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1,257,525. Thermal storage; central heating systems; liquid heaters. ELECTRICITY SUPPLY BOARD and W. J. CAMPBELL. 13 Feb., 1969 [14 Feb., 1968], No. 7861/69. Headings F4A and F4U. A central heating system receives its heat from a liquid thermal storage reservoir provided with a liquid circulation system for maintaining a desired temperature at the output point of the reservoir. As illustrated, the central heating system includes a pump 18 and an atmospheric connection 16, and draws its heat from a heat exchanger 15 located near the bottom of a water reservoir 11 which is heated by a heating element 12. A pump 14 is under the control of a thermostat 21 and, by returning water from the bottom of the reservoir to higher points, causes a downward migration of hot water in the reservoir to maintain a required temperature at the exchanger 15. Part of the water from the pump 14 is returned through a sparge pipe 24 which includes a thermostatic valve 25. The heating element is controlled by a thermostat 22 which is overridden by a pressure limiting switch 23. All but the lower part of the reservoir 11 is provided with thermal insulation 26. Omission of the insulation from the bottom of the reservoir ensures that, under static conditions, the temperature of the heat exchanger 15 does not rise to an excessive level. In a modification, Fig. 2 (not shown), the single reservoir is replaced by an upper and a lower reservoir connected by a pipe, the lower reservoir containing the sparge pipe and the heat exchanger. In another embodiment, Fig. 3 (not shown), water moving from the reservoir to the feedback pump passes through a heat exchanger which is located in a tank forming part of the central heating system.