Today only, March 28th, you can save 35% on these Arm & Hammer Enhanced Allergy & Odor Control Air Filters 4-Pack. Get a set for just $21.99! (regularly $33.99) That is just $5.50 each! The easiest way to take care of your furnace is to replace the air filter regularly. It is recommended to change your filter at least every 90 days. That means this 4-pack will last you an entire year, assuming you only have one furnace. Plus, this order will qualify for free delivery.