The built-in-storage type of solar water heater will be suitable for rural as well as urban areas of Vietnam, in general, and the Hanoi region, in particular (mainly for domestic purposes). In this paper, a thermal model for optimisation of the design parameters and estimation of the effect of various parameters on the thermal performance of this solar water healer is presented. The paper also presents the numerical computations with the climatic conditions of the Hanoi region for three cases: (i) using a horizontal collector; (ii) using a sloped collector without reflector; and (iii) using a sloped collector with reflectors. On the basis of the numerical computations, conclusions for the application of this solar water heater in Vietnam have been made.