When it comes to furnaces, the type and configuration of the cabinet will determine how many filters are necessary. For instance, a vertical HVAC unit with downward-moving air usually requires two filters. The cabinet may be designed to accept either a one- or two-inch filter, and the side rails may need to be removed in order to insert a two-inch size. In general, a single high-quality Merv 8 filter is usually enough.
However, it is important to consider the location of the closet and how air flows through the filter. The primary purpose of standard replaceable filters is to keep dust and other airborne particles out of the furnace air ducts and coils. When selecting a filter for your furnace, it is important to consider the size of the filter, the type of filter, and the airflow of your system. If you have a vertical HVAC unit with downward-moving air, you may need two filters.
If you have a horizontal unit with side-to-side airflow, you may only need one filter. Additionally, if you live in an area with high levels of dust or other airborne particles, you may want to consider using a higher quality filter such as a Merv 11 or Merv 13. It is also important to check your furnace manufacturer's instructions for specific recommendations on filter size and type. If you are unsure about which type of filter is best for your furnace, it is best to consult a professional HVAC technician for advice.