How It Ticks

Around the house · Exhibit 8

How does a toilet work?

Updated

A toilet flushes with no pump and no electricity. The handle lifts a rubber flapper at the bottom of the tank, up to 6 liters (1.6 gallons) of water drops into the bowl in a few seconds in a new American toilet, and that rush fills the curved pipe behind the bowl and starts a siphon, which pulls the bowl empty. Then a valve with a float refills the tank and shuts itself off when the water is back at its line. The bowl and its pipe are drawn further down, in the bucket section.

The common North American tank with a flapper, cut in half; the handle and its trip lever are top left. Break something with the buttons: each is one of the usual reasons a toilet keeps running, and the line under the drawing says how much water that fault would waste a day in the toy. The red ticks mark the rim of the overflow tube.

If your toilet keeps running, the usual causes and how to fix each one are below.

How does a toilet flush?

The handle turns a short arm inside the tank, which plumbers call the trip lever. A chain hangs from the arm to a rubber flapper that covers a hole in the bottom of the tank, usually 2 or 3 inches across (flappers are sold in both sizes), and pressing the handle lifts the flapper off that hole.

You let go of the handle right away, but the flapper stays up until the tank is nearly empty. Underneath it is a cup with air trapped inside, and that air makes it float. A patent application filed in 2003 explains the ordinary flapper this way: “the pocket of air within this cavity provides sufficient buoyancy to keep the flapper valve […] open until most of the water within the tank […] has passed through the drain hole […] into the bowl.” When the water is gone, so is the buoyancy, and the flapper drops back onto its seat.

The rush of water into the bowl does the flushing. Behind the bowl, a curved pipe called the trapway climbs to a crest and then falls away to the drain. Normally the bowl water sits level with the crest, and the water in the bend is what keeps sewer air out of the room. When the tank empties all at once, Kohler writes, “the surge of water to the bowl quickly fills the trapway and displaces the air space within the trapway. The resulting change in pressure causes a siphon to be formed.” Once the bend is full, the water falling down its far side drags the water behind it along, the way a garden hose keeps draining a tank once it gets going, so the bowl keeps emptying until air gets into the bend. That is the gurgle at the end of every flush.

In North America, the same American Standard patent says, siphonic toilets “are now viewed as the standard, accepted form of toilet.” Many European toilets work without one: they are washdown toilets, and the rush of water pushes the bowl’s contents over the crest and away. A patent from American Standard sorts gravity toilets into “wash down and siphonic” and notes that “in European markets, wash-down toilets are still more accepted and popular.”

In the toy, one flush sends 6.0 liters to the bowl in 3.8 seconds. I sized the tank to land on 6.0 liters on purpose, because that is the federal limit for a new tank toilet in the United States: 1.6 gallons per flush for toilets made since 1994 (the same rule allows 3.5 gallons for a different design, the blowout bowl). The American Standard patent below notes that California has gone further, to 1.28 gallons. Toilets with the EPA’s WaterSense label use 1.28 gallons or less, and older ones used “as much as 6 gallons per flush.” The EPA counts toilets as the biggest user of water indoors, “nearly 30 percent of an average home’s indoor water consumption.”

How does the toilet tank refill itself?

With a float. “As soon as the water level in the tank begins to drop, a float mounted on the fill valve begins to travel downward,” in Kohler’s description. The fill valve is tied to the float, so as the float goes down the valve opens, and water from the supply line pours back into the tank. As the tank fills, the float rises with it, and at its set height it closes the valve.

The bowl needs water too, because the siphon has just emptied it. That is the job of a thin refill tube that runs from the fill valve to the overflow tube, the open pipe standing up from the bottom of the tank. While the valve runs, part of its water goes down that tube and straight into the bowl. Fluidmaster, which makes fill valves, sets the tank’s water level “below top of Overflow Pipe” and recommends half an inch below, so that during normal use the overflow tube carries only the refill water and an emergency spill.

Why does my toilet keep running?

Almost always because water is getting from the tank to the bowl when it shouldn’t, and the fill valve keeps replacing it. The toy has buttons for three of the usual ways it happens; the other two aren’t in the toy.

The float is set too high. The fill valve shuts only when the float reaches its set height. If that height is above the top of the overflow tube, the water never gets there: it spills into the tube first, and the valve runs forever. In the toy the whole flow of the valve, 6 liters a minute, goes down the tube, which comes to 8,640 liters (about 2,280 gallons) a day. A real valve’s flow depends on your water pressure, and the toy’s is on the low side: New York City’s water department says water “can flow through an open fill valve at a rate of three to five gallons per minute”, so a real one can waste two or three times the toy’s figure. Fix: lower the float. Most fill valves have a screw or a clip on the float’s rod for this; set it so the water stops half an inch below the top of the overflow tube.

The flapper is worn. An old flapper goes hard or warps and stops sealing, so water seeps into the bowl all the time. In the toy I gave a worn flapper a gap of 2 square millimeters, a size I picked; that loses about 190 liters (50 gallons) a day. Fix: replace the flapper, a cheap part. Turn off the water at the valve behind the toilet, flush to empty the tank, unhook the old flapper from the pegs at the bottom of the overflow tube, and hook on a new one that fits your flush valve.

The chain is too short. A chain with no slack holds the flapper slightly off its seat. Korky, which makes flappers, recommends “only leaving 1 to 2 chain links in slack” and warns that this “prevents the flapper from being held open if there is too much chain tension.” How far a tight chain lifts a flapper depends on the tank; in the toy I made the gap ten times the worn flapper’s in area, which leaks about 1,900 liters (about 500 gallons) a day. Fix: move the chain’s clip on the handle’s arm until the chain hangs with a link or two of slack. Too much slack causes trouble of its own: Korky’s reason for the limit is that “loose chain links do not fall into the flush valve drain opening, which prevents the flapper from sealing.”

The refill tube is pushed too deep. The refill tube should be clipped to the top of the overflow tube. Shoved down inside it, its end sits lower than the water line in the tank, and it can keep siphoning tank water into the bowl after the valve shuts. Fluidmaster’s instructions for its 400A valve: “Do not shove refill tube down overflow pipe. This may cause significant water waste.” Fix: pull the tube up and clip it to the rim of the overflow tube.

The fill valve won’t shut off. Bits of grit from the supply line can get stuck in the valve, and the rubber seal under its cap wears out. Then water keeps coming in with the float at the right height, and the extra goes down the overflow tube, so from outside it looks just like a float set too high. Fluidmaster’s 400A instructions cover the case where the valve “WILL NOT TURN OFF”: take off the top cap and “check for debris,” and if the valve “has been in use for some time,” replace the seal.

If jiggling the handle stops the running, often the flapper had settled crooked or the chain had caught on something. It will happen again; check the chain’s slack and whether the flapper sits square on its seat.

The EPA says “nine percent of homes have leaks that waste 50 gallons or more per day,” counting every kind of leak in a home, faucets and pipes included, and singles out the flapper: a worn one “can cause your toilet to flush on its own or silently leak thousands of gallons a year.”

Why does my toilet run randomly for a few seconds?

That is a leak, sometimes called a phantom or ghost flush. Water seeps past a worn flapper, the tank level creeps down until the float drops far enough to open the valve, and the valve tops the tank up for a few seconds and stops. Nobody touched the handle. A refill tube pushed down into the overflow tube can do the same: Fluidmaster says “Siphoning occurs when the refill tube is pushed too far into the flush valves overflow pipe.” In the toy the worn flapper is a fairly big leak, 50 gallons a day, so the refill comes every 3.2 minutes; turn it on and watch the refills count go up. A slower leak tops the tank up less often, and the slower it is, the longer the gaps.

How can I tell if my toilet is leaking?

The EPA’s test needs food coloring and ten minutes: “Identify toilet leaks by placing a few drops of food coloring in the toilet tank. If any color shows up in the bowl after 10 minutes, you have a leak.” Don’t flush during those ten minutes, and do flush right after: the EPA adds “Be sure to flush immediately after the experiment to avoid staining the tank.”

A clean bowl doesn’t clear a very slow leak. Fluidmaster says the dye test is “great for verifying a faster leak but it does not always verify a slow leak.” If the tank still tops itself up now and then with nobody using it, it is leaking. (The other classic night noise, a chirp from the ceiling, is a smoke detector with a tired battery.)

Color in the bowl tells you water is getting from the tank to the bowl. It doesn’t tell you where, because water spilling into the overflow tube lands in the bowl too. Lift the tank lid to find out. If the water is up at the top of the overflow tube and running into it, the fill valve isn’t shutting off. Korky lists three reasons: the float “may be set too high, clogged with debris, or failing to shut off.” Lower the float first; if water still runs into the tube, clean or replace the valve. If the water is below the top of the tube, it is getting past the flapper, or the refill tube is pushed down inside the overflow tube and siphoning, or the overflow tube itself is cracked: Fluidmaster notes that “cracks in the plastic overflow pipes are not uncommon.”

How do you flush a toilet when the water is off?

Pour a bucket of water into the bowl, fast. A toilet doesn’t need electricity to flush, and it doesn’t need the tank either: all a flush takes is enough water arriving quickly enough to fill the trapway. When the water supply is off, the tank won’t refill, but a bucket does the same job.

Only do this if the bowl drains normally. If the water in the bowl is higher than usual, or the drains in the house are backing up, a bucket will overflow the bowl instead of flushing it.

Speed decides it. Pour slowly and the bowl rises a little. The extra spills over the crest and trickles down the bottom of the trapway, with air above it. Nothing gets pulled out. Pour fast and the water piles up at the crest faster than it can spill away, until it touches the top of the pipe. Now air can’t get past. The falling water drags the trapped air down, the trapway fills from end to end, and the siphon starts.

The bowl cut in half, with the crest of the trapway behind it and the trapway falling to the drain. The red mark is level with the top of the pipe at the crest: once the bowl water reaches it, the trapway fills. The dashed line shows how high the water rose on the pour before. Every pour starts with the bowl full to the crest, as a real one is after it refills.

In the toy, a gallon poured in under about 3.6 seconds flushes, and a slower pour doesn’t. That line comes out of three of the toy’s sizes: how wide the crest is, how big the bowl is and how tall the pipe is at the crest. How far the trapway drops doesn’t enter, because in this toy the pipe closes before the spill can outrun it. I picked those sizes myself, and your bowl is shaped differently, so take the number as a reason to pour quickly rather than as a rule.

Does a siphon work uphill?

Yes, over a hump, which is what the trapway is: the water climbs to the crest before it falls to the drain. The outlet has to end lower than the water surface the siphon starts from, and the hump has a height limit.

With ordinary water that limit is about 10 meters. Standard air pressure, 101,325 pascals, holds up a column of water about 10.3 meters (34 feet) tall. Near that height the pressure at the top of a siphon drops so low that the water boils into bubbles and the column breaks, a little short of the full figure. That limit holds for ordinary tap water, which has air dissolved in it: in 2015 researchers who had degassed their water beforehand ran a siphon 15 meters high at sea level. The crest of a trapway is a few inches high.

Does a toilet flush the other way in the southern hemisphere?

No. The toilet decides. The astrophysicist Neil deGrasse Tyson wrote in 1995 that for a toilet of normal size “the answer lies in the manufacturer’s design.” The shape of the bowl and the way water enters it, through angled holes under the rim or a jet near the bottom, set the direction, if the water swirls at all, and it goes the same way in Sydney and in Stockholm.

The Earth’s turning can steer a drain, but only when everything else is taken away. In a letter to Nature in 1962, the engineer Ascher Shapiro at MIT described the test: a round tub filled 6 inches deep with a deliberate clockwise swirl, left to settle for 24 hours, then drained through a small hole in the middle. It took about 20 minutes to empty. For the first 12 to 15 minutes nothing turned; then the water began to rotate counterclockwise, the way storms turn in the northern hemisphere. In 1965 a team in Sydney repeated the experiment and their water turned clockwise. A toilet bowl empties in seconds, and its rim holes or jet set any swirl long before the Earth’s turning could.

What do the toys simplify?

The tank’s sizes are a typical tank’s rather than one model’s, and eight of its numbers are my choices:

The water follows Torricelli’s law. For the refill interval the toy’s step-by-step arithmetic gives the same 193 seconds as the exact solution of that law (the published run below); nobody has checked it against a real tank. The counter leaves out the trickle the refill tube sends to the bowl.

The bowl is simpler still. It is treated as a basin that spills over the crest, the siphon starts the moment the water closes the pipe at the crest, and the push of the poured water itself is left out. Its sizes are mine too: a trapway 2 inches wide that drops 6 inches (15 cm), a crest as wide as the pipe, a bowl with 0.03 square meters of water surface, a gallon in the bucket, and a weir coefficient for the spill over the crest. Neither toy has the jets around the rim that rinse the sides of the bowl.

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