A furnace is an indoor unit used in most cases where there is a lot of cold weather. An AC Unit is an indoor unit used in hotter climates typically in southern states. All filters are designed to keep your unit clean and your air free of dust, dirt and debris, that passes through your HVAC system. Keeping dust particles out of your unit will improve your indoor air quality along with extending the life of your furnace or AC Unit. Higher quality filters trap smaller particles and help reduce allergens in the air. With the right filter you can improve your health, air quality and life of your HVAC system. It is very important to remember to change your filters regularly (every 4-6 weeks or as needed).