Four FAQs about Air-cleaner's MERV?

1.      What is MERV？

American National Standards Institute (ANSI) and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard 52.2, MERV is minimum efficiency reporting value.

Using KCl aerosol, and the particle counter shall have at least 12 channels. KCl particles within the particle size range of 0.3μm ~10.0μm shall be divided into 12 particle size ranges, and the filter efficiency with different particle size ranges is tested.

According to the test results, the filter shall be graded according to the following table.

E₁ is the average minimum PSE in the corresponding size range group of 0.30μm~1.0μm, E₂ is the average minimum PSE in the corresponding size range group of 1.0μm~3.0μm, E₃ is the average minimum PSE in the corresponding size range group of 3.0μm~10.0μm.

For MERV1-4, average arrestance also needs to be tested.

2.      Is the higher the MERV, the better the filter?  

It is wrong to say that the higher the MERV, the better the filter. MERV is designed to enable users to select appropriate filters according to the size of particles they want to remove. MERV system is to grade filters, not to comment on which filter is better.

The types and particle size distribution of pollutants in the air are regional, and are affected by local emission pollution sources, population density, geographical location and gas image conditions. Therefore, which MERV filter should be selected in different areas and for different purposes cannot be generalized. It should be selected according to the specific situation.

3.      Is the MERV grade the same for the same filter?

One foreign scholar used filters of the same specification (size, number of folds, etc.) made by the same worker in the same factory with the same materials and sent them to different laboratories for testing. Among them, six laboratories are independent third-party laboratories, and two are laboratories inside the factory. The test was conducted according to ASHRAE 52.2, and the test results are shown in the following table.

It can be found that the filter grade tested by 6 laboratories is MERV8, while that of the 2 laboratories is MERV7. The test result E₃ of laboratory H is 67%. If it is 3% more, it will become MERV8. It can be seen that MERV system still has some limitations. There are some similarities between the test of filter media, masks and filter elements. The test results of the same sample in different testing institutions or using different testing instruments will be different. Standards and methods specified in standards are tools to help us control product quality, but they cannot be equated with a ruler, and practicality is also important.

4.      Is the protection effect good if the MERV grade of the filter is appropriate？

The answer is no. Correct installation is also an important means to ensure the filter efficiency. The clearance between the filter and the filter housing will leak, reducing the filter efficiency after installation, especially the high efficiency filter. Therefore, in some occasions with high requirements for cleanliness level, after the filter is installed, it is also necessary to conduct on-site inspection, so as to ensure the filtration efficiency of the entire system.