The thermofluid characteristics of Al2O3–water nanofluid in the annulus of double-helical coiled tubes were experimentally and numerically carried out. The purpose was to investigate the effect of combined enhancement techniques of nanofluid and helicoid tube shape on the performance of a double tube heat exchanger. The effects of concentration of nanoparticles, Reynolds number, coil curvature ratio, and flow arrangement through the annulus of double-helical coiled tube were the main points of interest. Three coiled tube heat exchangers were manufactured and experimentally tested to study the design parameters on the performance of such a heat exchanger. A three-dimensional numerical computational fluid dynamic (CFD) model was developed to get additional insights on the thermal performance of double helically coiled tubes with nanofluid on a level of details not always available in experiments. It was found that the Al2O3–water nanofluid achieved an enhancement by 32% on the overall heat transfer coefficient. The heat exchanger effectiveness, heat transfer per unit pumping power, and the Nusselt number were also presented for different design parameters.