Backflow Prevention: Small Multi-Unit Property Frequently Asked Questions

A small multi-unit property has eight (8) or fewer dwelling units and may be exempt from backflow regulations.

Larger multifamily properties (9 or more units) represent an increased risk for backflow and cross‐connections and will not be exempt from backflow regulations.

Multi-unit properties with three to eight dwelling units may qualify for an exemption from the city’s backflow prevention installation and testing requirements. We typically do not track backflow assemblies at duplexes.

Properties must meet all of the criteria listed on the Small Multi-Unit Exemption Application to qualify. Please review the application and this FAQ carefully to determine if your property could be exempt. Submitting the application does not ensure exemption, and you must receive written approval of the exemption from Backflow Prevention Program staff. We may also need to conduct 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.

No, exemptions are not permanent. Any and all exemptions are subject to review, and exemptions may be retracted in light of any property modifications that introduce cross-connections or regulatory changes at the state, city or program level. If an imminent risk to the city’s drinking water is discovered that was not disclosed and/or misrepresented on the exemption form, the immediate installation and testing of an approved backflow prevention assembly will be required. Generally, changes that force compliance on previously exempt properties will be handled by notifying the responsible party and developing a compliance schedule.

Backflow prevention assemblies protect both the city’s water supply and the drinking water within the property itself. If an assembly is installed at a property that is granted an exemption, we strongly recommend annual testing, but the city’s Backflow Prevention Program will not need to track the assembly or its tests.

In cases where the property meets the exemption criteria in all areas except for the presence of a dedicated line (lawn irrigation or fire), the domestic service line will remain exempt from installation and/or annual testing. The specific dedicated line in question must still comply with backflow prevention requirements. For example, if a currently exempt single-family residence acquires a dedicated irrigation water line, that irrigation line will need an approved backflow prevention assembly installed and tested annually per state and city requirements.

In all circumstances, a commercial property must follow backflow regulations. If you have questions, please contact us at nobackflow@bouldercolorado.gov.

Hydronic heating systems circulate heated water, often mixed with chemical additives like antifreeze, throughout a property in order to heat the home when it’s cold. Water heaters are not considered hydronic heating systems as they simply heat water for domestic use. Boilers and heat exchangers are common hydronic heat sources.

The largest concern with boilers is that they sometimes use chemicals like antifreeze to keep lines from freezing. Further, chemical solutions may be run through boilers for cleaning. In either case, chemicals could get back into the drinking water supply during a backflow event. Installation and annual testing of an RPZ backflow prevention assembly may be required.

Sometimes boilers are isolated and protected with a non‐testable backflow prevention device, which does not meet state regulations for a testable backflow prevention assembly installed as “containment.” If the boiler is determined to be the only high-hazard cross-connection on the water line, installation and annual testing of an RPZ backflow prevention assembly may be required to isolate the boiler.

Process water becomes contaminated during an industrial process, such as photo processing or X‐ray development. Make‐up water can become contaminated when it is mixed with chemicals (like antifreeze) for the purpose of supplying boilers, cooling towers, solar water heating systems, or other sources of cooling and heating. Typical sources to look for include but are not limited to photo labs, darkrooms, X‐ray machines, hydronic heating systems, or solar water heating systems.

Three-story buildings are assumed to be about 36 feet in height (12 feet x 3 stories, above ground) Because the plumbing generally enters at the top of the basement, basements need not be considered in the overall calculation of stories. Similarly, attic space with no plumbing should be disregarded in any calculations.