One of the top problems we see in the industry today is the lack of airflow due to a dirty filter. When running an air conditioning unit, a lack of airflow can cause the unit to ice up and not function properly. It is recommended to change your filter every three months. This can reduce allergens as well as increase the efficiency and performance of your system. Most HVAC contractors will write the size of the filter on the exterior of the furnace.
If not, the size is printed on the filter. The filter in this photo is 16×25. You can get filters from any local hardware store or your local HVAC contractor. Many contractors offer annual maintenance programs and will leave extra filters so you can change them. You can also go to www.armstrongair.com and sign up for furnace change reminders.