It can be confusing when you look at all the types of AC filters on the market. There are different filtering media and it can be hard to compare one filter to another. The American Society of Heating , Refrigerating and Air-Conditioning Engineers (ASHRAE) has tried to make it easier by developing a test standard to quantify the efficiency of filters.
The ASHRAE standard measured the filters ability to remove airborne particles in the range of 0.3 and 10 microns. Based on the ability of the filtering media to stop particles at three different sizes, (0.3 to 1 micron, 1 to 3 microns, and 3 to 10 microns) a Minimum Efficiency Reporting Value (MERV) is assigned to the filter. The scale goes from 1 to 16 and is designed to represent the worst case performance of a filter and is a non-linear scale. As the MERV number increases so does pressure drop. This means the air flow is reduced throughout the system; and it must work harder and moves less air.
Most homes need a filter with a MERV rating in the medium range of no less than 5 but no more than 8. This medium efficiency level captures particles such as:
- dust mites
- cockroach debris
- sanding dust
- textile fibers
- carpet fibers
- mold, spores
- dust mite debris
- cat and dog dander and aerosol sprays such as hair spray, fabric protector and dusting aids.
You might think that a higher MERV rating would be better, but that is not the case. Remember that the higher the MERV rating, the smaller the pore size for air to flow through the filter. This creates more resistance in airflow making it inefficient. Reducing the airflow in your system can actually make the air quality in your home worse and put too much pressure on the fan in your AC damaging amount of pressure on the fan of your system.