What is a Backflow Preventer?


When you are taking care of your home’s plumbing, it is important to be aware of the different devices that help protect your system. One such device is the backflow preventer. This little-known but crucial piece of equipment helps ensure that dirty water doesn’t flow back into your clean water supply. In this article, we’ll show you how backflow preventers work and the types of backflow preventers that can be used.

What is a backflow preventer?

A backflow preventer is a device that is installed in your home’s plumbing system to keep contaminated water from flowing back into your home’s clean water supply. This cross-contamination is called backflow.

How does a backflow preventer work?

A backflow preventer works by using a series of valves to keep water flowing in one direction. When the water pressure changes, the valves close to prevent water from flowing back into your home’s clean water supply. Backflow preventers are valuable in areas where there is a cross-connection between the clean water supply and a contaminated water source.

What are the different types of backflow preventers?

There are five common types of backflow preventers:

  1. Air gap
  2. Reduced pressure-principle backflow prevention assembly
  3. Double-check valve assembly
  4. Pressure-type vacuum breaker
  5. Atmospheric-type vacuum breaker

Air gap

An air gap is a gap between the water outlet and the flood level rim of a receiving vessel that is open to the atmosphere. This physical separation ensures that there can be no backflow. An air gap is effective as long as it is at least twice the diameter of the water supply outlet.

Air gaps are the only method that can be used for dealing with hazards that are considered highly toxic. One important note is that because air gaps cause a break in the supply line, water pressure is lost and this needs to be considered when designing the plumbing system.

Reduced pressure-principle backflow prevention assembly

A reduced pressure-principle backflow prevention assembly (RPZ) is a device that consists of two check valves, a differential-pressure relief valve, and shut-off valves. These backflow preventers have test cocks and shut-off valves at each end. A test cock is used to check the pressure differential between the two sides of the device.

The area between the two check valves is typically called the zone of reduced pressure. This zone will have a lower water pressure than the line pressure. If the water pressure is not low enough in this zone, the differential-pressure relief valve will open and relieve the pressure. A reduced pressure-principle backflow prevention assembly is reliable in preventing both backpressure and back-siphonage.

Double-check valve assembly

A double-check valve assembly (DC) is a device that consists of two independently-working check valves, two shut-off valves, and two seated test cocks. The way a double-check valve assembly works is that each check valve prevents the flow of water in one direction. When both check valves are working properly, they will prevent backflow in either direction. However, if one check valve fails, the whole assembly becomes unreliable against backflow. Double-check valve assemblies should only be used to prevent contaminants.

Pressure vacuum breaker

A pressure vacuum breaker (PVB) is a backflow prevention device that consists of a valve that opens when water pressure drops. This valve allows air to enter the system and prevents a vacuum from forming. If the vacuum was able to form, it could cause contaminated water to be drawn into the system. Pressure vacuum breakers contain an independently operating

check valve and an independently operating air inlet valve. Pressure vacuum breakers are effective against both pollutants and contaminants, but can only handle back-siphonage and not backpressure.

Atmospheric vacuum breaker

An atmospheric vacuum breaker (AVB) is a backflow prevention device that consists of a float check, check seat, an air inlet port, and a shutoff valve. When water flows through the device, the float check floats upwards, sealing the air inlet port. When the air inlet port is sealed, water is able to flow freely. If there is an unexpected reduction in pressure, the float check will sink.

When the float check sinks, the air inlet port is unsealed. This allows air to enter the line and break the backflow of water. If the float check falls enough, it will land against the check seat. If this happens, water cannot flow upstream. This type of assembly is only effective against the back siphonage of both a pollutant and a contaminant.

What are the benefits of using a backflow preventer?

In addition to protecting your health, backflow preventers can also save you money by preventing damage to your plumbing system. Over time, backflow can cause pipes to corrode and break, leading to expensive repairs. By installing a backflow preventer, you can help to prolong the life of your plumbing system and avoid costly repairs.

What are the consequences of not having a backflow preventer?

If you do not have a backflow preventer, your home’s water supply could become contaminated with harmful bacteria and chemicals. This could lead to serious health problems for you and your family. In addition, if your plumbing system becomes contaminated, it can be very expensive to repair.

Do you need a backflow preventer?

Many municipalities require that homes have a backflow preventer installed, so check with your local building department to see if you are required to have one. Even if you are not required to have a backflow preventer, installing a backflow preventer is beneficial for protecting your health and your home’s plumbing system.

Where are backflow preventers installed?

Backflow preventers are typically installed where cross-connections might occur. These locations include:

  • Sinks
  • Toilets
  • Showers
  • Bathtubs
  • Laundry machines
  • Dishwashers
  • Water heaters
  • Pools
  • Spas
  • Landscaping irrigation systems
  • Fire sprinkler systems

How often should backflow preventers be tested?

Backflow preventers should be tested annually by a certified tester. The test includes opening and closing the device to ensure that it is functioning properly.

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