Want an easy way to choose the best filter for an HVAC system? Consider three factors: size, filtration efficiency and resistance to airflow. Choosing an air filter that is sized correctly, with an appropriate minimum efficiency reporting value (MERV) rating and reasonable pressure drop across the coil, will protect the HVAC equipment and improve indoor air quality.

Size: Furnace filters come in many sizes. Select a filter with the correct width, height and depth. All three are critical to ensure proper fit and prevent unfiltered air from bypassing the filter.

Filtration efficiency: Air filters improve indoor air quality, but their primary purpose is protecting HVAC equipment from dust, animal dander and grime. In systems with coils in the airstream (heat pumps, air conditioners and condensing gas furnaces), proper air filtration improves performance and prevents equipment damage. MERV is the accepted standard for measuring filtration performance, and is a component of the ASHRAE 52.2 standard. The rating, which ranges from one to 16 for residential applications, measures a filter’s ability to remove particles from 0.3 to 10 microns in size. (Human hair typically measures 50 to 100 microns.) The higher the rating, the greater the filtration capability.

Resistance to Airflow: The rule of resistance to airflow is the higher the filter’s MERV rating, the greater its airflow resistance, and the greater the airflow resistance, the less air moved by the HVAC fan. When airflow becomes too low, efficiency drops and equipment can be damaged. A filter’s resistance to airflow is measured in pressure drop across the filter in inches of water column (IWC). Unless the entire system is designed for filters with a large pressure drop, it is good practice to use filters with pressure drops of less than 0.10 IWC. In almost all cases this will limit your choices to filters with a MERV rating of less than eight. Don’t assume that a filter with a MERV of eight will have pressure drop of less than 0.10 IWC. Many hardware store filters have pressure drops in excess of 0.2 IWC, and are not suitable for residential equipment.