An investigation was performed of thermally stratified vertical cylindrical tanks equipped with diffusers and containing either heated or chilled water. Initial tests with a radial diffuser were performed in a scale model tank using flow visualization in addition to measurements of thermal performance. These were followed by thermal performance tests on a prototype 35,300 gal (133 m³) poured concrete tank that was alternately equipped with radial diffusers and an octagonal array of linear diffusers. The data and results demonstrate that tanks equipped with diffusers that produce an inlet Froude number of two or less stratify and perform satisfactorily. Both the scale model and prototype tank tests showed that mixing near the inlet diffuser during thermocline formation decreased significantly as the inlet Reynolds number was reduced; but only the results from the prototype tank were unambiguous in this regard. The scale model tank tests also showed that the velocity distribution in the supply pipe leading to a radial inlet diffuser should be axially symmetric to obtain good tank performance. A test on the scale model tank with inlet water temperature below 39. 2°F (4°C) demonstrated that a physical barrier would be required to maintain good stratification if the inlet water temperature dropped below this value. This paper discusses the tests performed on the scale model tank and the results obtained from these tests. Part II does the same for the prototype tank.