I have lived in our home for over 12 years. It has a Lennox heat pump with an AprilAire filter in the return line just above the inside heat exchanger. Originally it used loose pleated paper filters with a choice of particle removal efficiencies. We always had the one down from the best. I think it was a #9. There was a #11 too. We always had a lot of dust on our hardwood furniture and my wife was always dusting. So I went up to #11 to try and slow down the dust. No luck. Then I watched as the filter was being changed. I noticed that there was a layer of dust over the paper but it was much heavier along the edges. The only cause I could figure for that was that as the filter got tighter with dust it would start to push around the ends and carry dust with it. I complained to the service man and he tried to be sure it was as tight as possible. Still no luck. Then AprilAire came out with a box type filter that fit our unit and I had that installed. Still have dust. My conclusion is that the unit itself leaks too badly to be effective. I am a retired scientist and I used a condensation nuclei counter at work to measure particle densities in air. When I asked the service man if he had one so we could measure the dust levels on both sides of the filter, he had never heard of one. My only hope is the next heat pump will have a different kind of filter.