I'm in no way trying to put down your field of work, I just think that you're putting a "this is the way and that's all there is to it" point of view to things. Here's another article about pipe sizes. http://www.askthebuilder.com/NH007_-_Properly_Size_Your_Water_Lines.shtml Main line should be at least 3/4" but there can be different sizes branching off. No matter what, PIPE SIZE DOES NOT MATTER TO THE UNIT'S OPERATION.