Natural convection loops can occur when extracting energy from thermal storage with immersed “load-side” heat exchangers. To assist in heat exchanger design and annual performance simulations of such systems, this paper proposes modeling the natural convection loop with a comparatively simple “ersatz” thermosiphon loop (ETL). In a “real” thermosiphon loop, fluid in channels (such as pipes) flows in a closed loop under the influence of a net buoyancy head. The flow/temperature solution is derived by equating buoyancy head to total pressure drop. In the proposed approach, the “ersatz” flow channels are defined for each problem of interest based upon experiment, numerical solution, or other information. As a simplified model, the heat transfer and friction coefficients in the ETL model must be adjusted to fit known case(s). For the case of a horizontal shallow enclosure with temperature boundary conditions at both ends, it is shown that the ETL model adjusted via CFD results is a reasonable surrogate for the actual heat transfer predicted by an analytical solution. Over an order of magnitude variation of the heat transfer, the heat transfer deviates by about 8% on average.