I have a Utica tankless system that is fueled via gas. Whenever hot water is run, the water never stays hot for more than a few minutes. Initially the the water will be scolding, and then drop down to 70 degrees. The mixture valve has been replaced. New coils were put in, a new back pressure tank was installed, and $1100.00 later, they plumbers are scratching their heads!! The water still does not maintain temperature. Ideas?? Please help, I can't afford to have them keep doing trial and error on this.