An apparatus for heating a liquid, which apparatus comprises: a mixing chamber; dispensing means for dispensing metered amounts of first and second chemical reactants into the mixing chamber to form a reaction mixture so that the chemical reactants undergo an exothermic chemical reaction to generate heat and one or more reaction products; an electronic control device linked to the dispensing means for controlling the dispensing of the metered amounts of first and second chemical reactants; one or more pumps for moving the chemical reactants and reaction mixture around the apparatus; a heat exchanger having an inlet and an outlet for the reaction mixture and an inlet and an outlet for the said liquid, so that when said liquid passes through the heat exchanger it is heated by heat transfer from the reaction mixture; one or more monitoring stations for monitoring one or more physical or chemical parameters of the reaction mixture; the monitoring stations being arranged to communicate with the electronic control device; and a waste outlet for removing spent reaction mixture from the apparatus; wherein the mixing chamber, heat exchanger and the one or more monitoring stations are connected so as to form a loop; and characterized in that the electronic control device is programmed to cause the reaction mixture to be circulated around the loop at least twice, and optionally to cause the dispensing means to dispense further metered amounts of first and/or second chemical reactants into the mixing chamber; and/or to cause a proportion of the reaction mixture to be ejected through the waste outlet, in order to control the temperature of the reaction mixture passing through the heat exchanger. a mixing chamber; dispensing means for dispensing metered amounts of first and second chemical reactants into the mixing chamber to form a reaction mixture so that the chemical reactants undergo an exothermic chemical reaction to generate heat and one or more reaction products; an electronic control device linked to the dispensing means for controlling the dispensing of the metered amounts of first and second chemical reactants; one or more pumps for moving the chemical reactants and reaction mixture around the apparatus; a heat exchanger having an inlet and an outlet for the reaction mixture and an inlet and an outlet for the said liquid, so that when said liquid passes through the heat exchanger it is heated by heat transfer from the reaction mixture; one or more monitoring stations for monitoring one or more physical or chemical parameters of the reaction mixture; the monitoring stations being arranged to communicate with the electronic control device; and a waste outlet for removing spent reaction mixture from the apparatus; wherein the mixing chamber, heat exchanger and the one or more monitoring stations are connected so as to form a loop; and characterized in that the electronic control device is programmed to cause the reaction mixture to be circulated around the loop at least twice, and optionally to cause the dispensing means to dispense further metered amounts of first and/or second chemical reactants into the mixing chamber; and/or to cause a proportion of the reaction mixture to be ejected through the waste outlet, in order to control the temperature of the reaction mixture passing through the heat exchanger. wherein the mixing chamber, heat exchanger and the one or more monitoring stations are connected so as to form a loop; and characterized in that the electronic control device is programmed to cause the reaction mixture to be circulated around the loop at least twice, and optionally to cause the dispensing means to dispense further metered amounts of first and/or second chemical reactants into the mixing chamber; and/or to cause a proportion of the reaction mixture to be ejected through the waste outlet, in order to control the temperature of the reaction mixture passing through the heat exchanger.