Your plumbing system may have hundreds of different valves. But knowing where just one is can prevent thousands of dollars of damage in a home. The main water valve controls the flow of all water into your home and must work properly to prevent damage. First, find the valve. It's usually at the lowest level of your home near the street. Next test the valve to see if it turns off. Last, label a large tag with the words main valve and hang it from the valve handle to make sure all members of your household can find it before a flood sets in.