Most mechanical air systems will have a filter to clean air before it enters the space. The effectiveness of the filter is rated through a system called MERV (minimum efficiency value). A higher MERV value means better filtration, with each increased rating able to remove smaller and smaller particles. Buildings typically use MERV filters rated between 6 and 16. HEPA filters remove particles smaller than those typically captured by a MERV 16 filter. Mechanical systems will often include multiple filters. In some cases, this could include an outside air filter, a pre-filter, and a final filter section. Each of these filters may have a different MERV or HEPA rating. For input to the model, select the single highest level of filtration that is applied to both the outside air and the recirculated (or return) air. Typically, this is located downstream of a mixing section of an air handling unit (indoor or outdoor). The filtration performance rating may be determined from the building automation system (BAS), as built mechanical drawings, equipment specifications, or through the building engineer, operator, or maintenance supervisor.
There may also be instances where another method of air cleaning is used in the ductwork of the mechanical system. An example is the Delos Intellipure 2000IL. The Delos Intellipure 2000IL has been through vigorous performance testing that has been shown to achieve performance at or better than HEPA filtration. Other technologies can be used in-duct, such UVGI, electrostatic precipitation, bi-polar ionization or any number of others. In the instance of these non-MERV options, select a removal efficiency that best matches the selections available based on consultation with the manufacturer or the installing contractor/engineer. ParticleOne advises caution with respect to in-duct air cleaning technology where an understanding of how well they work is unknown or not independently verified. These systems often are brought to market without a universal method of test (MOT) from a trusted authority like ASHRAE. For these products, manufacturers are left to find their own path to demonstrate performance and assure users that they do not produce harmful byproducts. Some of these assessments are scientifically rigorous, while others are not.