A method of measuring the amount of magnetite captured by a magnetic filter in a central heating system, the magnetic filter including a separation chamber , an inlet to the chamber and an outlet from the chamber, and a magnetic element disposed within the chamber for attracting magnetic particles and removing the magnetic particles from the system water as it flows through the chamber, the magnetic element including a single stack of substantially cylindrical magnetic billets having north and south poles which are nominally on opposing flat faces of the billets, the billets being disposed with like poles facing each other, each billet being disposed on a central longitudinal axis of the magnetic filter, and the method comprising the steps of: providing a magnetometer positioned on the housing at a fixed point relative to the magnetic element, the fixed point being a predetermined distance from an end of the filter, and the magnetometer being of the type in which the output is directly proportional to the magnetic field strength, with a fixed but uncalibrated offset; measuring the output from the magnetometer when the filter is empty of magnetite; monitoring the output of the magnetometer; and characterised by issuing an indication on an output means when the output from the magnetometer exceeds or drops below a predetermined threshold, the predetermined threshold being calculated by adding a fixed offset to the output from the magnetometer measured when the filter is empty of magnetite. the magnetic filter including a separation chamber , an inlet to the chamber and an outlet from the chamber, and a magnetic element disposed within the chamber for attracting magnetic particles and removing the magnetic particles from the system water as it flows through the chamber, the magnetic element including a single stack of substantially cylindrical magnetic billets having north and south poles which are nominally on opposing flat faces of the billets, the billets being disposed with like poles facing each other, each billet being disposed on a central longitudinal axis of the magnetic filter, and the method comprising the steps of: providing a magnetometer positioned on the housing at a fixed point relative to the magnetic element, the fixed point being a predetermined distance from an end of the filter, and the magnetometer being of the type in which the output is directly proportional to the magnetic field strength, with a fixed but uncalibrated offset; measuring the output from the magnetometer when the filter is empty of magnetite; monitoring the output of the magnetometer; and characterised by issuing an indication on an output means when the output from the magnetometer exceeds or drops below a predetermined threshold, the predetermined threshold being calculated by adding a fixed offset to the output from the magnetometer measured when the filter is empty of magnetite. providing a magnetometer positioned on the housing at a fixed point relative to the magnetic element, the fixed point being a predetermined distance from an end of the filter, and the magnetometer being of the type in which the output is directly proportional to the magnetic field strength, with a fixed but uncalibrated offset; measuring the output from the magnetometer when the filter is empty of magnetite; monitoring the output of the magnetometer; and characterised by issuing an indication on an output means when the output from the magnetometer exceeds or drops below a predetermined threshold, the predetermined threshold being calculated by adding a fixed offset to the output from the magnetometer measured when the filter is empty of magnetite. <EOS>