A magnetic filter (10) for a central heating system comprises a filter body (12) and an external magnetic assembly (18). The magnetic assembly is in the form of a belt which is made up of multiple modules. The modules may be identical to each other. The number of modules in the belt can be selected according to the size of the filter body, allowing various different sizes of filter to be produced from a small number of different parts. The filter body may be the body of an air-and-dirt separator, so that an existing air-and-dirt separator can be upgraded to include a magnetic capturing capability.