A method for producing a supply of a heated fluid, which method comprises passing the fluid through a heat exchanger unit where it is heated by a heat source; wherein the heat source derives heat from the exothermic reaction of two or more chemical reactants; the heat exchanger unit comprising: (a) a heat exchanger element through which the fluid can flow; (b) a reaction chamber having at least one inlet (7, 9) through which reactants can be introduced into the reaction chamber, and at least one outlet through which spent reactant can be removed from the reaction chamber; (c) a first dosing unit for introducing a controlled amount of a first reactant through an inlet into the reaction chamber; (d) a second dosing unit for introducing a controlled amount of a second reactant through an inlet into the reaction chamber; wherein the first and second reactants react exothermically and the heat thereby produced is exchanged with the fluid passing through the heat exchanger element, the introduction of the first and second reactants into the reaction chamber being controlled to produce a required level of heating; (e) sensors (13, 15) which in use monitor the rates of flow of the first and second reactants into the reaction chamber; (f) one or more sensors for measuring the temperature of the fluid; (g) one or more reaction monitoring sensors for monitoring the extent of reaction between the reactants; and (h) a controller operatively linked to the one or more temperature-measuring sensors, the one or more reaction monitoring sensors and the sensors (13, 15) for monitoring the flow of the first and second reactants into the reaction chamber so as to produce a required level of heating of the fluid, and for controlling the flow of spent reactant out of the reaction chamber. (a) a heat exchanger element through which the fluid can flow; (b) a reaction chamber having at least one inlet (7, 9) through which reactants can be introduced into the reaction chamber, and at least one outlet through which spent reactant can be removed from the reaction chamber; (c) a first dosing unit for introducing a controlled amount of a first reactant through an inlet into the reaction chamber; (d) a second dosing unit for introducing a controlled amount of a second reactant through an inlet into the reaction chamber; wherein the first and second reactants react exothermically and the heat thereby produced is exchanged with the fluid passing through the heat exchanger element, the introduction of the first and second reactants into the reaction chamber being controlled to produce a required level of heating; (e) sensors (13, 15) which in use monitor the rates of flow of the first and second reactants into the reaction chamber; (f) one or more sensors for measuring the temperature of the fluid; (g) one or more reaction monitoring sensors for monitoring the extent of reaction between the reactants; and (h) a controller operatively linked to the one or more temperature-measuring sensors, the one or more reaction monitoring sensors and the sensors (13, 15) for monitoring the flow of the first and second reactants into the reaction chamber so as to produce a required level of heating of the fluid, and for controlling the flow of spent reactant out of the reaction chamber.