Abstract: A boiler is described comprising a housing with a flue gas inlet for receiving a hot gas flame and an outlet for venting a hot gas produced by the hot gas flame, and a tube assembly mounted within the housing. The tube assembly comprises a lower manifold for receiving a liquid, an upper manifold for receiving a heated version of the liquid; and, a plurality of tubes disposed between the lower and upper manifolds. Each tube is shaped to provide a hot gas pathway that travels in a parallel fashion along at least a lower portion of the longitudinal extent of each tube in the tube assembly to the outlet, whereby the liquid introduced into the lower manifold travels in the plurality of tubes and receives heat from the hot gas along the extent of the hot gas pathway.