The primary function of the pre-filter is to trap larger dust and particles. These allows the primary filtration to do its intended job easier. The pre-filter in the EverClear CM-11 air cleaners is washable, so you'll need to set up an schedule to keep the pre-filter clean.
The exact maintenance interval is determined by the specific application of the EverClear unit. In an office application, the prefilter could require cleaning every 2-3 months. Because of the different variables with each application, it is recommended that the prefilter be inspected every two weeks during the first couple of months of operation. When there is a noticeable accumulation of dust and dirt, clean the filter.