First of all, I do not understand why bathroom showers are designed NOT to have separate water line to be shut off and turn on. Second, for changing shower facuset or cartridge (moen), I know I must turn off main water valve.. (correct?).. My concern is ... What if there is something wrong during the installation, inproper insert, etc. So, as soon as I turn back on the Main water line, water may come splashing. Because of that, the whole house will be out of water. Of course, that is my speculation, I really want to know if there is a way to ensure that the rest of the house still have water supply even if that replacement failed.