In this study, we investigated ways of improving the performance of heat exchangers, which are the heat pumps for use in hot-water supply systems on the hot-water supply side. Therefore, we verified experimentally the pressure drop and the heat transfer characteristics within the coiled flow channel. Five smooth copper tubes and five internally helical-grooved copper tubes with an outside diameter of 12.7mm and coil diameters of 100 mm, 120 mm, 140 mm, 160mm and 180mm were used as the heat-transfer coiled tubes in the experiments. Experiments were conducted under conditions of constant isothermal heating and fluid flow inside coiled tubes with an inlet temperature of $20^{circ}C$, and the flow rates of the fluid flow inside the coiled tubes were adjusted and varied to change the Reynolds number within the range of 900 to 25,000. Based on the experimental values, we proposed prediction equations to systematically calculate the friction factor and the heat transfer coefficient for different curvature radius ratios.