An atmospheric vacuum breaker, also known as an atmospheric antisiphon, is one type of backflow prevention device. Two common examples are household hose bib vacuum breakers and frostproof wall hydrant faucets. An atmospheric vacuum breaker is generally a less reliable and effective backflow preventer than the backflow prevention assemblies we will discuss below.
An atmospheric vacuum breaker relies on air pressure to function rather than water pressure. It contains an air inlet valve that remains closed if water flows in the proper direction. If the flow of water reverses, the air inlet valve opens to prevent backsiphonage.
Cost may be a factor when consumers are considering an atmospheric vacuum breaker. Instead of being installed in one central location, atmospheric vacuum breakers are installed after every control valve on a pipe or on each zone of a sprinkler system. So for complex plumbing systems with many valves, atmospheric vacuum breaker assemblies are not typically economical solutions.
Several other limitations impede the efficacy of atmospheric vacuum breaker assemblies. For instance, these assemblies cannot protect against chemigation backflow — that is, the application of chemical pesticides or fertilizers via irrigation water. They also cannot be used in systems where they will be under constant pressure, such as in plumbing systems with downstream shutoff valves. For this same reason, though they protect against backsiphonage, they cannot protect against backpressure. Atmospheric vacuum breakers must also be installed at least 6 inches above any downstream piping.
Backflow prevention assemblies, on the other hand, contain all the requisite valves and shutoffs for testing, so they can and should undergo regular testing to ensure their effective function. Below are a few common types of backflow prevention assemblies:
The pressure vacuum breaker (PVB) assembly is one of the most common types of backflow preventers. Its design is straightforward and intuitive, and it is relatively economical to purchase and use, as well as easy to maintain and repair. A PVB assembly consists of a couple of different parts:
A PVB assembly also has an air inlet valve that opens when the internal system pressure is higher than the external pressure, thereby preventing backsiphonage.
These assemblies are typically installed vertically, at least a foot above the highest downstream point in the plumbing system.
One of the main disadvantages of PVB assemblies is that they are not typically suitable as a defense against backpressure — they protect against backsiphonage only. So consumers should not use these models where the likelihood of backpressure in the plumbing system is high. PVB assemblies also cannot protect against backflow of chemigation. They are, however, designed for use in both low- and high-hazard scenarios.
PVBs may sometimes leak water, but consumers can look for spillproof models for indoor plumbing installations to help prevent water leakage. Spill-resistant vacuum breakers (SVBs) solve the problem of leaky assemblies. They are similar in construction to PVBs, but they have additional diaphragm seals to keep water from spilling out of the air inlet when the assembly becomes pressurized.
Like regular PVBs, spill-resistant vacuum breakers protect against backsiphonage but not against backpressure.
A double check valve assembly (DCVA) is another common choice for backflow prevention in both indoor and outdoor plumbing systems. It is the most common type of backflow preventer for underground or in-line usage.
A double check valve typically contains an inlet shutoff valve and a valve body with two spring-loaded, independently operating check valves, four test valves and an outlet shutoff valve. A DCVA can be installed either horizontally or vertically — an in-line installation means that the valve will be installed parallel to the piping, whichever way the piping runs. Unlike with a PVB installation, a DCVA does not need to be installed at least a foot above the highest downstream point in the plumbing system. It does, however, require a foot of clearance underneath for maintenance purposes.
Unlike PVBs, DCVAs can protect against both backsiphonage and backpressure, but they typically cannot safeguard against chemigation backflow. They are designed for use only in low-hazard scenarios.
A reduced pressure principle (RP) backflow assembly, also known as a reduced pressure zone assembly, is one of the safest and most dependable backflow preventers on the market. It is also one of the most expensive and one of the most complex to install and maintain, so consumers will need to weigh these pros and cons carefully.
An RP assembly typically consists of an inlet shutoff valve, a pressure differential release valve that separates two independent spring-loaded check valves, four test valves and an outlet shutoff valve. It also contains a mechanically independent relief valve to maintain a low-pressure zone between the check valves.
These assemblies can be installed in a range of different configurations. Underground installation typically calls for straight, in-line installation, whereas above-ground installation allows for a more compact installation to conserve space. Ordinarily, the installation must be horizontal, though some assemblies can be specially configured for vertical installation. As with DCVAs, RP assemblies require a foot of clearance underneath for maintenance purposes.
Unlike other types of backflow preventers, RP assemblies can protect against chemigation backflow. They can also protect against backsiphonage and backpressure and are designed for use in both low-hazard and high-hazard scenarios.