I noticed a while ago that there is a huge difference between the less expensive furnace filters and Filtrete brand. The Filtrete filter picks up the dust/dirt/particles that turn the filter black which indicates (before it is actually black) that it needs to be changed. I check the filter, which has a place to put the date inserted, after 3 months, and, often it can remain a few more weeks. I feel confident that Filtrete filter works, even though it can be a bit costly. Good to buy in bulk and/or on sale.