Manufacturers have tightened down on who they will allow to install their product. All manufacturers say that in order to have a valid warranty, the installer MUST be a licensed plumber. It is important to clarify this point. Anyone can become a licensed plumbing contractor. However, to become a licensed PLUMBER, one must do the apprenticeship, and then pass a test to get the plumbing certificate. You must be a licensed PLUMBER to do plumbing for hire in Washington.