Anti Siphon Valve Vs. Backflow Preventer: What Is The Difference?

The anti siphon valve is another backflow-preventing mechanism used in various water systems. It is a safety mechanism that prevents contamination of water. Although it may not be a common occurrence, water backflow occurs due to freezing of pipes, pipeline bursts, and sudden high demands of water. Backflow-preventing mechanisms are also used in toilets to prevent contamination. To decide the best option for your water system application, you should compare an anti-siphon valve VS backflow preventer.

anti siphon valve

What is a Backflow Preventer?

A backflow preventer keeps water from traveling back to the supply to contaminate the public water system. It is often used in irrigation systems to keep the soil and other contaminants away from the water supply. It ensures that the water supply is clean, without any chemicals seeping into the water due to negative pressures. A backflow preventer is usually located near the connection of a tap to a hose. Different states have their own codes for backflow preventer installations. Check this with your local municipal authority to find the relevant backflow prevention requirements under ‘cross-connection control.’

What is an Anti-siphon Valve?

Anti-siphon valves are similar to one-way valves that let water flow only in one direction. These are mostly used in residential water supply systems. They are inexpensive and easy to install, unlike industrial backflow preventers. It functions using an in-built atmospheric vacuum breaker. It stops the suction that causes contaminated water to seep into the water system.

Limitations of Anti-siphon Valves

Despite being a popular backflow preventing mechanism, anti-siphon valves have certain limitations. Therefore, they cannot be used in all applications. The most prominent limitations are as follows.

  • These valves are usually installed six inches above the ground. Therefore, it is not ideal for use in underground irrigation systems. So, if you need to disguise the valve, you need to use some plants and bushes around it.
  • Anti-siphon valves require a direct connection with a water source. This acts as the hub for water in the location where the irrigation system connects to the hose.
  • This valve cannot be installed upstream of other backflow preventers. It needs to have a backflow preventer or a check valve installed upstream to prevent the installation of other anti-siphon valves downstream.
  • The building codes of some cities do not allow the use of anti-siphon valves. You need to check this with your local codes before the installation.
  • Anti-siphon valves cannot be installed in areas submerged underwater. So, this is not a wise choice for areas prone to flooding.

Is an Anti Siphon Valve Right for You?

The best way to find whether an anti-siphon valve is the best choice is by talking to an irrigation specialist. They will determine what kind of backflow prevention system is most suitable for your application. Even you can figure out the best choice yourself. Examine each backflow device and identify the functionality of each. Then you can choose the best fitting option for your application. But before you finalize it, check with your local ordinances whether it is approved in your area.

What Are the Other Backflow Prevention Options?

In addition to the usual backflow preventer valves and the anti-siphon valve, there are some other types. This includes:

  • Pressure Vacuum Breaker: This is quite similar to the anti-siphon valve, but you only need to use one valve. In the case of using anti-siphon valves, you need about six valves to cover an irrigation system.
  • Double Check Valve Assemblies: The advantage of this type is that it can be installed underground. It consists of a dual mechanism and is highly efficient.
  • Atmospheric Vacuum Breakers: These are relatively cheap, and you need just one valve to replace the anti-siphon valves. But it may not always be economical depending on the application.
  • Reduced Pressure Assembly: This system is ideal for areas that are extremely hazardous due to contaminants. It can handle high waste situations such as commercial farms.

Frequently Asked Questions

Is an Anti Siphon Valve Necessary?

Anti-siphon valves are commonly used in residences to prevent backflow. These are a popular choice since it is cheap and easy to install. So, it’s better to have one to be on the safe side so that you can have peace of mind. Don’t forget to check out the limitations of these valves to find whether it is the right choice for you.

Do I Really Need a Backflow Preventer?

You need to have a backflow preventer in situations where water could contact the soil or other contaminants. This is why backflow preventers are used in industrial sites and irrigation systems. The best way to figure out whether you need a backflow preventer in the system is to check the local codes. Contact your local authorities to find the regulations related to your state or city.

Is Anti-siphon and Vacuum Breaker the Same Thing?

The anti-siphon valve or the atmospheric vacuum breaker prevents back siphonage using a floating disc. When pressure increases, this disc rises and seals off the air inlet. Once it is depressurized, it drops down to allow air inside the downstream pipe. These AVBs come in plastic or brass and are either manual or electric.

What are Anti-siphon Faucets?

The faucet and the valve are the two types of anti-siphon mechanisms. The faucets come with in-built valves that allow only a one-way flow of water. They are also known as vacuum breakers as they prevent non-potable water from being sucked back in. 

How to Install an Anti Siphon Faucet?

  1. Turn off the main water supply and remove any hoses or attachments from the installation. 
  2. Turn on the spigot and allow the remaining water to drain. 
  3. Turn the spigot clockwise using an adjustable wrench to unscrew it.
  4. Use the plumber’s tape to cover the pipe threads of the anti-siphon valve.
  5. Place the pipe into the fitting where you removed the spigot and screw it. 
  6. Tighten the short pipe and the valve using a pipe wrench.
  7. Use more plumber’s tape on the pipe threads and screw the device. 
  8. Secure the valve to the pipe using an adjustable pipe wrench. 
  9. Turn the main water supply on and test the new faucet. 
  10. Run the anti-siphon valve for about 20 seconds to make sure that it is working properly.


Anti-siphon and backflow prevention devices are mostly installed in toilet models to prevent contamination of water. It blocks various contaminants from flowing back into the water supply during low-pressure situations. Find the most suitable backflow preventing option out of the valves depending on the application.