Last Updated on March 19, 2021 by HVAC
What You NEED To Know About MERV Ratings
MERV, which stands for Minimum Efficiency Reporting Value, was developed by ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) as a standardized rating system and it is used to assess the filtering power of an air conditioner or furnace filter.
How Does The MERV Rating System Work?
The ratings for MERV range from 1 to 20. 1 is the lowest filtration level, and 20 would be the highest. The filters with a MERV rating of between 16 to 20 are typically found only in cleanrooms, hospitals, or a nuclear power plants. The air filters for homes typically rate between MERV 5 and 13. We refuse to endorse any of the filters that have a lower rating than 8, and the filters below MERV 5 are not even worth mentioning.
MERV ratings are established by the effectiveness of the filter at filtering various-sized particles. More specifically, we are talking about 12 sized particles that range from 0.3 to 10 micrometers (µm) in diameter, created in lab environments. To reference what we are talking about, one strand of human hair is around 50 µm, while the smallest particles that the naked eye can see are around 40 µm. The 12 particles are further divided into 3 size ranges which include E1, E2, and E3, and the 4 subranges that also exist in each range.
E1 is the first range, which includes the particles sized 0.3 to 1.0 µm. E2 includes the particles sized 1.0 to 3.0 µm, while E3 includes the particles sized 3.0 to 10.0 µm.
Each of the filters goes through 6 tests for every particle size (72 tests in total) to determine the MERV rating of the filter. For every test, the particles used in the test are counted before they spray or pass through the actual filter before they are counted again. The particles that are counted after passing through the filter are translated into a “percentage” which represents the number of particles that were filtered successfully out the air.
The worst percentage from the 6 tests is chosen as an official measurement to determine the MERV rating of the filter. This is where the “minimum” in MERV is derived from.
MERV Rating Efficiency Chart:
|MERV Rating||Average Particle Size||Efficiency in Microns|
|MERV 1-4||3.0-10.0||less than 20%|
|3.0-10.0||85% or greater|
|3.0-10.0||90% or greater|
|1.0-3.0||90% or greater|
|MERV 16||0.3-1.0||75% or greater|
What Is Pressure Drop?
In basic terms, the pressure drop is best described as air resistance. Air filters are literal barriers between the HVAC system and the vents, slowing the capability of the air to pull through the vents to the actual system. How much the HVAC’s air-flow slows down by the filter is equal to its “pressure drop”.
Pressure drop will vary according to the filter. The more tightly woven filters make it more difficult for the air to successfully pass through. This also creates a lower airflow and an increased pressure drop. If there is an air filter built into the air return, there are no ways to get around pressure drop.
Even MERV 1 to 4 fiberglass filters will experience pressure drop, but it is usually minimal. These are cheaper filters that have minimal impact on airflow, but they also have very little effect on the air quality. The pressure drop is very low when it comes to fiberglass filters since the filters are very porous, which results in ineffective filtering. Finer particles will all pass through the fiberglass filters. and the larger particles will also filter out a lot less effective when compared to pleated filters. You can see how often should you change your furnace filter here.
Pleated filters that have a MERV rating of between 8 to 13, will filter smaller particles a lot more effectively as well as lower pressure drop (this is as close as you can get to MERVana). Even though the pleated filters have a pressure drop that is slightly higher initially, it is not that significant that it will cause any type of harm to an HVAC system, provided the filters are changed out regularly.
You can read about the differences between pleated filters vs fiberglass here.
Higher Does Not Mean Better
You may be under the assumption that higher MERV ratings make them automatically better, yet this is not the case. The MERV ratings that are higher means that the pores in the filter are smaller, making it harder for air to move through the HVAC filter. This can result in an increased airflow resistance that the system is able to handle, which can make it inefficient.
Reducing the airflow in the system can worse air-quality in a home, and putting too much pressure on the fans of an AC or furnace system. So it is worth your while to do some research. Find out the highest MERV-rated filter for your system that will still allow for the maximum amount of airflow.