A combination of a testing device and an automatic dosing device, incorporating: a testing device (10, 10', 10") for testing the level of a selected chemical in central heating system water in a central heating system circuit, the testing device comprising: a sample chamber for holding a sample of central heating system water to be tested, the sample chamber being adapted to connect to the central heating system circuit to allow fluid to pass between the central heating system circuit and the sample chamber; 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 by returning the central heating system water to the central heating system circuit; at least one valve (18, 18') for isolating the sample of central heating system water from the central heating system 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; and an automatic dosing device (50, 70, 90) for dosing central heating system water with the selected chemical, the dosing device (50, 60, 90) including a fitment (52, 72, 92) for permanently attaching the dosing device (50, 70, 90) into the central heating system circuit, a chemical reservoir (54, 74, 94), a passageway (56, 76, 96) in the fitment between the central heating system circuit and the chemical reservoir (54, 74, 94), a valve (58, 78, 98) in the passageway (56, 76, 96), and a means for introducing chemical from the chemical reservoir into the central heating system circuit, in which the testing device (10, 10', 10") is adapted to send an electronic signal to the dosing device (50, 70, 90) to cause the dosing device (50, 70, 90) to dose the central heating system circuit with chemical in response to a measurement on the testing device (10, 10', 10") indicating an insufficient chemical concentration. a sample chamber for holding a sample of central heating system water to be tested, the sample chamber being adapted to connect to the central heating system circuit to allow fluid to pass between the central heating system circuit and the sample chamber; 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 by returning the central heating system water to the central heating system circuit; at least one valve (18, 18') for isolating the sample of central heating system water from the central heating system 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; and an automatic dosing device (50, 70, 90) for dosing central heating system water with the selected chemical, the dosing device (50, 60, 90) including a fitment (52, 72, 92) for permanently attaching the dosing device (50, 70, 90) into the central heating system circuit, a chemical reservoir (54, 74, 94), a passageway (56, 76, 96) in the fitment between the central heating system circuit and the chemical reservoir (54, 74, 94), a valve (58, 78, 98) in the passageway (56, 76, 96), and a means for introducing chemical from the chemical reservoir into the central heating system circuit, in which the testing device (10, 10', 10") is adapted to send an electronic signal to the dosing device (50, 70, 90) to cause the dosing device (50, 70, 90) to dose the central heating system circuit with chemical in response to a measurement on the testing device (10, 10', 10") indicating an insufficient chemical concentration.