Installing and maintaining a high-quality HVAC system in your home is perhaps one of the most important (though often overlooked) aspects of being a property owner. HVAC stands for Heating, Ventilation, and Air Conditioning, and it affects the air quality of your indoor space as well as your overall health and comfort.
Here are two of the most important questions that you will need to answer when choosing an HVAC system for your property:
- What kind of air filter should you opt for?
- What is the MERV rating of an air filter?
Some of the different types of air filters that you can choose from include electrostatic filters, HEPA filters, UV light filters, washable filters, pleated filters, and spun glass filters.
However, in this article, we will be focusing largely on the second question – what is a MERV rating and what is its purpose?
The Minimum Efficiency Reporting Value (MERV) of an air filter (from 1 to 20) measures how much dust, pollen, mold spores, and other contaminants pass through that filter to enter the indoor airstream. Filters rated MERV 7-12 are typically considered to be suitable for residential HVAC systems.
The MERV scale was designed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). In general, the higher the MERV rating on a filter, the more particles it will catch. As a result, air filters with higher MERV ratings can more effectively remove contaminants from the air that passes through them.
In technical terms, the MERV rating represents the worst possible performance of a filter at removing particles that are between 0.3 to 10 microns (µm) in size.
This means that, before being assigned a MERV rating, a filter must undergo six tests for every particle size between 0.3 and 10 µm. The total number of particles being used will be counted before each test begins.
During the test, air containing these particles will be sprayed through the filter. Later, the remaining particles will be counted once again, to determine how many particles were successfully removed from the air that passed through the filter.
The final particle count, compared to the total number of particles originally counted at the beginning of the test, will then be converted into a percentage which represents how efficiently the filter can remove particles from the air.
Out of the six tests, the worst percentage is chosen as the official MERV rating.
This is where the system derives its name, as it measures the minimum efficiency of a given filter.
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The Different MERV Ratings & What They Mean
To choose the right air filter for your HVAC system, you need to understand what the different MERV ratings actually mean, and how each rating corresponds with your filtration needs. Here’s a chart that breaks it all down:
|MERV Rating||Application||Contaminants Filtered||Minimum Particle Size|
|MERV 1-4||Typically used for window air conditioners and residential furnaces. Can also be used as a pre-filter in commercial buildings.||They can filter out sawdust, pollen, textile fibers, and dust mites.||Filters out particles that are 10µm or larger, but is not very effective for smaller particles.|
|MERV 5-7||Used for paint booths, industrial workplaces, and some of the better residential buildings.||These filters remove contaminants like mold spores, dust mites, household dust, carpet fibers, pollen, lint, and concrete dust.||Filters out particles that are 3 to 10 µm in size.|
|MERV 9-12||Commonly used in hospital laboratories, as well as residential and commercial buildings that require above-average air quality.||They filter out harmful particles like legionella, lead dust, mold spores, concrete dust, textile fibers, and coal dust, as well as the dust generated from nebulizers and humidifiers.||Filters out particles that are 1 to 3 µm in size.|
|MERV 13-16||These filters are used in buildings that require world-class air filtration, like smoking lounges, hospital inpatient care units, and general surgery facilities. They can also be installed in homes if the resident(s) suffer from severe allergies.||In addition to all the common contaminants that can be removed by MERV 1-12, they can filter out tobacco smoke, bacteria, automobile exhaust, pet dander, sneeze particles, insecticide dust and cosmetic dust.||Filters out particles that are between 0.3 and1 µm in size.|
|MERV 17-20||Typically found in pharmaceutical manufacturing facilities, scientific laboratories and clean rooms, orthopedic surgery rooms, and buildings where carcinogenic and radioactive materials are stored.||Can filter all the particles listed for MERV 1-16. Additionally, they can remove carbon dust, virus carriers, radon progeny, sea salt, combustion smoke, legionella, and microscopic allergens from the airstream.||Filters out particles that are 0.3 µm in size, or less.|
Which MERV Rating Is Right For Your HVAC System?
So far, we’ve discussed how a higher MERV rating means that your air filter is better at removing contaminants from the indoor airstream. This might lead you to think that a filter with a higher MERV rating is always better. However, this is not necessarily the case. There is a reason why filters rated MERV 15 and above are rarely ever used in residential HVAC systems.
The higher the MERV rating of an air filter, the smaller its pores will be. These tightly woven filters with small pores are more effective at catching airborne contaminants, but they also make it harder for air to pass through. This phenomenon is known as “pressure drop,” and we will discuss it at greater length below.
You should follow the recommendations of your HVAC manufacturer when choosing an air filter. If you use a filter with a higher MERV rating than your HVAC system can handle, it could actually decrease indoor air quality and lower the efficiency of the system.
The higher-rated air filter, with its smaller pores, will create greater resistance to air flow. This, in turn, might put too much strain on the fan of your furnace or air conditioner.
Therefore, before upgrading the air filter of your HVAC system, you should consult an experienced HVAC professional. They will help you discover the highest MERV-rated filter that would still allow for adequate airflow in your system. (Click here to learn more about the ways in which you can finance your HVAC upgrade.)
The Problem Of Pressure Drop
The air filter forms a barrier between the vents and your HVAC system. This barrier offers a certain amount of air resistance, which slows down the pace at which air can be pulled through the vents and into the HVAC system. This air resistance is known as pressure drop because higher resistance causes a drop in air pressure when the HVAC system is in use.
The amount of pressure drop caused by a particular filter is equivalent to the extent to which it slows down the passage of air from the vents to the HVAC system. (Click here to learn more about the different elements of home ventilation.)
Air filters with a higher MERV rating are more tightly woven and have smaller pores, in order to effectively catch any contaminants or impurities. However, the smaller pores also make it harder for air to pass through these filters, leading to a decrease in air flow, which in turn enhances pressure drop.
There is no way to completely avoid pressure drop as long as you have an air filter installed in your HVAC system. However, filters with a MERV rating of 1-4 are extremely porous, and therefore cause very little pressure drop. Unfortunately, this also makes them quite ineffective at filtering.
Pleated filters with a MERV rating of 8-12 can effectively filter out most types of contaminants, but they cause a relatively higher initial pressure drop. As long as you change your filters every two to three months, however, this pressure drop is generally manageable and will not cause any harm to your HVAC system.
But you should keep in mind that, over time, mold, dust, and dander will accumulate in your air filter, further reducing air flow. Needless to say, this will cause an increase in pressure drop, which might affect your indoor air quality and even damage your HVAC system, if left unattended. This is why air filters with a MERV rating of 7 and above need to be replaced every few months, to avoid hampering the functionality of your HVAC system.
When Is A Higher MERV Rating Appropriate?
Allergies, respiratory issues, and other such health concerns are some of the major reasons why people want to upgrade to an air filter with a higher MERV rating. If you are allergic to pet dander or cosmetic dust, for instance, you might have to install an air filter rated 13 and above, which is usually not found in residential buildings.
For the best results, you should consult an HVAC technician to determine the appropriate level of filtration for your needs. You will need to choose an air filter that will effectively filter out potential allergens, without damaging your HVAC system or decreasing the air quality in your home.
A MERV 11 filter might be adequate for pet owners who have mild allergies, and are looking for an air filter that will keep some of the pet dander out of their living space. On the other hand, those who suffer from severe allergies or have multiple pets at home might require an air filter with a MERV rating of 13 or higher.
People with respiratory conditions like asthma and pulmonary fibrosis might also require higher levels of filtration, especially if they live in an area with high levels of smog and air pollution. This is because smoke particles and automobile fumes are extremely small, and can be quite hazardous to those with preexisting respiratory issues.
If you do not own pets, live in a particularly polluted area, or suffer from allergies, asthma, or other such health conditions, then you will be fine with an air filter that has a MERV rating of 8 to 10. A MERV 8 filter will effectively remove all the basic contaminants like dust, pollen, bacteria, and mold from your indoor air.
According to most HVAC experts, under normal circumstances, there is little noticeable difference in indoor air quality once the MERV rating is higher than 7.
The purpose of MERV ratings is to help homeowners choose the most appropriate (and efficient) air filter option for their HVAC system. Residential HVAC systems typically use air filters rated between MERV 7 and 12, to remove the common contaminants found in homes, like lint, sawdust, pollen, and mold spores.
Filters with a MERV rating of 13 and higher are usually found in hospitals, laboratories, and pharmaceutical manufacturing facilities, although they can also be installed by homeowners with severe allergies or respiratory conditions.