A pair of Brink Renovent HR 250/325 Medium / Large (Without Bypass) G4 Replacement Filters. 531101 535001 Size - 237mm x 415mm It is recommended filters are changed every 6-12 months. They can also be gently vacuum cleaned to extend the life. These filters are manufactured, in the UK, as replacement filters. They are in no way associated with the MVHR air system manufacturer. The use of manufacturer names is solely for the purposes of identifying the correct filters.