Abstract: A boiler comprising a long tank lying on its side and having a fire box at one end which is adjacent to a recessed bulkhead that is spaced from a partition in the end of the tank. The top of the tank and two sides thereof extend beyond the bulkhead around the fire box to provide water filled side and top portions for the fire box. The other end of the tank has a rear bulkhead which lies adjacent to a hot gas manifold that extends beyond the rear bulkheads to form a hot gas chamber below a smoke stack. The tank has a number of large diameter tubes extending horizontally between and through the two tank bulkheads to conduct hot gases from the fire box to the manifold chamber and smoke stack.