Even though furnace filters is a seasonal item, there are still enough searches during the spring and summer months to be profitable all year round. I would suggest targeting the keywords "furnace filter" from and SEO prospective, because you will have an easier time getting your site to rank well due to the lack of competition.
All AdSense niches that I profile will use the following rating system that describes the average payout per click:
- $------ $.01 to $.25
- $$----- $.26 to $.50
- $$$---- $.51 to $.75
- $$$$--- $.75 to $1.00
- $$$$$-- $1.00+
Your results should be very similiar to those posted here, but your results may vary due to competition since this post was posted (MFA sites), smart pricing on your AdSense account, and or bad traffic sources.