This study describes the characteristics of a panel geometry obtained by a novel manufacturing process for the development of ultra-compact flue gas-water condensing heat exchangers. In this process two stainless steel sheets are prepared and stacked and then brazed together in a special pattern outlining a desired configuration of water ducts and connecting manifolds. By injecting pressurized fluid to the clearance between the bonded sheets, the sheets are deformed and the enlarged clearance becomes a flow pass of water. By stacking several such bonded panels together, the clearances between them work as flue-gas passages. The principle of using this process is that panels can be produced more economically and with sufficient control over the parameters of the obtained geometry as well as the strength. Because of this manufacturing process, the cross-section of the water duct obtains a special noncircular shape, so the heat transfer characteristics must be specifically assessed for the new geometry, in order to qualify the new design. Through CFD analysis, we evaluate the heat transfer performance of the water duct in the novel heat exchanger by comparing with the heat transfer performance of a standard circular duct which has the same hydrodynamic diameter as the investigated water duct.