Backflow Prevention on Public Water Systems

What is it, why is it important, & should we care?

Just as it sounds, a backflow preventer prevents water from flowing backward out of your house into the main water supply system.

Why would this happen?

Water usually flows towards your plumbing devices such as sinks, showers, laundry, etc. In the case of a fire nearby, a fire hydrant would be tapped by the fire department to provide sufficient water to put out the fire. Since the pressure and flow of water used during a fire can be enormous, there is the potential that the water would be “pulled” backward out of your house into the main water line.

So, if water were to flow out of your house, plumbing fixtures that normally rely on positive pressure, such as water heaters, boilers, etc., would be under negative pressure and could be damaged. Also, contaminated water in your system, especially pools or hot tubs with chemicals involved, would be pulled back into the main system and contaminate everyone’s water.

As a result of these concerns, the American Water Works Association recommended that all municipalities create a cross-connection (or backflow prevention) plan for public water supplies. The State of Maine, in the process of updating codes for MUBEC (Maine Uniform Building and Energy Code), refers to the International Residential Code (2009) and the Maine Plumbing Code, which require that plumbing fixtures shall be provided with backflow prevention.

New homes obviously must be provided with backflow prevention. However, if the home is on a private well, this is not an issue. Water supply backflow preventers are only needed on public water supply systems.

What about existing homes?

With the new code in place and the concerns presented by AWWA, many communities began to require backflow preventers in all homes on their water systems, regardless of when they were built. A couple of years ago, some municipalities in the area began to send out letters to the residents, requiring installing a backflow preventer on existing homes.

This includes Bangor, Brewer, and Hampden. For more information on a typical system requirement, see Bangor’s program here. Unfortunately, not all water districts in all communities have instituted this requirement.

For a home inspection, how do we handle this?

On any home on a public water system, Perkins Home Services, LLC, checks to see if there is a backflow preventer in place, and we note on our report if verified or if it is lacking. If there is no backflow preventer, we suggest that one be installed, even if the water district in that area does not require it. Chances are, shortly, they will, and the new owner should be aware that this may be a future cost for them.

How much does it cost?

Installing a backflow preventer is relatively inexpensive, usually in the neighborhood of $150 – $200.


The installation of backflow preventers helps guarantee that all of us on public water have safe drinking water.

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