An insert for a condensing heat exchanger, having: a body extending aft from a forward end to an aft end, and defining: a body exterior surface; a forward segment that extends aft from the forward end of the insert to a first axial location between the forward and aft ends of the insert, along the forward segment the body exterior surface is without openings; a middle segment that extends aft from the first axial location to a second axial location, along the middle segment the body exterior surface is cylindrical; and an aft segment that extends aft from the second axial location to the aft end of the insert, along the aft segment the body exterior surface of the body is cylindrical and defines axially extending grooves, and the grooves are spaced apart from each other and extend forward from the aft end of the insert to the middle segment.