Understanding Backflow Valves

As engineers, we are often asked to make sure that different systems, like plumbing systems, are safe and reliable.

The backflow valve is one of the most important parts of a plumbing system.

It keeps dirty water from flowing back into clean water sources.

Without this small but powerful tool, the health and safety of many people could be in danger.

In this blog post, we will learn more about backflow valves, including why they are important, what kinds there are, and how they work.

Whether you are an experienced engineer or want to become one, you need to know how backflow valves work to make sure that plumbing systems in all kinds of buildings and facilities are safe and reliable.

Introduction to Backflow Valves

Formal definition:

A type of check valve in a drainage pipe; reversal of flow causes the valve to close, thereby cutting off flow.

Backflow prevention devices are needed to keep water from getting contaminated and to keep people from getting sick.

There are many different kinds of devices that stop water from flowing backwards.

  • Air gaps: Air gaps are used in sinks to prevent debris from entering pipes.

A vertical space between the end of a water supply outlet and the flood rim of a receiving vessel is called an air gap.

  • Vacuum breakers: Vacuum breakers contain a check valve that opens an air vent when the system loses pressure.

When the air vent is open, it breaks the vacuum and stops the water from getting in.

Back siphonage can not happen because of this process.

  • Check valves: Check valves force water to flow in only one direction and keep toxic chemicals out of sanitary pipes.
  • Atmospheric vacuum breakers (AVB): AVBs prevent backflow in hose, faucet, and spigot applications.

AVBs have a check valve that lets air out when the pressure in the system drops.

When the air vent is open, it breaks the vacuum and seals off the water supply line to stop backsiphonage.

Backwater Valves

Backwater valves are made to stop sewage or wastewater from flowing back into a building.

They are put on sewer lines and only let waste water or sewage flow in one direction, away from the building.

Sudden, heavy rain can flood city sewer lines, causing sewage or wastewater to flow back toward the building.

If sewage or other waste water starts to flow backwards into the building, the backwater valve closes its flap to stop it.

It is important to remember that building codes and regulations do not always require backwater valves, and their installation may depend on where they are and what is going on.

But putting in a backwater valve when the house is first built can prevent hundreds of dollars worth of damage from sewer water backing up into the house.

Installation and Permit Requirements

Before installing a backflow or backwater valve, a homeowner should look into local building codes and regulations to find out if any permits are needed and what kind of equipment is recommended.

Retrofitting a valve can be expensive, so if possible, it is best to install it when the building is first built.

Also, it is important to know that the costs of installing a backwater valve depend on the type of valve and where it is.

When a house is first built, it costs about $500 to install a backwater valve (At the time of writing), but it costs more to add one later because you have to cut into the pipes.

The Importance of Backflow Valves in Ensuring Safe Water Supply

Still hard to understand? Let me change the point of view a bit:

Are you sick of drinking water that is clean and safe? Do you like getting sick from food poisoning or other illnesses that come from water? If that is the case, you do not need a backflow valve in your plumbing system at all.

Why try to stop water from flowing backwards and getting dirty if you can just live on the edge? But if you want to stay healthy and make sure your water supply is safe, like the rest of us, read on to learn about the unsung hero of plumbing: the backflow valve.

Okay, that was just a joke made to look like a TV ad.

Now let's go back to the explanation.

Types of Backflow Valves

Check Valves

Check valves have a hinged plug that lets water go through the pipe but stops it from coming back.

They are a simple way to stop water from flowing in the wrong direction, which is called "backflow".

Check valves are used in low-risk situations and are an important part of systems that stop backflow.

They are used in different places on fire protection systems and do not have the fail-safe design of backflow preventers.

Backflow Preventers

Backflow preventers put more pressure on a pipe to make sure water can only flow in one direction.

With their fail-safe design, they are made to fully protect potable water.

A check valve, on the other hand, is used in low-risk situations to stop water from flowing backward, but it does not have the fail-safe design of backflow preventers.

Backflow preventers are used in places where there is a high risk of backflow, and their fail-safe design keeps potable water safe.

Backflow is one of the biggest dangers to the quality of our local water supply, so it is important to install the right equipment to stop it.


Types of Backflow Preventers

There are four kinds of devices that stop water from flowing backwards:

  • Pressure vacuum breaker (PVB): PVBs are the most common and inexpensive type of whole-system backflow preventer.
  • Reduced pressure zone (RPZ): RPZs provide protection against backsiphonage and backpressure and can withstand high or low-level scenarios.

They can handle dangerous situations like a fire.

  • Double check assembly (DCA): DCAs have all the valves and shutoffs needed to test them, so they can be tested regularly to make sure they work well.

They are often used in plumbing systems that are underground or inside.

  • Atmospheric vacuum breaker (AVB): AVBs are installed in each zone of the sprinkler system, immediately following the zone control valve.

Factors to Consider When Selecting a Backflow Preventer

Choosing the right backflow preventer is based on a number of things, such as:

  • Local water authority requirements.
  • Degree of hazard posed by cross-connection.
  • Potential for cross-contamination between gases and water supply.
  • High or low-level scenarios.
  • Indoor or outdoor plumbing systems.

How Backflow Valves Work

Backflow preventer valves come in different styles, such as check valves, air gap backflow preventers, and reduced pressure zone (RPZ) backflow preventers.

Check valves keep water from going in the wrong direction and are used in low-risk situations.

They use a plug with a hinge that lets water go through the pipe but stops it from coming back.

Backflow preventers with an air gap put a physical gap between the water source and the place where the water is used.

This stops water from flowing backwards.

RPZ backflow preventers are used in high-risk situations and keep potable water safe because they are designed to work even if something goes wrong.

With their fail-safe design, they are made to fully protect potable water.

RPZs protect against backsiphonage and backpressure, and they can handle both high-level and low-level situations.

They work well in dangerous situations like a fire, while check valves are usually used in low-risk ones.

How Backflow Valves Work

Backflow preventer valves stop water from flowing back into the system by using a check valve or a series of valves.

Some backflow preventers also have an air gap to add another layer of protection against contamination.

These things only let water flow in one direction, so dirty water can not get back into the clean water supply.

Backflow preventers need to be tested and maintained regularly to make sure they are working well and protecting the public water supply.

Every year, homeowners should check their backflow preventers for wear and problems.

By law, businesses must have their devices checked for wear and damage every year.

Installation and Maintenance of Backflow Valves

Backflow preventers are important for any business or commercial building because they stop dirty water from flowing back into the public water supply.

Backflow preventers come in two main types: double check valve assemblies (DCV) and reduced pressure zone assemblies (RPZ).


It is important to follow best practices when installing a backflow preventer to make sure it works well, lasts long, and keeps people safe.

The way these things are installed can have a big effect on them.

It is best to talk to a professional who can help you choose the right type of device for your building and check the parts before installing it.

The minimum requirements for installing backflow preventers are underlined, and everything else is a suggestion or something to think about.

Before installing the assembly, it is also recommended to put a strainer on the side where the water comes in, unless the line is for fire protection.

Backflow preventers should also be set up with unions and isolation valves on both ends of the assembly so that the assembly can be taken apart and fixed.

When a backflow assembly is added, it may change the pressure, flow, or other hydraulic functions of the system.

The installation of a backflow assembly could also cause thermal expansion.

The person who uses or installs the water system is responsible for making sure the minimum requirements for system operation can be met.

Backflow assembly installations for retrofits or replacements must meet the current state and city installation requirements.

All RPZ devices must have at least 18 inches of space above the ground.


To keep a backflow prevention assembly in good shape, it needs to be checked at the right times.

Every three years, DCV systems should be checked and fixed if they need to be.

Every year, RPZ models should be tested, and every five years, they should be rebuilt.

The best way to keep a backflow prevention program going for a long time is to schedule regular preventive maintenance.

If something is wrong with the system when it is being tested, it needs to be fixed right away because backflow can make drinking water unsafe and cause major plumbing problems.

Internal seals, springs, and moving parts in mechanical backflow prevention assemblies can get clogged, wear out, or get tired over time.

With proper care and testing once a year, they can last for many years, but they will need to be replaced at some point.

What is a Backflow Preventer and How Does It Work

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Use cases

Used in:Description:
Residential Plumbing:Backflow valves are often put in homes to keep dirty water from flowing back into the water supply for the whole neighborhood.This is especially important in homes with irrigation systems, pools, or other places where water could be contaminated.
Commercial Buildings:Backflow valves are important in commercial buildings, especially those that deal with food, drinks, or medical supplies.These valves help stop the backflow of dirty water that could spread illnesses that come from food or water.
Industrial Facilities:Industrial facilities, like chemical plants and factories, often use a lot of water to do their jobs.Backflow valves are used to keep the water supply from getting contaminated, which could be very bad for public health.
Fire Sprinkler Systems:Backflow valves are put in fire sprinkler systems to stop dirty water from getting into the system and possibly damaging the equipment.
Irrigation Systems:Backflow valves are used in irrigation systems to keep dirty water from going back into the water supply for the city.This is important because irrigation systems often use fertilizers, pesticides, and other chemicals that could get into the water supply.
Swimming Pools and Hot Tubs:Backflow valves are put in swimming pools and hot tubs to keep dirty water from flowing back into the water supply for the city.This is especially important in public places where a lot of people might drink water that has been tainted.
Hospitals and Healthcare Facilities:Backflow valves are put in hospitals and other health care facilities to keep bacteria and other pathogens from getting into the water supply.


As we finish talking about backflow valves, one thing becomes very clear: you can not say enough about how important they are.

Even though they are a small part of a plumbing system, they are very important for making sure that our water supply is safe and reliable.

As engineers, it is our job to not only know how important backflow valves are, but also to push for them to be installed and maintained correctly in all kinds of buildings and facilities.

Backflow valves are important, but they should also serve as a reminder of how interconnected we all are as a society.

Installing a backflow valve in a building can help protect not only the people who live there, but also the rest of the community, from getting sick from water.

It is a small step, but it can make a big difference in the health of the public.

In the end, backflow valves are important for a lot more than just engineering.

It reminds us that we are all responsible for the health and safety of the people around us.

So, the next time you turn on the tap for a glass of water, take a moment to think about the backflow valve, which is a small but important part of keeping our water supply safe.

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