What are MERV ratings and why are they important?

MERV ratings provide customers a standard for comparing filters’ ability to trap small particles. MERV ratings are not related to the ability to remove chemicals for gases. This rating was developed by ASHRAE, the engineering association for the heating and cooling industry. Beware of filters that do not provide a MERV rating, since this is the industry standard. However, don’t count on MERV ratings to tell you what is better.

A higher MERV rating does not automatically mean better home air quality. There are three reasons for this. One is that the MERV rating is not indicative of the lifetime performance of an air filter. Two is called electrostatic degredation, which has caused MERV 14 filters to perform worse than MERV 8 filters within a few weeks of installation. Finally, there is the problem of air flow resistriction, which reduces the speed of air filtration.

A MERV rating is not indicative of how a filter performs over time. A filter is rated on the lowest of 6 readings, so it could have a lower first reading and then outperform much higher rated filters for 85% of its use. For example, the MERV 11 filters which we offered had an initial reading of 82% efficiency at 3 microns and an average of 97% for the last 5 readings. The same test for our MERV 8 Duo showed 73% initially and an average of 99.2% for the remainder. Both average 95% over the life of the filter. Which is better? See ASHRAE test curves at the bottom of “Proven Results” .

This leads us to the second issue- electrostatic degredation. A preword to the ASHRAE standard 52.2 which defines MERV values is thatelectrostatically ehanced filters may perform worse in home conditions than in the lab. This is because homes have vapors and conditions which can rapidly degrade the electrostatic charge used to obtain high MERV ratings. We’ve seen MERV 14 filters degrade from 80% effective to 40% effectiveness on 1 micron particles in 10 weeks. By losing electrostatic charge, high rated filters can become less effective than lower rated filters, yet maintain higher air flow restriction.

Higher air flow resistriction reduces air flow, which means that less air is cleaned over a given period of time. This creates a breaking point since the net effectiveness of air filtration is a function of effectiveness and air flow. Taken to the extreme, a sheet of plastic over your filter will stop 100% of everything, but your air quality will get worse because there is no air flow.

Considering these three issues, until we see in home results that convince us of high MERV rated filters (beyond 8/9), we don’t recommend their use. We offer electrostatic MERV 11 filters, but it our opinion that our MERV 8 Duo with mechanical filtration will outperform MERV 11s and that our MERV 7s create the same net impact on air quality but with better home comfort and energy efficiency.

MERV ratings range from 1 to 16. The highest rated 1″ filter we know of is MERV 12. Higher ratings are typically HEPA filters which may be more than 12″ deep. While higher MERV ratings offer better particulate pollution, they can also create problems for home air systems.

The Trade-Off Between MERV Ratings and Your Central Air System’s Performance

One inch deep filters with high MERV ratings are typically frowned upon by heating and cooling technicians because these filters restrict airflow and reduce circulation. Reduced airflow causes several problems with home comfort and HVAC equipment.

  1. Filtration: Air is filtered less often because recirculation is slowed down.
  2. Home Comfort: Adequate airflow is needed to keep your home at the right temperature and humidity. Highly rated filters may cause inadequate air flow.
  3. Potential damage to HVAC Equipment: Condensors have to work harder to overcome reduced airflow. This often causes condensors to freeze and fail. We have seen this happen many times, even with a high quality $25 MERV 12 filter made by a well regarded company.

A recent study of hospitals actually concluded that the rooms had better air quality using MERV 7 filters than MERV 12.

Our Recommendation: It Depends

These concerns and findings led us to recommend using our MERV 11 SafeHome Allergen Filter in returns where you are already using a high rated allergen filter or know that one can be used. We use a high number of pleats to decrease air resistance, but the MERV 12 filter that caused our home’s system to freeze up had the most pleats per foot that we could buy.

Our MERV 11 filter is best if you are concerned about asthma or allergies. If you don’t know whether your system can handle a MERV 11 filter, call your HVAC company. Modifications that can help include increasing the size of the air return grill or installing a stronger or variable speed fan.

MERV definitions

MERV 7 filters remove more than 50% of airborne particles between 3.0 and 10.0 microns. This includes pollen, dust mite debris, mold spores, dust and dander.

MERV 11 filters remove more than 85% of airborne particles between 3.0 and 10.0 microns and more than 65% of airborne particles between 1.0 and 3.0 microns. This includes tobacco smoke particles, pollen, dust mite debris, mold spores, dust and dander.