One of the most common questions we get is "What MERV Rating should I use?" and for commercial HVAC air filters this question is one of those easy questions with a semi-complicated answer. There are a lot of variables when selecting a MERV rating, and in some cases looking only at MERV may not be enough....
Understanding MERV: There are some inherent flaws with the MERV rating system, which is discussed more in depth in our article "Why ISO 16890 is the Future for HVAC Filters" but here is a quick summary. ASHRAE 52.2 created MERV as a way to systemically test air filter particle removal at different particle sizes - and that's it. There is no correlation of the particle sizes tested and what the filter is being challanged with in real life. What this means is you get filters with different values that have very little difference in real-world IAQ/impact. Functionally in real world general HVAC filtration there is no discernable difference between MERV-8, 9, 10, and 11. As always there are exceptions to this general rule, but outside of those specific applications your building won't notice a difference between 8 & 11. Here's a chart that takes the count of particles that are in ambient air and how much two of the most common filter efficiencies actually remove:
Goal of Filtration: With that understading, we hvae to determine what your goal for air filtration is; which fall into two basic catagories: 1) Protection of HVAC Equipment or 2) Provide Clean Indoor Air Quality. For equipment protection, the most important things we are protecting are energy recovery devices (like heat wheels, enthalpic cores) and the heating and cooling coils. These parts of your system have been tested to filter the air at approximately MERV-5, so the general recommendation by ASHRAE and other organizations involved in that testing have said using a MERV Rating of 6 or higher will keep your equipment reasonably clean.
IAQ Focus: When trying to boost the Indoor Air Quality there are a couple more variables that need to be addressed, but the general rule is using a minimum of MERV-13. The reason that MERV-13 is the starting point is based on what it takes out of typical ambient air, it is at this efficency that just over 50% of all airborne particles are removed from the air. Increasing to 14, the single pass efficiency of all particles is increased to 75% and even though we are asked a lot about HEPA filters there is almost never a need for a real HEPA filter in general HVAC applications. In urban areas a carbon filter is usually beneficial as it will remove the molecular pollutants most commonly found in city environments like Ozone, Nitrogen Dioxide, and Sulfer Dioxide.
Your Equipment May Hold You Back: Now that you hopefully have a better idea of what you want to do with air filtration in your facility it is important to note that you may be limited to what is possible based on the design of your HVAC Equipment. When buildings are designed and equipment is selected the air filter section is often overlooked which means it is an easy place for the companies involved to go cheap and "save money" to offer a lower bid in hopes of winning the job. This often leads to only MERV-8 filters being able to be used unless an overhaul of the filter section is done.