Abstract: A waterwall heat transfer system has a tube block secured to a tube assembly. The tube assembly includes a plurality of parallel tubes connected together by a membrane. The tube block has a base section and a plurality of spaced ridges extending upward from the base section, the upper surface of at least one of the spaced ridges defining a generally horizontal surface. The ridges are spaced to define channels therebetween with the height of at least one of the ridges selected to provide a seat for the membrane of the assembly.