A circuit for filtering a liquid, comprising a storage vessel (10), a pump (17), a consumer device (14), a filter element (23) and a controller (24). The circuit maybe used in systems where liquid, e.g. water, oil or fuel, is filtered and at least part thereof is returned to the storage vessel. Liquid is pumped in the circuit from the storage vessel by the pump through lines (11, 13, 15, 16), filtered by filter (23) and supplied to the consumer device. Excess liquid not required by the consumer device (14) is returned via return line (15) to the controller (24) which directs the liquid either to the filter or the storage vessel—depending on liquid temperature. The controller (24) and the filter are arranged in a common housing (18) having connectors for an unfiltered liquid inlet (19), a filtered liquid outlet (20), a liquid return (21) and a liquid outlet (22). The connectors (19–22) enable rapid assembly of the circuit.