Off-peak electric liquid heating system employing regulatable heat pipe

In a liquid heating system for heating a first liquid, said system including a heat store to be heated to a relatively high temperature by off-peak electricity, and said heat store being thermally connected to a vessel containing the first liquid to be heated to a temperature lower than said relatively high temperature, wherein the improvement comprises a controllable heat pipe providing said thermal connection, the heat pipe including an evaporator zone in thermal contact with the heat store and a condenser zone in thermal contact with the vessel, the zones joined by at least one duct, the evaporator zone, condenser zone and the at least one duct being hermetically sealed and containing a predetermined quantity of a volatile second liquid, arranged so that after the second liquid is evaporated in the evaporator zone, it passes through said at least one duct to the condenser zone where it is condensed, and from whence the second liquid returns to the evaporator zone through said at least one duct, the quantity of second liquid being chosen to be sufficiently small so that, in use, the rate of heat transfer from the evaporator zone to the condenser zone is determined by the rate of return flow of the second liquid towards the evaporator zone rather than the rate of flow of the evaporated second liquid towards the condenser zone, or the rate of heat transfer to the evaporator zone or from the condenser zone, and control means being provided directly responsive to the pressure of the evaporated second liquid in said condenser zone for controlling the rate of return flow of the second liquid to the evaporator zone.