Abstract: An energy saving device for use in connection with a heat-insulated storage vessel in a domestic hot-water system is disclosed. When there is no demand for hot water, water contained in the vessel is heated in an electric through-flow heater and circulated by gravity, with the heated water rising into the vessel through an outflow pipe of the vessel and the water to be heated being supplied to the heater from the lower portion of the vessel through a jet pump, a vortex diode and a valve which are connected in series between the through-flow heater and a water inlet pipe to the vessel. As soon as a hot water faucet is opened, water from a supply enters the jet pump, and the jet pump causes water to flow through the water inlet pipe into the lower portion of the vessel. At the same time water of medium temperature from the upper portion of the vessel flows through the heater and is brought to high temperature before flowing to the faucet.