Abstract: A check valve sub assembly for series connection in a dual pipe string that extends downhole in a borehole for permitting flow of fluid in one direction through the annular passageway of the dual pipe string and preventing flow of fluid in the other direction. The check valve sub assembly includes an inner tube and an outer annular member for connection into series relationship respective to the inner and outer dual pipe so that fluid flow through the inner pipe is conducted through said inner tube and is maintained separate from the annulus.