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Introduction

The discussion below is about MERV rating for air filters. You use air filters in air cleaners which are also known as air purifiers. Air conditioners and furnaces also use MERV rated filters.

What is MERV or the Minimum Efficiency Reporting Value?

MERV or Minimum Efficiency Reporting Value measures the performance of large air purifiers in cleaning a whole house or building. It demonstrates how efficient an air filter is at catching particles of various sizes. The higher the MERV the better the filtration properties of a filter.

Clean Air Delivery Ratings or CADR measures the performance in smaller portable air purifiers.

Link to CADR Article

The Environmental Protection Agency defines MERV as:

Minimum Efficiency Reporting Values, or MERVs, report a filter’s ability to capture larger particles between 0.3 and 10 microns (µm).

Environments Protection Agency

MERV measures the performance of different filters. It then goes on a say that the higher the filters rating the better it is at trapping certain particles.

Some of the common air contaminants are pollen, bacteria, pet hair, dust mites, carpet fibres and tobacco smoke.

How were MERV Ratings created?

Created by the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE), in 1987. They were based on older standards created by the ASHRAE called “Standard 52” which was the first formal method of testing air purifiers.

Donald Newell states that the previous standards tested the following properties of a filter:

1. Particle removal capacity, 2. Resistant to airflow, 3. Expected operation life

Donald Newell’s paper

But MERV only measure the first of these qualities which is particle removal capacity.

MERV rating measure the effectiveness of a filter to trap particles if different sizes.

How the MERV Ratings are Calculated

MERV values can range from 1 to 20. The higher the value the less particles or contaminants can pass through the filter. MERV ratings are determined by completing a series of tests for each air cleaner. Twelve categories of different sized particles introduced into the test air. The 12 categories divided into 3 size ranges called E1, E2 and E3. Each of these ranges has 4 further sub ranges.

• E1 0.3 to 1.0 µm.
• E2 1.0 to 3.0 µm
• E3 3.0 to 10.0 µm.

The size of the particles which range from the smallest diameter to 0.3 to the largest 10 micrometers (µm). For comparison, a human hair is about 50 µm in diameter. The particles then pass through the filter and the density of the particles recorded before and after passing through the filter. The size of particles ranges from 0.3-0.4 micrometers (microns) to larger particles from 7 to 10 micrometers. The lab completes the test six times. So, there are six results for each of the 12 categories, a total of 72 tests in all. The final count is a plus the total number of particles in the air originally converts into a percentage. The MERV is determined by the worst achieving result. That is why it is known as the minimum ERV. Here’s how Air Filter MERV Ratings relate to the size of particle trapped:

• MERV 1 to 4— <20% of large particles (3 to 10 microns) captured
• 5 to 7—Between 20% and 69% of large particles captured
• 8 to 10—Approximately 85% of large particles and 50% of small particles (1.0 to 3.0 microns) captured
• 11 to 16—More than 90% of large particles and between 70% and 90% of small particles captured
• 17 to 20—Even the smallest particles (< 0.3 microns) captured

Is a higher MERV rating better?

Not in every case. It is not so simple as the higher the value the better the filter is for you. A filter with a higher MERV rating has smaller pores. This makes it more difficult for the air to flow through. This can create more resistance in the running of your HVAC (Heating, Ventilation and Air Conditioning system) which can make it more inefficient to run. Reducing the air flow generated can worsen the air quality in your home and damage the fan on your air conditioning unit.

You need to choose the highest MERV rating recommended for your machine. If you are using a filter with a higher MERV rating than that recommended by the manufacturer this means that you HVAC system must work harder to maintain the same airflow. This could put more pressure on your system and possibly damaging it. Therefore, you should always use the filter with the MERV rating recommended by the manufacturer.

Filter with a MERV rating of 7 to 12 are suitable for removing most particles in a domestic situation. A MERV rating 8 can filter out contaminants such as pollen, saw dust and mold spores. Hospital generally use filters with a MERV rating of 13 to 16. A MERV rating 13 with smaller pores can filter out bacteria and tobacco smoke. Clean rooms, surgical operating rooms and areas requiring absolute cleanliness generally use MERV rating of 17 to 20. Filters with higher MERV rating filter require more frequent replacing. Although lower rated MERV filters are less efficient over all they allow more air flow which can help your HVAC system.

MERV Rating Charts

There are various MERV rating charts available. This chart shows the pollutant types filtered out at each MERV rating. Link: http://www.mechreps.com/PDF/Merv_Rating_Chart.pdf

What MERV rating filter would you use in your home?

The filter in a HVAC system traps dust, allergens, and bacteria. and mold. The MERV rating shows how effectively a filter prevents dust and other particles passing through a filter into the airstream. Medium efficiency filter from 7 to 13 are best at removing small to large particles. These can be as effective as HEPA filter art controlling indoor air particles. These filters are generally more expensive than HEPA filters.

The air filters are quieter and allow more airflow in the HVAC fans than HEPA filters because they have less airflow resistance. ASHRAE recommend fitting of rating 13 for HVAC systems home air conditioning. But check your system can cope with this filter. As a higher rating filter will mean less air flow and higher electricity costs. If your system can not cope with this rating, go for the highest rating recommended for your HVAC system. HEPA filters help improve air quality. HEPA stands for High Efficiency Particulate Air. Link to HEPA Page You need to change your MERV filter more regularly with higher MERV rate filters.

Otherwise, they become clogged which then restricts the air flow. This makes them less efficient and can damage the machine. A higher MEV rated filter is usually thicker which means the HVAC unit must work harder to pump the air around the system. So, the energy costs are more expensive. MERV 5 to 8 rating is generally good to remove dust, mold spores and pollen. If you suffer from allergies a higher rated MERV between 10 -12 would be good. As this filter can trap particles as low as 1 micron such as pollen, mold spores and automobile exhaust.

The filter has two main types, pleated and non-pleated. The pleated is usually a higher MERV rating and more expensive. A MERV rating of between 10 -12 should be sufficient to remove allergy causing particles such as dust, pollen, and pet dander. Some of the better-quality air filter have a carbon layer which is useful to remove odours such as pet smells.

Pre-Filters vs. Final Filters

Final filters are the primary filter in an HAVC system or in standalone air conditioners. If used the pre-filters filter out the larger particles in the air flow and the final filter filters out the remaining smaller particles. This has the advantage of prolonging the life of the expensive final filter.

How often should I change my air filter?

How regularly you change your air filter can depend on several factors: 1. Air filter model 2. The quality of indoor air 3. Number of people in the household 4. Number of Pets 5. Level of pollution outside the home Manufactures usually recommend that you replace basic filters every 3 months. If you suffer from allergies, you can change them more frequently.

How to compare air cleaners?

The MOD method is the only measure to particle reduction by high grade filters. The Federal Government use this method. This is the MOD system used by the military for high grade air filters. 

However, there are two private trade associations which have set standards for the rating of removal of particles from the air by system or portable air cleaners.

They are the ASHRAE and the Association of Home Appliance Manufacturers (AHAM) The standards measure how effective an air cleaner is in removing particles from the air, used in comparing different models. These standards only measure the effectiveness of removing particles from the air. There are no standards for how effective air cleaners are at removing gaseous pollutants from the air. There is no conclusive research to show how effective different models of air cleaner are at removing these pollutants.

What is pressure drop?

Pressure drop is air resistance. Your air filter is a barrier between your HVAC system and your vents. That slows down the amount of air that can pass through your vents to your system. The amount air flows slowed by the filter is its pressure drops. The pressure drop varies according to how tightly woven is the fabric of the filter. The more tightly woven the harder for the air to pass through and therefore the greater the pressure drop. All filters will result in some pressure drop.

Fiberglass filters with a MERV rating of 1-4 have some pressure drop. But as there is only a small drop in air flow this has little effect on air quality. This is because they are highly porous letting small particles pass through the filter and only trapping some of the larger particles. Pleated filters have a higher MERV rating of between 8 and 15. These are pleated filters which can filter out small particles effectively with only a slight pressure drop.

These filters do initially have an initial pressure drop but if you change the filters regularly this does not harm your HVAC systems. As you use the filter after a time dust, dirt and mold will become trapped in your filer reducing air flow and increasing pressure drop. Replacing your filter every two to three months will retain your HVAC system performance.

The MERV filter recommended for preventing the transmission of airborne illnesses such as COVID-19?

Although ASHRAE does not recommend a specific MERV rating to cope with preventing COVID infections it does suggest upgrading systems to MERV rating 13 or highest possible achievable in emergency plans. 

HVAC systems may decrease people’s exposure to airborne pathogens that spread COVID 19 and other diseases. Most HVAC system come fitted with filters of MERV rating of 8 and above. A filter with MERV rating of 13 and above can trap viruses. Upgrading to a HVAC filter rating filter could improve your systems ability to trap viruses. HVAC systems can be adapted to fit these filters. You can also consider running your filter either continuously or more frequently. In some systems you can run the fan without heating or cooling. Remember that air cleaners alone will not stop CIVID 19 viruses.

Limitations of the MERV Rating System

The MERV rating can only predict the size of particles it removes from the air. But gases are also a pollutant and the MERV rating is of no use for them. Newell states that air filters:

are likely to perform worse than predicted because of various installation conditions.

Donald Newell’s paper

MERV rating assigned under perfect laboratory conditions and may not be accurate in real world situations. This is known as the installation affect.

MERV rating are only applicable to air cleaner used in whole building and are not effective for small portable air cleaners.

Conclusion

The MERV Rating is a useful way to determine the capacity of large air cleaners. Calculated by testing filters. The worst-case performance of the test then used to calculate the MERV rating. There are other factors to consider when buying an air filtration system. You should not go just for the system with the largest MERV.