A novel heat exchanger architecture was investigated, in which each fluid pathway transitions through the heat exchanger from tube-flow to shell-flow, or from shell-flow to tube-flow. The heat exchangers (HX) consisted of two plenums filled with tube bundles and a solid core of Counterflow channels Sandwiched between Plenums (CSP). To allow a high packing in the core region, a non-circular, e.g., triangular, cross-section tube shape was employed. In this exploratory study of the proposed CSP HXs, prototypes were fabricated by Laser Powder Bed Fusion additive manufacturing. Several challenges posed by the laser-powder-bed fabrication were presented and design solutions for additive manufacturing were discussed. Two different CSP HX were 3D printed, one in which the core region was placed midway between the end plates in the HX, and another one in which the core region was offset toward one end of the HX. The performance of the CSP HXs was evaluated against an off-the-shelf 5 kW shell-and-tube heat exchanger, considered as the baseline heat exchanger. Each of the baseline HX and six CSP HXs configurations were tested at three different flow rates for R245fa (cold fluid) while inlet conditions for the water (hot fluid) were not changed, for a total of 21 tests. It was found that the heat load for three CSP HXs configurations shows excellent thermal performance as compared to that of the baseline HX. The overall heat transfer coefficient for two CSP HX configurations was higher than that of the baseline by 16 to 32%. Although the proposed HXs did not outperform the commercial baseline HX in all aspects, the reported thermohydraulic performance in this first embodiment of the proposed HX concept indicated that the CSP concept offers a new path towards more efficient and more compact heat exchangers.