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When browsing for air filters, you should just go for the filter with the highest rating, right? Not always. Many people don’t consider how a filter with a high MERV rating will affect other parts of the HVAC system.
The primary concern when upgrading to a higher MERV rated filter is the reduction in airflow in the system. The reduction in airflow can have far-reaching implications in terms of performance and efficiency on your entire HVAC system.
Does a higher MERV rating affect airflow?
Generally, a filter with a higher MERV rating will reduce airflow. However, there are many other factors at play such as the size of the filter and the type of blower motor in your HVAC system.
Filters for central HVAC systems are usually the primary form of particle removal for residential and commercial buildings.
Much attention is giving to the filter’s efficiency at removing particles and cleaning the air. But using a filter also introduces a barrier that can cause reductions in airflow.
Could using stronger filters lead to inadequate airflow? It seems obvious that a stronger air filter will reduce the airflow in your HVAC system. However, considering the airflow reduction caused by a high MERV filter is usually an afterthought for most homeowners.
Even if high MERV filters are used, will it substantially affect your system? One study found that while there is a link between using high-efficiency filters and increased energy use, for most residential systems the impact is not significant.
When it comes to air filters affecting airflow, there are three factors at play:
- The size of the air filter. The size of the air filter has a huge impact on how much airflow goes through your HVAC system. This is mainly because a filter with a large face area will minimize pressure drops.
- The efficiency of the air filter. The MERV rating has an indirect impact on airflow. Generally, filters with higher MERV values will have less airflow.
- Blower motor type. Whether you have a PSC or ECM blower motor has implications on airflow.
When faced with filter pressure drops PSC motors will maintain less airflow but use the same amount of energy.
ECMs will maintain the same airflow but use more energy.
This is very important because if your system has an ECM blower motor, then you will see likely see minimal airflow impact when using a high MERV rated filter. Instead, your blower motor will use more energy to maintain airflow.
How filter size affects airflow
The size of your air filter has a huge effect on the airflow through your HVAC system. Assuming all else is equal, the larger the filter, the more airflow through your HVAC system.
This applies to both the length, width, as well as depth of the filter.
In a pleated filter, the deeper the filter, the longer its pleats. The surface area of the filter that is used for filtering air is called the face area.
A filter with more face area is effectively a larger filter size. So this means that the filter will produce less pressure drop.
Filter pressure drop and MERV rating
The pressure drop across an air filter will determine how negatively the filter affects airflow. Higher pressure drop = less airflow.
The Air Conditioning Contractors of America assumes that the pressure drop across a filter is 0.10 in. w.c. However, one study found that actual pressure drops in home installations were on average 0.28 in. w.c.
Even though stronger filters produce higher pressure drops, there are some ways that airflow can be maximized while using high MERV-rated filters.
In fact, even filters with high MERV ratings can be used in some systems with a minimal pressure drop. The trick is that the filter’s face area needs to be maximized in order to obtain the lowest filter pressure drop as possible.
There are a couple of different ways to maximize your air filters’ face area:
- Increase the size (width) of your air filters. This includes maximizing the cross-sectional area of the slot that you put your air filter in.
For instance, if an air filter’s slot is in the return register of an HVAC system, some homeowners might elect to modify the return register to a larger size to accommodate a larger filter.
- Use an air filter with a larger depth. For instance, a 2-in. pleated air filter will have a larger face area than a 1-in. pleated air filter.
The air filter slot on your home’s HVAC system determines the depth of the air filter that you can use. In order to use a deeper air filter, your air filter’s slot will need to be large enough to accommodate the filter.
How Total External Static Pressure (TESP) affects airflow
The total external static pressure or TESP is the pressure difference across your entire HVAC system. The total external static pressure is measured by taking the difference of the pressure between the supply and return registers.
Why is TESP important? It is an indicator of how much airflow is going through your system. Generally, higher TESP = more airflow.
How do air filters contribute to total external static pressure?
Air filters contribute to TESP by reducing the total external static pressure in the system. This means that using an air filter will reduce the amount of airflow in your HVAC system.
Most residential homes are designed to have no less than 0.5 in. w.c. of TESP. Since the air filters in many residential homes contribute to more than half of that pressure, it is clear that air filters are responsible for a large part of the pressure drop in an HVAC system.
Since using an air filter is not optional, you’ll need to find a filter that meets both your air filtration needs and airflow requirements.
The diagram below shows a typical pressure distribution of a residential HVAC system.
In the diagram, the total external static pressure is represented by ΔPreturn and ΔPsupply. You can clearly see how the air filter reduces the total external static pressure by adding to pressure drop.
How different types of blower motors affect airflow
The type of blower that you have in your furnace or air conditioner will largely determine if there will be a reduction of airflow in the system.
Permanent Split Capacitor (PSC) Motor
Permanent split capacitor (PSC) motors are used in most standard, economy-priced furnaces. In the presence of airflow restriction, PSC motors will consume relatively the same amount of power but provide less airflow.
Even though the amount of power that a PSC consumes remains unaffected, the reduction in airflow can cause an overall increase in energy use. This is due to increased runtimes from the reduction of heating/cooling capacity.
When using a high MERV rated filter, you will likely see a slight reduction of airflow when using a PSC motor. As the filter gets dirty and more airflow is restricted, the PSC motor will deliver even less airflow.
Electrically Commutated Motor (ECM)
Electrically Commutated Motors (ECM) are newer and more energy-efficient than PSC motors. ECMs consist of a brushless DC motor combined with a microprocessor to control its speed.
The result is that when airflow restriction is introduced, the ECM will speed up to maintain the same amount of airflow. Doing so increases the energy use of an ECM.
However, ECMs are already very energy-efficient, so they will on average use less energy than a PSC, even with a dirty filter.
The impact of dirty air filters on airflow
When your air filter gets dirty, the pressure drop across the filter will increase. This will cause a reduction in airflow in your system.
Did you know that a dirty filter actually filters out more particles? The implications of having a dirty air filter are similar to using a high MERV filter. As the air filter clogs, it is able to trap smaller particles in its dirty membrane.
I recommend changing your air filters every 1-2 months to maintain peak performance and energy efficiency levels.