What is a Backflow Valve and why would you want one?

What is a Backflow Valve?

A Backflow Valve or backflow preventer is a device that can be installed on your property’s water and sewer pipes that only allows water/sewage to flow in one direction. Its job is to prevent “backflow” which is when contaminated water or sewage flows back into your property’s pipes rather than away as it is designed to do normally.

Why would you want a Backflow Valve installed?


You may be surprised to know that your sewer line can backup into your home even if you are not actively using it. We had a client who was on vacation when the sewer line backed up from the main sewer in the street. This created an awful mess and an unwelcome expense. There was extensive damage inside the home as well as the furnishings. A Backflow Valve will prevent this type of scenario.

Please note that not all homes will not need a Backflow Valve, but some do. Per the City of San Jose, “A building needs backwater valve protection if the floor elevation of the lowest plumbing fixture in the building is less than one foot above the next upstream manhole cover of the public or private sewer system.” See Preventing Sewer Backups, City of San Jose.


Backflow valves for water service are a good idea on properties where there is potential for cross connection or backflow into the drinking water supply. In some cases, they will be required by your City, County, or Water provider.

Possible water contamination can come from any source or system containing non-potable water or other substances. Some examples are: irrigation pipes, wells, creeks, and even your garden hose has the potential for water to flow back into your home (see tip below). Not all homes needs a backflow valve but if you have any concerns, you can call your water utility company.

Note/Tip: Do not attach an insecticide or fertilizer sprayer to your garden hose unless you also have a backflow preventer installed in line with the hose.

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