How often should carbon filters (Can Fans) be replaced? People I have talked to say every 12 months, but I've had mine for years and still no smell, by the way I use mine in a sealed room to scrub (clean) the air. My rooms stay clean, but just curious as to how long they are known to last? I have Can 150s which are the big boys and they are heavy and a pain in the ass to move. Not to mention this is a costly but must for a smell and air control in a commercial set up.