Plumbing · Lesson 4 of 13Beginner10 min read

Fixtures and How They Work

Fixtures are where you meet the plumbing. A toilet is a little machine built from a float, a valve and a siphon. A faucet is a valve you can grab. A shower valve quietly protects you from scalding. Once you know how they work, most of their problems make sense.

The toilet: a float, a valve and a siphon

The tank holds the water for the next flush. Push the handle and a lever and chain lift the flapper, a rubber plug over the flush valve at the bottom of the tank. EPA describes the flapper as the part that controls the flow of water to the bowl. Tank water rushes down into the bowl, through holes under the rim and often through a jet near the bottom.

The bowl drains through the trapway, a curved passage molded into the toilet that rises before it falls. Normally water sits in the bowl up to the top of that hump, which makes the toilet’s own trap. When the rush of water from the tank fills the trapway, it starts a siphon. The water going down the long side is heavier than the water coming up the short side, so the pressure at the top of the bend drops below the air pressure on the bowl, and the bowl empties fast. When the bowl is nearly empty, air gets in and breaks the siphon. That’s the gurgle at the end of a flush.

Meanwhile the tank refills. As its water level drops, a float drops too and opens the fill valve. Fresh water flows in, and a small refill tube sends some of it down the overflow tube to top up the bowl and restore the trap seal. The float rises with the water, held up by the buoyant force that OpenStax’s physics text describes (Archimedes’ principle), and at the set level it closes the valve. The overflow tube is a safety outlet: if the fill valve fails to shut off, water spills into the bowl instead of onto the floor.

Press the handle to flush, then try the faults. The tank and bowl volumes follow the gallons per flush you pick; the timing is sped up for a clear view.

Toilet parts
Part	What it does	Signs of trouble
Handle, lever and chain	Lift the flapper when you push the handle.	A chain too short holds the flapper open; too long, it tangles under it.
Flapper	Seals the flush valve; lifts to start a flush.	A worn flapper leaks, so the toilet runs or refills on its own.
Fill valve and float	Refill the tank and shut off at the set level.	Set too high, water runs into the overflow tube nonstop.
Refill tube	Sends water down the overflow tube to refill the bowl.	Bowl water level too low after a flush.
Overflow tube	Lets extra water spill into the bowl instead of the floor.	Water trickling into it means the level is too high.
Trapway	The toilet’s built-in trap and siphon path.	Weak flushes, clogs.
Wax or foam ring	Seals the toilet’s outlet to the floor drain.	Water at the base, sewer smell, a rocking toilet.

How many gallons a flush

EPA says older, inefficient toilets can use as much as 6 gallons per flush. The current federal standard is 1.6 gallons, and WaterSense labeled toilets use 1.28 gallons or less while passing tests for how well they clear the bowl. Toilets account for nearly 30 percent of an average home’s indoor water use, and EPA estimates that replacing old toilets with WaterSense models can save a home nearly 13,000 gallons a year.

Flappers wear out. EPA suggests replacing yours at least once every five years. If a toilet runs or refills by itself, Fix a Running Toilet walks you through the dye test, the chain and the float.

Faucets: valves you can grab

Every faucet is a valve that opens a path for water and closes it off again. Older two-handle faucets often use a compression valve: turning the handle screws a stem down until a rubber washer presses onto a metal seat. These take several turns to close, and a worn washer lets the faucet drip. EPA’s WaterSense pages note that leaky faucets can often be fixed by checking the washers and gaskets and replacing worn ones.

Most newer faucets are washerless. A cartridge faucet slides or turns a cylinder with openings in it; O-rings make the seal. A ball faucet uses a slotted ball under a single handle, with springs and rubber seats. A ceramic disc faucet uses two hard, polished discs with holes; turning the handle slides one over the other, and water flows where the holes line up. A quarter turn takes it from off to full.

At the tip of most faucets is an aerator, a small screen that mixes air into the stream. A clogged aerator can make a faucet’s stream weak, and EPA lists cleaning it regularly among its steps for reducing lead in tap water (Lesson 7).

Simplified sections. Many single-handle faucets use a cartridge, a ball or ceramic discs; older two-handle faucets often use compression valves. Take the old part to the store to match it.

Showers and anti-scald valves

A shower valve mixes hot and cold. The danger is a sudden change: someone flushes a toilet, the cold side’s pressure drops, and the mix turns scalding. Two kinds of valve guard against that. A pressure-balancing valve has a spool that senses the hot and cold pressures and cuts back the hot when the cold drops. A thermostatic valve senses the temperature of the mixed water itself and adjusts to hold it steady.

HUD’s standard for manufactured homes requires shower and tub-shower valves to be pressure-balancing, thermostatic or a combination, with the limit stop set to no more than 120°F. The CDC adds that if a water heater is set above 120°F, thermostatic mixing valves at faucets and showers help prevent scalding (Lesson 5). Your local code sets the rules where you live.

Simplified. Both guard against a sudden blast of hot water. HUD’s standard for manufactured homes requires one of these types (or a combination) in showers, with the handle limit set no higher than 120°F.

Sinks, tubs and the overflow

Under a sink, a straight pipe called the tailpiece drops from the drain into the trap. Most bathroom sinks and tubs also have an overflow: a hole near the rim that lets water escape down the drain before it spills. That’s why you plug the overflow with a wet rag before plunging a sink; otherwise the push escapes through it instead of reaching the clog (Unclog a Sink or Toilet). Tubs work the same way, with a stopper linkage behind the overflow plate.

Dishwasher and clothes washer hookups

A dishwasher and a clothes washer each have a supply line, a shutoff valve and a drain. Their supply connections must be protected from backflow; HUD’s manufactured home standard, for example, requires a fixed air gap built into the appliance. The drain hose from a clothes washer usually hooks into a standpipe, a pipe open at the top with its own trap below, so the washer’s pump can dump water fast without siphoning the trap or letting sewer gas out. Quick-closing appliance valves are a common cause of water hammer (Lesson 9).

You canYou can do these

Do the toilet dye test and replace a worn flapper (Fix a Running Toilet).
Unscrew and clean a faucet aerator or a showerhead with scale on it; EPA suggests white vinegar or a cleaning product.
Close a fixture’s shutoff valve before you work on it, and open a faucet to let the pressure out.

StopCall a plumber for

Valves inside the wall, such as a shower valve, and anything that means opening a wall or cutting into supply or drain pipes. A permit and inspection may be required, and the work may need a licensed plumber where you live.

Check yourself

Tap an answer to check it, or open “Show the answer.”

What starts the siphon that empties a toilet bowl?
Show the answer
Water from the tank filling the trapway. The rush of tank water fills the trapway, and the heavier water in the long leg pulls the bowl empty.
A toilet refills by itself every so often. What’s the most likely cause?
Show the answer
A leaking flapper. Water leaks past a worn flapper, the tank level drops, and the fill valve turns on to top it up.
What’s the most a WaterSense labeled toilet uses per flush?
Show the answer
1.28 gallons. 1.28 gallons or less. The federal standard is 1.6.
Someone flushes a toilet and the shower stays the same temperature. Which valve senses the hot and cold pressures to do that?
Show the answer
A pressure-balancing valve. A pressure-balancing valve cuts the hot to match when the cold pressure drops.
Why plug a sink’s overflow hole before plunging?
Show the answer
So the push goes to the clog instead of out the overflow. An open overflow lets the pressure escape.

Go deeper

Being an Adult 101: Fix a Running Toilet Fix a running toilet, step by step.
U.S. EPA WaterSense: Residential Toilets WaterSense toilets and how they’re tested.
OpenStax College Physics 2e, 11.7 Archimedes' Principle Why the float floats: Archimedes’ principle.
Lesson 5: Water Heaters Next: where the hot water comes from.

Sources

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A free course from Teddy Rook Book. It explains how plumbing works in general and follows U.S. sources; your local plumbing code, the authority having jurisdiction and manufacturers’ instructions are the final word.