What is backflow?
Backflow is the unwanted flow of non-potable water source in the reverse direction into a potable system. This contaminated water remains in water lines until it is drained from another fixture in your home.
It can be a serious health risk for the contamination of potable water supplies with foul water, especially when used for drinking, cooking or bathing. For this reason, building codes mandate a series of measures and backflow prevention devices to prevent backflow.
What can cause backflow?
Backflow occurs for one of two reasons, either back pressure or back siphonage.
Back pressure is the result of a higher pressure in the system than in its supply, i.e. the system pressure has been increased by some means. This usually occurs in unvented heating systems, where thermal expansion increases the pressure.
Back siphonage is the result of supply pressure being lowered below that of the system. This occurs when a supply is interrupted or drained down. When there is a sudden drop in water pressure in your area (usually during the elimination of a fire, or concurrent use of large amounts of water by too many people at one time) the drop in pressure is what then pulls water that has already been expelled back into the main water supply.
What are the effects of backflow?
When backflow occurs, the water drawn back into your main water supply may be contaminated. This contaminated water remains in water lines until it is drained from another fixture in your home. A serious health hazard could result when this contaminated water is used for drinking, cooking or bathing.
For an in-depth look at the health concerns regarding backflow contamination, download and read this report.
Who is affected by backflow?
Short answer: Everyone!
All water pipes entering your home, the coffee shop on the corner, your child’s school, your local hospital, shopping centers, apartment buildings and more are all connected to the same common water source. Once contaminated water is pulled back into the pipes in the wrong direction, it has the opportunity to mix with the clean drinking water, thus contaminating the entire town’s water supply.
There are over 10,000 reported cases of backflow contamination each year. Some cases can even be fatal.
How can I prevent backflow or back-siphonage?
A backflow prevention assembly is used to protect water supplies from contamination or pollution due to backflow.
What is a Backflow Prevention Assembly?
In many countries where regulations allow for the possibility of backflow, approved backflow prevention assemblies are required by law and must be installed in accordance with plumbing or building codes.
A typical backflow assembly has test cocks and shut-off valves and must be tested each year, if relocated or repaired, and when installed.
In the United States, the Environmental Protection Agency (EPA) holds local water suppliers responsible for maintaining a certain amount of purity in potable water systems. Many states and/or local municipalities require annual testing of backflow prevention assemblies. A check valve is a common form of backflow prevention. However, in most cases, the law requires a double check (DC), an Reduced Pressure Principle Device(RP) device or an air gap when backflow prevention is mandated.
The simplest, and most effective way to provide backflow prevention is to provide an air gap. An air gap is simply a space between any device that opens to a plumbing system (like a valve or faucet) and any place where water can collect or pool.
What are the various types of backflow preventers?
Double Check Valve Assembly (DCVA)
In the case of usage in water supply, it consists of two check valves assembled in series. This employs two operating principles: firstly one check valve will still act, even if the other is jammed wide open. Secondly the closure of one valve reduces the pressure differential across the other, allowing a more reliable seal and avoiding even minor leakage.
Small valves may be so compact as to be barely noticeable, particularly when they are integrated into the bodies of existing taps (faucets). Larger check valves may be installed with ball valves at the ends, for isolation and testing. Often, test cocks (very small ball valves) are in place to attach test equipment for evaluating whether the double check assembly is still functional.
The double check valve assembly is suitable for prevention of back pressure and back siphonage, but is not suitable for high hazard applications. It is commonly used on lawn irrigation, fire sprinkler and combi-boiler systems. If the hazard is higher, even a relatively low hazard such as using antifreeze in the fire sprinkler system, then a more reliable check valve such as a Reduced pressure zone device may be mandated.
Reduced Pressure Principle Back-flow Prevention Device (RPD)
The RPD, found on high-hazard systems such as lawn irrigation, boiler makeup, chiller makeup, and domestic water assemblies, uses check valves to prevent water from flowing backwards through the pipes. A check valve is a disc or flap that can be pushed open when water flows in the normal direction, allowing water to flow through the pipe. But the check valve is loaded with a spring and closes when water attempts to flow in the opposite direction through the pipe.
The RPD is very similar to a double check valve assembly. The RPD has shut-off valves on either end of the assembly and two spring-loaded check valves in the middle like the double check valve assembly. In addition, the RPD contains a hydraulically operating, spring-loaded pressure differential relief valve between the two check valves. This addition makes the RPD suitable for cross-connection control when the back-flow could create a danger to public health.
The pressure differential relief valve is a a valve which opens and closes in response to differences in pressure on either side of the valve. There are two pipes leading to the relief valve. If the water pressure is equal in both pipes, then the valve remains closed. However, if the water pressure is greater in one pipe than in the other, the relief valve opens and allows water to flow out into an outflow pipe. This is a way of channeling water away from the cross-connection during high pressure back-flow so that the high pressure does not break through the check valves and allow contaminated water into the potable water system.
When the water flows through the R.P.D. in the normal direction, water forces the check valves open just as it does in a double check valve assembly. In addition, some water flows down two small pipes which lead to either side of the pressure differential relief valve. Since pressure is being applied equally to both sides of the valve, the relief valve remains closed.
However, when water flows in the reverse direction through the R.P.D., the check valves close. If the back-flow pressure is great, some water will break through the first check valve and flow toward the second. Some of this water will also flow toward the relief valve from above. Since no water is flowing toward the other side of the relief valve, the pressure on the valve will be uneven. This will force the relief valve to open, allowing water to run out into a backup system.
Atmospheric vacuum breaker (AVB)
An ABV is usually constructed of brass and resembles a 90-degree elbow with a hood on its top to allow air to enter the water system if a siphon attempts to form. Inside this elbow is a poppet valve that is held “up” by the water pressure found in the system, closing the air entrance to the device. If the pressure in the “upstream side” is reduced to atmospheric pressure or below, the poppet valve drops and allows air to enter the system, breaking the siphon.
These devices, since they work on atmospheric principles, cannot be installed in an enclosure containing air contaminants. As air is a fluid, those contaminants could be drawn into the device, thus fouling the pipes. AVBs must be installed a minimum of 6 inches above the highest usage point in the system (i.e. sprinkler, drip emitter, etc.) Underground installation of AVBs is entirely ineffectual in providing backflow protection as groundwater in the underground vault could be drawn into the water system, contaminating it.
The AVB should not be used for high hazard applications and must not be used with continuous pressure on the device, as the poppet would likely stick and the AVB would no longer function properly. A shutoff valve should never be placed downstream of any AVB, as this would result in continuous pressure on the AVB. The AVB is not a testable device.
Pressure Vacuum Breaker (PVB)
A PVB is similar to an atmospheric vacuum breaker (AVB), except that the PVB contains a spring-loaded poppet. This makes it acceptable for applications that are high hazard or where valves are downstream. Pressure vacuum breakers must be protected from freezing when installed outdoors. PVBs usually have test cocks, to which specially-calibrated gauges are attached, in order to ensure that they are functioning properly.
How do I know if I need a backflow prevention device?
In your residence or business, any mechanically controlled use of water (lawn irrigation system, fire sprinkler system, community pool, etc.) requires a backflow preventer as a necessary accessory to your plumbing system to prevent contamination of the public water supply.
What is cross-connection and cross-connection control?
Cross-connections are defined as actual or potential connections between a potable water supply and a non-potable source, where it is possible for a contaminant to enter the drinking water supply.
Control of cross-connections can be achieved through thorough knowledge, vigilance and education. All municipalities with public water supply systems should have cross-connection control programs. Those responsible for institutional or private water supplies should also be familiar with the dangers of cross-connections and should exercise careful surveillance of their systems.
While there are cross-connection control programs in place in many locales, many others lack such programs. There is evidence that inconsistent implementation of such control programs can result in public health risk. The problem is a dynamic one, because piping systems are continually being installed, altered, or extended.