22 December Close up of a reduced pressure zone device backflow preventer

Types of Backflow Preventers

Posted by PlumbWize

Backflow preventers are devices installed in the pipes of your home or building to ensure that water flows in one direction and never the opposite. These kinds of devices protect potable water supplies against contamination from various kinds of cross-connections, thereby keeping drinking water safe and clean for everyone. If you would like to learn more about the types of backflow preventers, how they work, and what they’re typically used for, then you’re in the right place! 

Let’s get started!

Close up of a woman drinking a glass of water

How Do Backflow Preventers Work and Why Do I Need One?

Under normal circumstances, wherein your plumbing system is in tip-top shape and your home or building is providing a steady supply of water, backflows don’t happen even without a backflow preventer. This is because the water pressure, by itself, would be flowing strong enough through your pipe network – thereby keeping water still or flowing in one direction. 

However, incidents such as breaks in the main water line or when a fire hydrant is used can reduce the water pressure in your home or building. When water pressure is reduced or fails completely, water won’t be pushed forward into your home and fixtures – instead, it will flow backwards into the city lines. Thus, without a backflow preventer, contaminants such as fertilizers, chlorine, soap, and human waste can be siphoned back into the public drinking supply due to the negative pressure. 

These incidents of contamination typically happen in “cross-connections” or connections between two water systems wherein one is potable and the other isn’t. Here are some common examples of cross-connections in the home:

  • Garden hose connected to the hose bib with the other end touching a non-potable source of water (e.g. a pool, puddle, or bucket)
  • Makeup water in the boiler that has oil and rust, thereby non-potable and dangerous if consumed
  • Water conditioning equipment that’s directly connected to the sanitary sewer 
  • Older bathtubs with the tub spout below the flood rim

What are the Types of Backflow Preventers?

There are numerous types of backflow preventers with their own mechanisms and unique applications. Your plumber will be able to advise you on the best type for your home and what is required by your municipality. In this section, we will talk about the different kinds of backflow preventers, how they work, and where you can typically find them. 

1. Air Gap 

Unlike other types of backflow preventers, an air gap is completely non-mechanical. For sinks and bathtubs, an air gap can be created just by putting the faucet high enough above the flood rim of the sink or tub. In situations that this can’t be done, an air gap device is used. Air gap devices are common for water softeners and dishwashers. The air gap, whether achieved through proper fixture placement or using a device, provides a physical break between the water source and the container of non-potable water.

2. Hose Bib Vacuum Breaker 

As implied by the name, a hose bib vacuum breaker is typically used for outdoor faucets where a hose connects. This threaded device can be screwed onto the faucet and prevents backflow with its single spring-loaded check valve. The valve opens and closes depending on the water pressure, thereby allowing water to flow only one way, preventing the back-siphonage of non-potable water (e.g. from a pool, puddle, bucket of soapy water) via the hose’s end.

3. Atmospheric Vacuum Breaker 

The atmospheric vacuum breaker or (AVB) is typically made of brass and is bent at a 90-degree angle. Inside an AVB, there is a poppet valve that is held up and out of the way by normal water pressure. In this state, air cannot enter. However, if the pressure is reduced and backflow is about to happen, the poppet valve is dropped and blocks the line. Along with this action, air is allowed into the system to break up any kind of back-siphonage. 

Because these devices work with atmospheric principles, they cannot be installed in an environment that has air contaminants that should not get into the water. Another limitation of this device is that the potable water line (the line that’s protected) needs to be upstream from the non-potable water source. The most common use for these kinds of valves is for irrigation systems.

4. Pressure Vacuum Breaker 

Pressure vacuum breakers (PVBs) have a very similar mechanism to AVBs. The difference, however, is that PVBs have a spring-loaded poppet valve. Because of this difference, PVBs can be installed in systems where the potable water line (the line that’s protected) is downstream from the non-potable water source. These devices usually have test cocks where calibrated gauges are attached to ensure that the device is working properly. The most common application for PVBs is in preventing backflow in underground sprinkler systems.

5. Double Check Valve 

A double check valve or double check assembly (DCA) is a device that has two spring-loaded check valves that are placed in a series. Since these two valves operate independently, one of the main advantages of having this backflow preventer is that even when one valve fails, the other can take over. Moreover, if both valves are functioning, the closure of the first valve reduces the pressure differential across the other valve, resulting in a tighter and more reliable seal against backflows. The most common application for DCAs is preventing backflow in fire sprinkler systems. 

A vacuum breaker used for an irrigation system

6. Reduced Pressure Zone Device 

A reduced pressure zone device (RZPD) is a device used specifically to prevent health hazards. RZPDs are equipped with two independently-action spring-loaded check valves. In between these two valves is a pressure-monitored chamber called the “zone.” The pressure in this zone is maintained by a differential pressure relief valve. This third valve will open to the atmosphere in the event that both check valves fail. The two independent valves can also take over for each other in case one of them fails – similar to how double check valves work. 

In cases when backflow will result in significant harm, these devices are considered suitable for the job – because they can effectively and reliably prevent back pressure and back-siphonage. The redundantly secure design of this device makes it suitable for protecting drinking water. 

Who Can I Call for Backflow Preventer Installation or Repair?

If you need backflow preventers installed in your home/building or if you need to repair the backflow preventers that you currently have, then get in touch with a trusted plumber. At PlumbWize, we ensure quality service for all your plumbing needs. We make it a point to get the job done right the first time and give you reliable and long-lasting plumbing solutions. 

If you have any questions for us or would like to get started with backflow preventer installation, repair, or replacement, please feel free to give us a call!


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