An air filter assembly has a fan, pre-filter, a main filter, and a control circuit which generates various signals to the user as to when the pre-filter and the main filter should be replaced. The control circuit detects when the pre-filter should be replaced from two conditions: a first pre-filter replacement condition based decreased air flow, and a second pre-filter replacement condition based on cumulative operating time. The control circuit detects when the main filter should be replaced based on a formula containing a number of the occurrences of the first pre-filter replacement condition and a number of occurrences of the second pre-filter replacement condition. The control circuit also detects a main filter replacement condition based on cumulative operating time of main filter. The main filter can have a fuse that is blown when the main filter should be replaced.