The Razaire MINI can still be used as a fan to draw air away from your burning surface even if the filters are removed or no longer effective. The Primary Filter should be replaced every other time you replace the Prefilter. This works out to roughly 50-100 hours between Primary Filter changes. This is a basic guideline only. You may find that you want (or need) to replace filters more frequently or less frequently, depending on how effective you feel the filter is at removing smoke while you are burning.