Changing your furnace filter should be done every 2-3 months. It's a simple task, but often overlooked, especially if you've never done it before. Follow these simple steps below, and you'll soon be a pro at changing your home's furnace filter.
Step 1: Locate Filter and Determine Filter Size
The first step is to figure out what size filter you need. The most common filter size is 20" by 24", but you need to verify what size your furnace requires. The best way to determine your filter size is by pulling out the old filter and looking at the number printed on top. It will include three numbers. Almost all filters are located in the cold air return to the furnace. Some filters are inside the furnace body, but most are located in the duct going to the furnace. 
Step 2: Buy New Filter
Filters can be found at your local hardware store, big box stores such as Target or Walmart, and online. Be sure to purchase the correct size.
Step 3: Change Filter
On the edge of each filter is an arrow that indicates the direction of airflow. The arrow should point towards the furnace when you install it.
If your filter is located in the body of the furnace, open the narrow door on the side of the furnace and remove the old filter. Slide in the new filter with the arrow pointing toward the larger part of the furnace. If your filter is located in the ductwork, as shown in the next photos, slide out the metal slide exposing the old filter, remove it, and slide in the new filter with the arrow pointing in the direction of air flow (toward the furnace).
Step 4: Verify Placement
Verify your new filter is seated in rails made for holding it, or resting on the supports so that it covers the entire cross-section of the return air duct. Regular changes of your furnace filter will keep your house cleaner and make your furnace more efficient saving you energy. To learn more about the ratings of filters and what the different “MERV” and efficiency numbers mean, check out our post about furnace air filters.