Backflow Prevention

Backflow is the undesired, reversed flow of water or other substances into the drinking water supply. Backflow can occur at any cross-connection between potable and non-potable water lines and can contaminate the water supply, presenting a public health risk.

Backflow can occur when there is a loss of pressure in the public water supply system, which draws the water from a private water system into the public water system. This is called back siphonage and can occur at any time, such as during a large water main break or firefighting.

Backflow can also occur when the pressure in a private system exceeds the pressure in the public distribution system, causing the normal flow to reverse. This is called backpressure and can occur for multiple reasons, such as a pump used to increase the water pressure within a building’s plumbing system to reach a higher floor.

A cross-connection is any actual or potential connection between the public drinking water system and any other system that could accidentally introduce a contaminant back into the public water supply. An example is a piping arrangement or equipment that allows the drinking water to come into contact with non-potable liquids, solids, or gases. Cross-connections can lead to contamination of the city’s drinking water supply and impact public health if not adequately protected by backflow prevention assemblies.

We typically does not track backflow assemblies at duplexes. However, for multi-unit properties with three to eight units, we have developed an application to determine whether backflow prevention is required.

Please review this small multi-unit property exemption application carefully, to determine if your property is required to comply. Email any questions to nobackflow@bouldercolorado.gov. Submitting the form does not ensure exemption and you must receive written approval of the exemption from Backflow Prevention Program staff. We may also perform an on-site cross-connection survey to assess whether an exemption can be granted. Please note that backflow assemblies may be required by plumbing code, even though the backflow program does not track them per state regulations.

There are three main types of backflow prevention assemblies. Email nobackflow@bouldercolorado.gov if you have questions about what type to install.

• Reduced Pressure Zone (RPZ): These assemblies are the standard and are considered to provide the highest degree of protection among the testable assemblies. These assemblies will dump water, so drains must be installed and sized according to plumbing code (increasing the drain size may help prevent flooding). RPZs must be installed 12 inches above grade and will need to be protected from freezing and flooding.
• Double Check (DC): Generally considered to protect against low hazards. These assemblies normally don’t dump water. They may be placed below grade in certain circumstances and should still be protected from freezing and flooding.
• Pressure Vacuum Breaker (PVB): Used specifically for outdoor irrigation systems. These systems can dump water. Assemblies must be installed 12 inches above the highest sprinkler head, which cannot have continuous backpressure, and they will need to be protected from freezing and flooding.

• Containment assemblies: We track the installation and annual testing of "containment" assemblies. Containment assemblies must be installed after the meter but prior to any plumbing branches. It is possible that two containment assemblies may be required at a property.
• Isolation assemblies: Other assemblies may be required inside a property by plumbing code. These are called "isolation" assemblies because they isolate internal hazards. In most cases, the backflow program does not track these assemblies. It is strongly recommended that isolation assemblies are tested annually along with containment assemblies, as they can keep water inside the property safe for consumption.