The MERV 8 pleated filter is the most commonly used air filter in the United States. Possibly a better way to say this is the 2" deep pleat is the most commonly used, as manufacturers continue to take advantage of the ASHRAE 52.2/MERV testing system and now many of these are labeled as MERV 10 (Which is discussed in our post "MERV 8 and MERV 10 Comparison") Also with more synthetic charged medias developed some manufacturers even market their 2" deep pleated air filters as MERV 11 or MERV 13 - which is a little misleading as well. Why are these so popular?
The major reason is that HVAC system design in the US has included 2" filter racks/frames for over 50 years - and the status quo doesn't appear to be changing anytime soon. This is what lead to air filter manufacturers to try and invent ways to get higher and higher MERV out of a 2" deep product, but even with current technology we are limited at a true efficiency of somewhere around MERV 8 or MERV 9 and can temporarily enhance that as high as 13.
Most facilities view the 8 as "enough" which if all you are trying to do is keep the equipment clean it is sufficient. ASHRAE has done some testing and found that coils in an HVAC system are somewhere around MERV 6, so to keep them mostly clean and functioning properly have lead to most specifications with a minimum of MERV 7. When you couple this with the fact that most of the US has ver clean air compared to other parts of the world, the percieved need to use a high efficiency product is minimal as simply increasing the outside air ratio will dilute the dirtier indoor air.
Fighting us though is that outside air in many parts of the country (including us here in the Upper Midwest) is expensive to condition, leading to more recircualted air as a way to save energy. An often cited statistic from the EPA is that indoor air is 2-100 times more dirty than outdoor air, with many of these particle being sub-micron (also referred as PM1) in size. Here's a graph that shows the impact of filteration on the concentration of these sub-micron particles:
These small PM1/sub-micron particles travel deep into lung tissue and can even penetrate into the blood stream, making them the most dangerous of all particles. They aren't quite as dangerous as molecular pollutants - but that's a different post (found here). This is a clear illustration of why MERV 8 simply isn't enough when we start recirculating air at 50% or more.
Another big reason so many 2" deep filters are purchased in teh US is that they are sacraficial - similar to Zinc Plates on a boat. It is an inexpensive part that can reasonably be affored to be replaced on a regular basis to protect more expensive parts of the overall system. The industry has taken this sacraficial nature to heart as there has been a race in recent years to make the cheapest pleated filter on teh market. Some technologies like kraft board frames have mostly failed as when tehy get wet the collapse almost immediately, while others like synthetic medias and "self-supported" structures seem to have taken a foothold.
Nowhere else in the air filter industry has commoditization been more clear than in 2" and 4" deep filters. Becasue manufacturer's know these are percieved as sacraficial and to be replaced commonly they have designed products to only last a couple months. These cheaper variations have lead to big contracts being awarded to those products and maintenance professionals now expecting this low-quality from 2" deep filters. There have been some manufacturer's that have taken a different line to creat long-lasting 2" pleats, like Camfil's Dual 9 that is guaranteed to last an entire year in most applications.
By representing over a dozen filter manufacturers it is interesting to see who comes out with what, and why eash is pitched as the best - which makes me glad we maintain an independant outlet where we get to hear from all manufacturers and see all their product in the field to be able to talk about how they all really work. To learn more or ask us some questions about your facilities air filter applications, simply click here to contact us!