The advantages of solar thermosyphon hot water systems in terms of simplicity, cost and performance have long been recognised in domestic hot water heating applications. However, the use of solar thermosyphons in cold regions of the globe is limited due to the lack of effective and reliable freeze protection. One technique for reliable freeze protection is to use a heat exchanger inside or outside the storage tank and an anti-freeze fluid in the collector loop. The performance of vertical and horizontal shell and tube heat exchanger-based storage tanks has been examined extensively. However, there is no published information on the performance of horizontal concentric heat exchangers for such applications. The present paper addresses the basic hydrodynamic and heat transfer phenomena of the flow in the annular gap between the two horizontal concentric cylinders using Fluent CFD code. The computational results include heat transfer rates and descriptions of the temperature and velocity distributions in the annular gap between the cylinders.