Check valves can be installed on hot and cold water supply lines (even in pipes under the floor or behind walls). Most plumbers will recommend installing check valves on both hot and cold water supply lines where damage could occur. If your home is connected to a well with an air chamber, you might not need more than one check valve. But make sure that if there is already a check valve installed, it's working properly first; before installing another one. Make sure any check valve you install is designed for indoor plumbing use: They're made of plastic or brass. Brass ones are more durable, but they cost more too.