Abstract: In a heat storage device comprising a container (40) which contains a salt hydrate, the heat of fusion of which is utilized to provide high heat capacity in relation to the volume, oil is circulated by a pump (50) through a number of channels (62, 65) leading to outlet openings (55) distributed in the salt hydrate. A slide valve (58, 69), the sliding valve body (58) of which is influenced by the pressure on the delivery side of the pump, automatically controls the distribution of oil to the channels in such a manner that solidification of the salt hydrate during heat discharge from the storage takes place in a regular progressive way so that the oil can always find non-blocked outlet openings to flow through.