Recent advanced technologies in boiler manufacturing bring about highly efficient and compact water-tube boilers for gas firing or oil firing. These boilers have very simple water-circulation system, which often brings about flow instabilities followed by critical heat flux, CHF, problems. This paper describes such an oscillatory flow (CHF) at relatively low mass flux and at low pressure. CHF is a decreasing function of the amplitude and period of the flow oscillation, and reaches almost 40 % of the steady state value. When the tube wall is thin, CHF is mainly controlled by the two-phase flow dynamics and is well simulated using a simple lumped-parameter model of boiling channel. At very low mass flux, CHF depends significantly on the how orientation, and is closely related to the flow pattern transition.