The distribution of temperature according to the height in the heat storage stratified water tanks designed for the heating of premises is uneven. In order to determine possible thermal energy amount stored in the tank it is necessary to know the residual water temperature in the tank. In the experiments with vertical thermal energy storage tanks of 0.31, 0.9 and 1.5 m diameter, we determined that at a duration of charging of 8 h, and discharging of 16 h, the value of temperature transitional zone height at 90% probability is 0.47 +/- 0.03 m. Using analytical methods we worked out carry-overs for the determination of residual temperature of heat accumulating tanks with external and internal heat sources. It was determined that in the heat storage tanks with internal heat source fitted at the bottom of the tank the residual water temperature is higher, and the amount of accumulated thermal energy is lower than in the tanks with external heat source.