How Bathtub Plumbing Works

The Mysteries of Bathtub Plumbing Revealed

The bathtub is a plumbing fixture people use every day, but they don't often think about how it works. Out of sight, out of mind, and the pipes behind the wall remain a mystery. But because people depend on the bathtub so much, no one should wait until something goes wrong to understand how it works.

Your Home's Plumbing System

Man Working on Plumbing System

Mostly hidden out of sight, an intricate network of plastic and metal pipes resides in the walls and floors of a house. That's what the plumbing system is. The purpose of this system is to deliver water, either hot or cold, and remove waste.

It starts at the water main, which is the pipe used to supply the entire home with water, usually from a municipal treatment source. The water main is a plastic or copper pipe ½" to 1 ½" in diameter. 

From there, the water flows throughout the house via copper, plastic, or PEX pipes. Water goes to the water heater, sinks, toilets, tubs and appliances like washing machines or dishwashers. Each fixture connects to the water line by supply lines and valves that can control the water flow.

Most fixtures that receive water also have drains. Drain pipes are typically plastic and tend to be wider than supply lines. All drains flow downward using gravity, and lead to the main sewer line.

The main sewer line is a large plastic pipe underneath the house that shuttles all wastewater away from the home to a municipal treatment facility.

The Plumbing in the Bathroom


Bathrooms typically have three main fixtures: the sink, the toilet and the bathtub. All three connect to a water supply line, and each has its own drain.

The sink and tub both draw water from the water heater, allowing the adjustment of the temperature of the water. The toilet uses only cold, untreated water.

The drains for all three fixtures connect to one larger drain pipe underneath the floor, which sends the wastewater down to the main sewer line. This drain pipe is also attached to a vent stack, a vertical pipe that runs up inside the wall to the roof. The vent stack allows the necessary suction for the wastewater to flow downward.

Understanding Bathtub Plumbing

Man Caulking Bathtub

As mentioned, the bathtub has two main features: the water supply and the drain, and it's good to understand its plumbing in a little more detail.

There are two water supply lines to the bathtub: one for cold water, straight from the main water supply, and one for hot water, which comes from the water heater. These are laid underneath the bathtub and are commonly made of copper, cream-colored plastic CPVC, or red and blue PEX tubing. Each has its own shutoff valve in case of an emergency.

The two supply lines meet at the faucet, where the handle or handles to control the temperature of the water. Turning the handles open valves that allow water to pass through. The water then issues from the bathtub spout or is sent up through a single vertical pipe to the showerhead.

After use, the dirty wastewater goes down the tub drain, through a P trap, and to the larger drain pipe under the floor. The entire process takes less than a minute and is made possible by basic physics.

About A&A Plumbing

Family owned and operated since 2007, A&A Plumbing is a trusted source for residential and commercial plumbing services in San Antonio and the surrounding areas. For upfront pricing, prompt and tidy service, and guaranteed results, trust A&A Plumbing for all plumbing needs!