A fluid system is provided comprising: a heater having an inlet and an outlet; a storage vessel; storage vessel heating means for heating the fluid in the storage vessel; a mixing valve having a first inlet for receiving fluid to be heated from a fluid supply, a second inlet for receiving fluid from the storage vessel, and an outlet for supplying fluid to the inlet of the heater; and a controller wherein the controller is arranged to monitor the heater's performance and to operate the mixing valve to blend the fluid from the fluid supply with fluid from the storage vessel, for example, when a demand on the heater exceeds a threshold value.