Many HVAC systems will include at least one filter bank. This is called the primary filter, and the efficiency (rating) of these filters depends on the space the HVAC unit is serving. In a standard AHU configuration, the primary filter bank sits upstream of the coils and the supply air fan. They help filter dirt, dust, debris, and other contaminants before the air is conditioned and supplied to the space.
Other HVAC systems will also have a secondary filter bank installed. This is commonly called the “final” filter. Unlike the primary filter, the final filter often sits downstream of the coils and the supply air fan. Its purpose is to catch what the primary filter couldn’t, which is often smaller particulates and even pathogens.
It is not uncommon to see HEPA filters installed in final filter banks for critical spaces, like protective environments or operating rooms.