Water should only ever flow in one direction into your home and never in the other direction. When excess water causes a pressure shift in your pipe, water can start to move backwards and flow into your main water supply. That poses the health risk of your drinking water being contaminated.
John the plumber Ottawa here. One of the questions we are asked often is what is a backflow preventer and do I need one in my home? This is a question that most homeowners will need a bit of assistance with, and we are here to help.