Backflow Prevention: An essential part of any domestic water system

By Peter Kraut

Backflow prevention protects the water we drink from the hazards connected to the potable system, including pollution (low hazard) and contamination (high hazard). It is required to separate private water supplies from public water supplies and to isolate individual systems or equipment within private water supplies. Backflow prevention became law under the Safe Drinking Water Act in 1974 and was amended in 1986 and 1996. The requirements specified in this act, however, are widely misunderstood by plumbers, engineers and code officials. With dozens of classifications and hundreds of models, it is not surprising that so much confusion exists.

Air gaps

The air gap (ASME A112.1.2) is one of the most effective means of protecting the water supply. An air gap is merely the elevation difference between the faucet and the fixture. A lavatory, for example is fixed at no less than 1” above the flood level rim of the basin. A toilet has a fill assembly 1” higher than its overflow. But homeowners and designers alike often combine faucets with vessel sinks and replace the fill assembly on toilets. They could be thinking about aesthetics, or they may be rightly concerned about water conservation. Either way, when backflow prevention is overlooked, the results can be disastrous.

Vacuum breakers

Even less thought is given to the simple hose connection vacuum breaker (ASSE 1011). This device should only be used to protect against back siphonage, although some jurisdictions will allow it for low head back-pressure. The integral type of vacuum breaker is preferred. If it is being installed on an existing device, the non-removable type with a break-off set screw should be used. This deters the user from removing it later. The user may not know that vacuum breakers are not very effective when a valve is installed downstream. It is not unusual for a homeowner to leave a dirty hose on with a garden sprayer (valve) downstream. In an industrial application, where pressure washers and other pumps are likely to be connected to hose bibs, consider an industrial loop to be protected with a reduced pressure principle backflow prevention device (to be discussed later).

Integral vacuum breakers are also used on mop sink spouts with threaded hose ends. In a kitchen, an elevated vacuum breaker (ASSE 1001) is used to protect the water supply from a pre-rinse sprayer. It should be installed in accordance with its listing, typically 6” above the flood level rim of the fixture and 6” above the highest point that the sprayer can be raised. Irrigation too, is typically isolated with vacuum breakers. One for each zone is used to ensure that there are no valves downstream. Unfortunately, all too often they are installed at a low point in the landscaping. What the installer has overlooked is that the water in sprinkler piping above is standing on the vacuum breaker, which should be located 6” above the highest sprinkler so that water drains out at the end of the cycle.

These devices discharge water. In the case of hose bibs, draining to grade is generally acceptable. In the case of mop sinks, draining to the basin below will suffice. In the case of an independent elevated vacuum breaker, be sure to locate it over a fixture or drain board. Pressure vacuum breakers (ASSE 1020) and spill-resistant vacuum breakers (ASSE 1056) are also available. The latter may help eliminate nuisance water. A good example of an application for these is a swimming pool fill valve located below the water line. Note that both types need to be accessible for annual testing.

Dual check valves

To protect public water supplies from low hazards in places such as single family residences, a dual check valve (ASSE 1024) is used. These devices are typically buried with the meter. Similar vented devices (ASSE 1022) are used to isolate carbonated beverage dispensers and cappuccino machines. It is often necessary to provide manufacturer’s information about these devices to the authority having jurisdiction (AHJ). If such a parts diagram is not available, additional external protection will need to be provided.

Double check valves

Better protection for low hazards can be achieved with a double check valve (ASSE 1015). These valves do not discharge water, so they can easily be installed indoors. This is beneficial in freezing climates. In the past, it was common to install them in underground vaults, but this practice is diminishing, due to problems with vault construction, such as rainwater intrusion. Moreover, vault installations are difficult to access for annual testing.

For a long time, double check valves have been the standard for isolating fire protection systems. They protect against back siphonage and back pressure and are available with a “detector” meter to assist utility providers in monitoring water use. This small meter is fed by a ¾" pipe outside the main waterway. As a fire protection system, it should not register flow outside of testing or an actual fire. This allows for a reliable water supply in an emergency, without the obstruction of a water meter and its internal components. Another advantage of double check valves in fire protection systems is the low pressure drop, typically between 3 and 8 psi.

Double check valves are quite large. A typical assembly includes an elbow on the supply pipe, an OS&Y valve, two check valves, a second OS&Y valve and an elbow down to the distribution pipe. A good rule of thumb for determining length is 12:1. This means that a 3" double check valve is approximately 3 feet long from elbow to elbow. A 4" device is approximately 4 feet long and an 8" device is approximately 8 feet long.

Reduced pressure principle backflow preventers

A reduced pressure principle backflow device (ASSE 1013) protects against high and low hazards in back pressure and back siphonage conditions. There are many names and even more acronyms for these devices. RPPBFP, RPBFD, RPBP, RP and RPZ have all been used on plans. RPZ (reduced pressure zone) refers to a region of low pressure within the device. Essentially, a reduced pressure principle backflow preventer is a double check valve with an atmospheric vent between the two check valves. This vent keeps the middle region 2 psi lower than the supply pressure at all times and provides protection even when both check valves are fouled.

Since the middle zone is vented, this device will discharge water. This makes installation in a vault prohibitive and installation in freezing climates a little more difficult. Installation requirements vary by jurisdiction but, generally speaking, the valve is installed at least 30" above grade with at least 12" clear below the assembly. There should be a clear, level platform for testing at least 30” deep on one side of the device. When installed indoors, a drain pan or manufacturer’s drain kit should be installed and piped indirectly to the sanitary sewer or to the outdoors where allowed.

Lengths are similar to double check valves. To reduce the space requirements, manufacturers have designed many configurations of these devices and double check valves. These include vertical installations, u-shaped devices and z-shaped devices. Stainless steel models provide compact sizes, lighter weights and, often, lower pressure drops. Many of these devices are also listed for compliance with lead-free laws.

With the reliability of reduced pressure principle devices, it’s no wonder that administrative authorities are requesting them everywhere. They are the silver bullets of backflow prevention. Some jurisdictions have begun requiring them in fire protection systems. Because these devices can have a pressure drop up to 10 psi greater than a similarly sized double check valve, retrofit into existing systems is quite difficult without significant redesign. Doing so often necessitates the installation of a fire pump, making the fire protection system less reliable.

Using reduced pressure principle devices has also become commonplace for equipment inside a building. An ASSE 1022 device, internal to many carbonators, meets backflow requirements without the need to install a separate device on the water connection. Misuse of vacuum breakers has also built a lack of trust in their effectiveness. Proper specification and clear installation details will improve their reliability. Nevertheless, many authorities still require reduced pressure principle devices everywhere. Often, this is simply due to a lack of understanding. Building one’s library with all referenced standards for backflow prevention can help reduce confusion while still protecting our valuable water supplies.

Peter A. Kraut, P.E., is a licensed mechanical engineer in 22 states. He founded South Coast Engineering Group, near Los Angeles, Calif. in 2001. He can be reached at 818/224-2700 or at pkraut@socoeng.com.