The critical heat flux (CHF) phenomenon has been investigated for a stable flow of water in vertical tubes at low pressure and velocity conditions. Experiments have been performed with round tubes with diameters of 0.006 m and 0.0088 m for mass fluxes below 220 kg/m2 s under atmospheric pressure. Experimental results are discussed with special reference to the physical mechanism leading to the CHF at very low flow conditions. Finally a new design correlation is proposed based on the combination of the present data and other CHF data available in the literature. Assessments show reasonable agreement of predictions with experiments.