The easiest way to take care of your furnace is to replace the air filter regularly! Walmart has the Filtrete Allergen Defense Micro Particle Furnace Air Filter 3-Pack for just $17.88! (regularly $26.64) That is just $5.96 per filter! It is recommended to change your filter at least every 90 days. That means this 3-pack will last you 9 months. Be sure to select free in-store pickup to avoid shipping fees. Otherwise orders of $35 or more will ship free.