An experimental investigation of the natural convection heat transfer from helical coiled tubes in water was performed. The outside Nusselt number was correlated to the Rayleigh number using different characteristic lengths. The relationship was based on a power law equation. The constants in the equation are presented for each of the different characteristic lengths used. The best correlation was using the total height of the coil as the characteristic length. The developed models were then used to develop a prediction model to predict the outlet temperature of a fluid flowing through a helically coiled heat exchanger, given the inlet temperature, bath temperature, coil dimensions, and fluid flow rate. The predicted outlet temperature was compared to measured values from an experimental setup. The results of the predicted temperatures were close to the experimental values and suggest that the method presented here has promise as a method of predicting outlet temperatures from similarly dimensioned heat exchangers.