When it comes to selecting an air filter, the MERV rating is an important factor to consider. MERV stands for Minimum Efficiency Reporting Value and is a measure of the filter's ability to capture particles of different sizes. Generally, a filter with a higher MERV rating will reduce airflow, but there are many other factors at play, such as the size of the filter and the type of fan motor in your HVAC system. Research shows that, in general, HVAC systems with high MERV filters have a greater pressure drop across the entire filter. Using an air filter with a MERV rating that is too high is just as bad as using one that is too low.
Air filters with higher MERV ratings can filter more, but the thickness of the filter material can restrict airflow. Restricted airflow can decrease comfort, increase energy use, and accelerate wear and tear on HVAC components. In particular, using an air cleaner with a MERV rating that is too high can damage the compressor, heat exchanger, and air conditioner coil. The filter area and pressure drop analysis illustrates how HVAC experts are considering all parameters that can be adjusted, without driving costs to unacceptable levels. Since using an air filter is not optional, you will need to find a filter that meets both your air filtration needs and your airflow requirements.
The MERV classification recommendations are based on outdoor concentrations of particulate air pollutants (PM2), averaged in 100 of the world's most populous cities. Most homeowners don't take into account the reduced airflow that comes with using a high MERV filter. MERV 8 air filters are very common because they are quite a complete option, so you might think that they are excellent in price but poor in performance. Using an oven filter prevents accumulation of dust, hair, and other particles from entering the blower fan. For example, general filtration with a MERV of 8 will filter down to approximately 10 microns, whereas a 13 MERV filter will have only approximately 0.3 microns of passage. One thing to keep in mind is that a MERV 11 air filter may need to be changed a little more frequently than a MERV 8 air filter. Through this site and the trails it led me along, I have learned that two MERV 11 filters can have the same filtering capacity, but one could allow more air to be transferred.
However, considering the reduction in airflow caused by a high MERV filter is often an afterthought for most homeowners. While ASHRAE recommends MERV 13 and 14, it's best to select a filter with the highest possible MERV rating for your specific HVAC system. They are similar to fiberglass filters, but have a higher resistance to airflow and a better ability to stop dust. It's best to comply with oven manufacturer's recommendations or consult an HVAC professional to determine exactly which MERV rating is best for your specific system.
