler valve were to jam so that it never cut off? Theoretically, the tank would overflow and flood the bathroom. But the overflow tube is there to prevent that from happening, directing the extra water into the bowl instead of onto the floor.

Now that you have seen all the parts, you can understand the complete mechanism:

­What happens after it flushes? Read How Sewer Systems and Septic Tanks Work for a complete rundown.