Natural convection heat exchangers are valuable devices in solar domestic hot water systems, but they are difficult to model. The output pressures and temperatures they develop are complex functions of the flow and temperatures that they are subjected to, and these latter quantities-which vary over each hour in the year-are themselves dependent on the exchanger outputs. Yet to optimize the exchanger and to be truly confident of its performance, one needs good models which can be embedded into total simulation codes, like WATSUN or TRNSYS. This paper gives a suitable exchanger model and outlines how it can be used to model a total system. The model calls for a set of steady state experiments to be carried out on the exchanger in question, characterizing the exchanger. The experiments produce curves of collector effectiveness and shear pressure drop, both plotted against flow rate. A suitable experimental apparatus is described, and the results obtained for a particular exchanger are reported. Also reported are transient experiments that verify the applicability of the steady state experiments to real, time-varying conditions.