Nuaire Drimaster filters should be replaced every 3 to 5 years. If your unit is near a main road / high pollen area you may want to change more often. These replacement filters will fit all models of Drimaster manufactured after 2001. If you notice that your Nuaire Drimaster is not performing as effectively as when first installed - Condensation or mould reoccurring, this is an indication that the filters could be blocked or restricted. Therefore the unit supply of fresh replacement air is reduced and will become less effective - Service your unit and replace the filter. PLEASE NOTE - Product dimensions are approximate. These are direct replacements for the Nuaire filters supplied with the Drimaster units.