I was really amazed how much of a difference a good air filter will make as far as how good your furnace will run. My furnace atually mal-functioned as far as it wouldn't ignite the burners from what I assume was a clogged filter. A mistake on my part as I couldn't believe how much this filter actually caught and how quick it took. We do have a few cats and I assume between the litter dust and the cat hair the filter was a mess when I took it out. In just 3 months it seemed to be enough to cause this problem! It's amazing how much of a difference such a relatively inexpensive item can change the furnace's efficiency if not its function altogether!