Air Purifier Spares - AOS2261 Carbon & HEPA Filter Carbon filter to be changed every 6 months, HEPA filter to be changed once a year Price: £43.07 incVAT IN STOCK NOW. FREE MAINLAND UK DELIVERY IN 2-3 WORKING DAYS. This filter set includes 2 x carbon filters and 1 x HEPA filter, giving you a years worth of filters.