The Truth About Higher MERV Ratings and Air Flow

As an HVAC expert, I have spent countless hours researching and studying the effects of MERV ratings on air flow. And one question that often comes up is whether a higher MERV rating decreases air flow. The short answer is yes, but it's not something to be overly concerned about unless you are dealing with extreme circumstances. First, let's define what a MERV rating is. MERV stands for Minimum Efficiency Reporting Value and it measures the effectiveness of an air filter in removing particles from the air.

The higher the MERV rating, the better the filter is at trapping pollutants and dust. However, with this increased effectiveness comes greater resistance, which means less airflow. But before we dive into the impact of higher MERV ratings on air flow, let's address a common misconception. Many people believe that higher MERV filters are not compatible with modern HVAC systems. This is simply not true.

In fact, millions of homeowners rely on higher MERV filters without any issues. The main concern with high-efficiency air filters is that they can become clogged if not changed regularly. However, if you stay on top of filter changes, you are unlikely to experience any problems with your air conditioning system. Research has shown that air conditioning systems with filters with a high MERV content do have a greater pressure drop throughout the filter. This means that the air has to work harder to pass through the filter, resulting in decreased air flow. This is a common finding in multiple studies. If you want to remove more particles from the air, you can opt for a filter with a different material.

These filters have smaller pores, allowing them to trap more contaminants from the air stream. However, this also means they have higher resistance and can decrease air flow. So, what is the ideal MERV rating for your air filter? It ultimately depends on your priorities. If you prioritize air quality and have to deal with asthma, severe allergies, or other similar circumstances, then a MERV 13 or higher filter is recommended. These filters are highly effective at removing pollutants from the air, but they also have a greater impact on air flow. In fact, studies have shown that airflow in high MERV filters can be reduced by 7% to 11% compared to low MERV filters.

For example, an 8 MERV filter will filter down to approximately 10 microns, while a 13 MERV filter will only allow about 0.3 microns through. This is a significant difference in particle size and can have an impact on air flow. One consequence of using a highly effective air filter is that it can become loaded with pollutants quickly and increase the pressure drop. The extent of this increase may vary depending on your home and air conditioning system, but with modern technology, the impact on air flow is usually marginal. Now, let's address the two types of motors commonly used in HVAC systems - permanent split capacitor (PSC) and electronic commutation motor (ECM). For those with PSC motors, the increased resistance from a high-efficiency filter can result in a sharp drop in pressure and decreased air flow.

However, for those with ECM motors, the controls typically increase motor speed as pressure increases, maintaining a relatively constant airflow. So, what can you do to use a high MERV filter without experiencing a significant drop in air flow (for PSC fans) or increased energy use (for ECM fans)? The solution is quite simple - regularly change your filters. By doing so, you can reap the benefits of a high-efficiency filter without sacrificing air flow. Some HVAC technicians prefer low-efficiency filters because they have lower resistance to airflow. However, this also means they are less effective at removing particles from the air. It's important to strike a balance between air flow and air quality. On average, it is recommended that most homes and commercial spaces have a filter with a MERV rating of at least 5 and 12 for high-end filtration.

Low-efficiency filters typically fall within the MERV 1-4 range, while high-efficiency filters are MERV 13 and above. Through my research and experience, I have learned that two MERV 11 filters can have the same filtering capacity, but one can allow more air to pass through. This highlights the importance of regularly changing your filters to maintain optimal air flow and air quality.