The answer to 'what is backflow preventer' is a device that is installed in a plumbing system to prevent the reverse flow of water. Backflow preventers are essential to protect drinking water supplies from contamination by preventing water from flowing back into the public water system or a home's plumbing. They are typically installed where the public water supply connects to a home or building, and are required by building codes in many areas.
Backflow preventers work by allowing water to flow in only one direction, from the public water supply into the home or building. If the water pressure in the home drops or there is a loss of pressure in the public water system, the backflow preventer automatically closes to prevent contaminated water from being drawn back into the clean water supply.