An experimental study has been made on steady state natural convection heat transfer from vertical helical coiled tubes. Average heat transfer coefficients were obtained for turbulent natural convection to water. The experiments have been carried out for four coil diameter to tube diameter ratios, for five and ten coil turns, and for five pitch to outer diameter ratios. The data are correlated with the Rayleigh number for two different coil sets. The heat transfer coefficient decreases with coil length for tube diameter d0 = 0.012 m, but increases with coil length for d0 = 0.008 m. A critical D/d0 is obtained for a maximum heat transfer coefficient for tube diameter of 0.012 m with either five or ten coil turns.