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What is Backflow?

Backflow is the unwanted reverse flow of non-potable (dirty) water that contaminates drinking water with things, such as chemicals, pesticides, and human waste due to a sudden or unexpected change in water pressure. This can occur two ways:

  1. ​Back-pressure – This occurs when there is higher pressure in the dirty water system than the water supply, forcing the dirty water back through the water pipes.

  2. Back-siphonage – A significant drop of pressure in a water delivery system creates a suction that pulls possibly contaminated water into the system, e.g. water main breaks.

Double Check Assembly Backflow Preventer

How Do you Prevent Backflow?

Backflow prevention devices keeps water flowing in one direction and prevents contaminated water or chemicals from flowing back into the drinking water supply if there is a sudden or unexpected change in water pressure. They are installed at strategic locations in the plumbing system wherever there is a risk of contaminated fluids entering the water supply pipes.

How Do Backflow Prevention Devices Work?

Double check valves are one of the common backflow prevention devices. They are designed with a series of springs that open and close the chambers based on the water pressure flowing through the system.

Under normal flow, the spring loaded check valves will open to allow water to flow from one end to another. In the diagram example, the clean water supply (left side) flows through and out of the system to the potentially contaminated water (right side).

The valve is made of multiple chambers that reduce the water pressure from one chamber to another as water flows through.

If reverse flow/backflow were to occur and the contaminated water pressure had higher pressure than the previous chamber, then the springs will close the chambers to prevent backflow.

Why is Backflow Testing Needed?

Backflow prevention device can fail. Debris buildup can prevent the springs from closing the chambers, thus allowing the contaminated water to backflow into the drinking water.

Backflow testing is to ensure your backflow device is properly operating. In New York City, the building owner must schedule testing with a certified tester at least once a year and file appropriate forms with Department of Environmental Protection (DEP). For more information if you require backflow testing in New York City, please visit the NYC Environmental Protection Backflow Prevention Outreach Program website.


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