A model for water level dynamics in the drum-riser-downcomer loop of a natural circulation drum-type boiler is presented. The model is based on basic conservation rules of mass, momentum, and energy, together with well-known constitutional equations. Steam water mixture in a drum is divided into three sub-volumes; water, steam above and below water level, and a mass balance relation is applied to each sub-volume. The amount of steam under water level is predicted using two constitutional equations for condensation rate and rise velocity of steam. The constitutional equations entail uncertain values such as superficial velocity of water and average size of steam bubbles in a drum. The superficial velocity of water is assumed to be zero and its effect is compensated by a mechanistic model on movement of the water level. Average size of steam bubbles is assumed to be an arbitrary value and its effect on the dynamics is investigated through sensitivity analysis. The model enables one to investigate the water level dynamics for changes in steam demand and/or heating rate simply from basic design values. Simulation results are compared with those in the literature in which an empirical model for the steam bubble dynamics is employed. The presented model shows a reasonable prediction of water level for a change in steam demand. (c) 2005 Elsevier Ltd. All rights reserved.