I buy them to make my Wife a Home Air Cleaner using a box fan and 4 filters. Set the Fan on top. They work great as over time they will darken with all the dust and pollutants pulled from the air in our house!
I have Carrier -6 seer furnace. I use this filter for last 2 years. Buy 4 at a time and replace every 6 months. Fit, Form and Function is great. I am systems engineer.