How Does a Toilet Work?

Contemporary design toilet and vanity.
credit: Victorian Plumbing
Whether the design is upscale or standard, all gravity fed toilets work the same way.

Some of the appliances you use every day and take for granted are technological miracles, and your toilet is one of these. As the most visible part of a universal sanitary plumbing and waste disposal system that has been some 200 years in the making, the toilet is such an important amenity that no dwelling would be considered livable without one. Whether it empties into a municipal sewage system or into a private septic system, a toilet performs two critical functions: It gets rid of what you put into it completely and quickly, often using less than the maximum-allowed 1.28 gallons per flush; and it blocks unsanitary and flammable sewage gases from entering your home.

The principle of a gravity flush toilet is easy to understand, but it's the engineering of modern toilets that make them work so well. The internal trap is carefully designed to maximize water flow. The waste and vent pipes also play a role in efficient waste elimination. Once you understand how your toilet works, you'll know what it takes to keep it tuned and ready for business.

What's Inside that Porcelain Throne?

Cutaway of a standard toilet.
credit: Universal Plumbing and Sewer
A good flush depends on the rapid transfer of water from the tank to the bowl.

Water is the fuel inside the porcelain enclosure of your home's waste elimination machine. Without water, a toilet would be like a flashlight without a battery or a car without gas.

Water enters the tank from the home's water supply through a fill valve connected to a flexible supply tube. The fill valve, which is one of two valves in the tank, has two outlets. One sends water directly into the tank and the other sends water through a hose into the overflow tube to maintain the water level in the bowl, which is an important sanitary precaution. A ballcock or cup-style float (sometimes called a Fluidmaster float after the company that introduced it) rides on the surface of the tank water and shuts off the fill valve when the water is about 1 inch below the mouth of the overflow tube. When the water level in the tank drops during a flush cycle, the float device also turns the fill valve back on to refill the tank.

The flush valve is the other indispensable component inside the tank. Actuated by the flush handle, it consists of a rubber flapper or a plastic canister that lifts off the flush opening in the bottom of the tank. When it lifts, it does so completely, giving the water an unobstructed path into the bowl. This unobstructed path is crucial, because a complete flush depends on a large volume of water flowing quickly.

The water enters the bowl through holes in the rim. As water from the tank drops down and fills the bowl, the water level there rises above the top of the built-in trap and spills into the waste line. It completely fills the waste pipe as it flows, and that motion creates the suction that empties the bowl. This is why a successful flush requires a large amount of water flowing quickly.

What Can Go Wrong?

Simple toilet repair.
credit: Lowe's
You adjust the tank water level by adjusting the float.

Short of a major blockage in the waste line or a crack in the porcelain, toilet problems are easy to fix, and most homeowners can take care of them. The bulk of problems that cause poor flushing or waste water occur in the tank and are related to the float and the flapper (or canister).

If the toilet doesn't flush completely, it may be because the water level in the bowl isn't high enough. You can adjust this by shortening the rod connecting a cup-style float to the fill valve or shortening the armature supporting a ball float by screwing the ball clockwise. In addition, some fill valves have screws that you can adjust with a screwdriver. Turn them counterclockwise to reduce the sensitivity of the valve to the float and raise the water level. Conversely, one way to stop a toilet from "running" and pouring water down the overflow tube is to lower the tank level by reversing these procedures.

The flapper or canister can also be responsible for poor flushing. If the chain connecting to the flush handle is too long, the flapper won't raise all the way, which causes water to flow too slowly to create the suction needed for flushing. Remedy this by shortening the chain a link or two. Don't make the chain too short, though, or the flapper won't seat firmly, and the toilet will leak. That also causes the fill valve to run all the time, or it may cycle on and off, turning your toilet into what plumbers call a "phantom flusher."

Waste Pipes, Vents and the Pressure-Assist Option

Pressure-assist toilet.
credit: Moen Brothers Plumbing & Drain
A pressure-assist toilet tank eliminates gravity-feed flushing problems.

Proper installation of waste and vent pipes is critical to proper toilet operation. The waste pipes can't be too wide or steep, or the water coming from the toilet won't fill them and create suction. On the other hand, the waste pipe needs enough of a downward angle to allow water to flow.

Poor plumbing is the main reason why modern low-flow toilets don't always work in older houses, and pressure-assist toilets are a solution for such situations. Instead of a flapper-style flush valve, they have a tank filled with pressurized air that compresses when water is forced into it at regular household water pressure. During the flush cycle, the pressure forces water into the bowl more quickly than gravity can do the job, thus filling the waste pipes and creating a complete flush.

Every toilet waste line needs to be vented as per the plumbing code. Venting is a method by which outside air is allowed into the drain pipes so the suction created by the flowing water doesn't stop the water flow or siphon water out of the traps. Vents can get blocked for various reasons, and when that happens, the toilet may flush sluggishly and display the symptoms of a clogged waste pipe. However, try as you might, you can't improve the situation with a plunger or auger. The vent clog could be due to ice or debris build-up in the vent stack opening on the roof, and you'll need to clear vent blockages in order to get things back in tip-top working order.

Chris Deziel

Chris Deziel

Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at