December 7


How Does A Sink Work? Complete Plumbing Guide

By Robin Hatch

December 7, 2020

Having a basic conceptual understanding of how the plumbing in your bathroom works and making basic repairs is an essential skill for everyone. It can be a lifesaver in a plumbing problem.

How sink plumbing works is a basic flow of water from one component to another. Here’s how a sink works.

Water comes into the system from the main water supply by the water supply tubes which run to the faucet. When the faucet opens, the water flows into the sink, down the drain, through the sink trap, and into the wastewater pipe in the wall that carries the water away.

Many components work together to ensure that the water stays where it should be, flows in the right direction, stops when it should, flows when it should, is reliably supplied, and is safely drained to the correct waste management infrastructure.

How A Bathroom Sink Works

Let’s lift the hood, so to speak, and take a closer look at some of the most commonly found and most essential parts of bathroom sink plumbing, how the water supply and flow works, as well as what to do if a component fails.

Water Inlets

Water inlets allow the water to flow from the municipal water supply or external water tank into the bathroom sink plumbing system.

The first link in the chain of bathroom sink plumbing is one, or two, shut-off valves.

These shut-off valves are located near the sink and are attached to the water supply tubes that lead directly to the sink or the main water shut-off valve for the building.

These shut-off valves are convenient because they allow the water flowing into the bathroom to be locally shut off in case of component failure, damage, or leaks that need to be repaired.

The water is then let into the water supply tubes that connect to the water faucets.

The faucets act as stop-valves to keep the water held behind them until they are opened to let the water flow through them into the sink basin.

Hot Water

The hot water supply comes from either a hot water tank or a tank-less water heating system such as a gas water heater or radiator.

The water is heated before it arrives at the hot water supply pipe at the sink.

The water arrives in the bathroom plumbing system via the internal plumbing system of the building, flows through the shut-off valve, and then through the hot water supply tube to the hot water faucet.

The faucet stops the water until the faucet is opened to allow the water to flow through.

Cold Water

The cold water supply is pumped into the building from an external source such as the municipal water supply or an external water tank.

The water pressure in the supply system pumps the water into the bathroom plumbing system, through the shut-off valve, and into the cold water supply tube within the system, and then is also held at bay by the stop-valve that is the cold water faucet.

When the faucet is opened, the water is allowed to flow freely into the basin and is then drained away into the wastewater system along with the hot water and any other liquids that happen to be washed down the sink.

Water Faucets

The water faucets in your bathroom sink act as stop-valves to prevent the water from flowing freely.

The faucets have a water-tight rubber washer that seals them shut, and the clamp of the faucet, which is the handle that you turn to allow water to flow through the faucet, applies pressure to the rubber washer that prevents water from flowing through it.

When the handles of the faucets are turned open, the pressure applied to the washer is released, and the stop valve is opened, allowing the water to flow freely into the sink.

See our complete bathroom sink buying guide here for additional parts.

Water Outlets

All of the water that flows into the sink, whether hot or cold, flows out of the sink via the same outlet, the drain at the bottom of the sink.

The water drains through the plug hole or pop-up stopper down to a clever little plumbing invention known as the “sink trap,” which is simply a pipe with a U-bend in it that traps a small amount of water within the pipe that prevents the smells from the wastewater and sewage systems from wafting into your home.

Once the wastewater flows down the drain and through the sink trap, it flows through the trap arm, the straight piece of pipe that leads the water out into the main wastewater line in the wall or floor of the bathroom.

The main wastewater line then carries the water out of the bathroom, out of your home, and into the central municipal wastewater management system to be processed, cleaned, or recycled.

Plumbing Components

The main components that are used in the plumbing system for the bathroom sink are:

  • Water faucets
  • Hot water supply pipe
  • Coldwater supply pipe
  • Drain outlet
  • The sink trap consists of a U-bend pipe and the trap arm
  • Drainpipe or pop-up stopper
  • A Shut off valve to turn off the water supply
  • Various washers, nuts, screws, clips, and rods

These are the most significant, most commonly used, and most crucial bathroom sink plumbing assembly components.

These listed components are most commonly used in many countries for bathroom sink plumbing systems, but not all bathroom sinks are the same, and there may be different components used in your bathroom.

Some countries even use unique plumbing systems, so your bathroom sink system may have some outstanding components not mentioned on this list.

Read more about the pop-up stopper here and how they work here.

If any of these components fail or break, there is usually a simple solution to replacing or repairing the piece.

If anything goes awry with the plumbing system for your bathroom sink, turn off the water supply at the shut-off valve before you begin to disassemble any components from the system to diagnose the problem; otherwise, you may find yourself sprayed with cold water from the head to toe.

If a component in the system fails, there are always replacement parts available at your local plumbing supply store. Be sure to take the broken part with you when purchasing a new one to ensure that you buy the correct component for your system.

This is a good practice if any of the components in the system fail, from the most significant parts, right down to the most miniature washers.


The bathroom sink plumbing system is not very difficult to understand, and with some simple analysis, almost anyone would be able to figure out how it works.

The main municipal supply line supplies the water that flows to your bathroom sink in your area or from an external water tank.

The water is pumped into the system by either internal water pumps, gravity, or water pressure from the main municipal line into the bathroom plumbing system, where it is then held in the water supply tubes, being held back by the seals in the faucets until the faucets are open allowing the water to flow and be used.

It is helpful to understand how the plumbing system for your particular bathroom sink works so that in the unfortunate event of a plumbing problem, you can identify the cause of the issue and either repair it yourself or give correct diagnostics to a professional plumber over the phone.

Most of the components used in bathroom sink plumbing are available at your local hardware store, as well as the tools required to repair or change them.

Learning how this plumbing system works is a great skill that is useful for everyone.

Spend some time investigating your bathroom sink and the plumbing used, and then do some research into the components you find in use. It may well save you a lot of time, frustration, and money in the future.

We’ve compiled a complete list of frequently asked bathroom questions for new and experienced homeowners.

Our team of experts answers these questions to help assist you through DIY projects and home repairs.

Robin Hatch

About the author

For over a decade, Robin has been a real estate agent, interior design specialist, and mother. Through her trials and tribulations, she wanted to create the perfect website to help you save money and make your home look beautiful.

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