This paper presents an intensive experimental-numerical study of heat flow in a saturated porous domain. A temperature and a flow rate range compared to that existing in a typical deep low-enthalpy hydrothermal system is studied. Two main issues are examined: the effect of fluid density and viscosity on heat flow, and the significance and effect of thermal dispersion. Laboratory experiments on a saturated sand layer surrounded by two impermeable clay layers, subjected to different flow rates under cold and hot injection scenarios, and for both vertical and horizontal flow directions, are conducted. Atemperature range between 20°C and 60°C is studied. The finite element method is utilized to analyze the experimental results. Backcalculations, comparing the numerical results to the experimental results, are conducted to quantify the magnitude of thermal dispersion. A constitutive model describing thermal dispersion in terms of fluid density, viscosity and pore geometry, taking into consideration different injection scenarios, is developed. This study demonstrates the importance of taking the variation of formation water density and viscosity with temperature into consideration in predicting the lifetime of deep low-enthalpy geothermal systems. It shows that if ignored, the lifetime of a system with hot injection will be overestimated, and that with cold injection, will be underestimated.