A testing device for testing the level of a selected chemical in central heating system water in a central heating system circuit comprises: a sample chamber for holding a sample of central heating system water, the sample chamber being connected to the central heating system circuit; means for controlling filling of the sample chamber with central heating system water from the central heating system circuit, and emptying of the sample chamber; at least one valve for isolating the sample of central heating system water from the heating circuit during testing; and optical testing apparatus including a light source and a detector, for measuring an optical property of the sample of central heating system water isolated within the sample chamber and thereby making a determination as to whether or not the level of the selected chemical in the water is greater than a predetermined threshold level.