A control system for one or more ablutionary devices (1;6), comprising: a mixer valve having a first inlet and a second inlet configured to receive a supply of hot and cold water, and an outlet configured to output cold water, hot water or a mixture thereof as an output stream for supplying water to the one or more ablutionary devices (1;6) downstream of the mixer valve; a flow rate sensor arranged to generate a flow rate signal indicative of the rate of flow of water flowing in the output stream; and a controller configured to: control the mixer valve during use according to one or more target output water properties of the output stream, the one or more target output water properties being set by a user of the one or more ablutionary devices (1;6); obtain one or more current operating signals related to the flow rate of the output stream produced by the mixer valve, wherein the one or more current operating signals include a current flow rate signal obtained from the flow rate sensor and the corresponding historical operating signal is a historical flow rate signal previously obtained from the flow rate sensor; compare each of the one or more current operating signals to a corresponding historical operating signal, the historical operating signal being an operating signal previously obtained by the controller; and generate a diagnostic signal based on the comparison. a mixer valve having a first inlet and a second inlet configured to receive a supply of hot and cold water, and an outlet configured to output cold water, hot water or a mixture thereof as an output stream for supplying water to the one or more ablutionary devices (1;6) downstream of the mixer valve; a flow rate sensor arranged to generate a flow rate signal indicative of the rate of flow of water flowing in the output stream; and a controller configured to: control the mixer valve during use according to one or more target output water properties of the output stream, the one or more target output water properties being set by a user of the one or more ablutionary devices (1;6); obtain one or more current operating signals related to the flow rate of the output stream produced by the mixer valve, wherein the one or more current operating signals include a current flow rate signal obtained from the flow rate sensor and the corresponding historical operating signal is a historical flow rate signal previously obtained from the flow rate sensor; compare each of the one or more current operating signals to a corresponding historical operating signal, the historical operating signal being an operating signal previously obtained by the controller; and generate a diagnostic signal based on the comparison. control the mixer valve during use according to one or more target output water properties of the output stream, the one or more target output water properties being set by a user of the one or more ablutionary devices (1;6); obtain one or more current operating signals related to the flow rate of the output stream produced by the mixer valve, wherein the one or more current operating signals include a current flow rate signal obtained from the flow rate sensor and the corresponding historical operating signal is a historical flow rate signal previously obtained from the flow rate sensor; compare each of the one or more current operating signals to a corresponding historical operating signal, the historical operating signal being an operating signal previously obtained by the controller; and generate a diagnostic signal based on the comparison.