Home heating, ventilation, and air-conditioning (HVAC) systems that have a central air handler and ducting should be equipped with air filters. The purpose of air filters is to remove particulates, such as dust, from the air stream. This serves three primary purposes:

1. Filters improve indoor air quality for occupant respiratory health. This is key not only in daily life, but especially when outdoor air quality is low such as during extreme heat events or during regional wildfires.
2. Filters protect the HVAC equipment from degradation by keeping internal components clean of particulate build-up that could cause lower equipment efficiency, reduced reliability, and diminished heat transfer.
3. Filters help maintain a cleaner indoor environment by removing particulates that would otherwise settle on furniture and floors.

HVAC filters are typically located upstream of other components. This means they are usually in the return duct adjacent to the air handler or on the back of the return register grille(s). This allows them to trap particulates in the air pulled into the return ducts by the air handler, before the particulates reach the coils, fan, or supply ducting.  Figure 1 shows the lower portion of a vertical upflow air handler for a heat pump. Return air flows down the return ducting (silver) installed to the left of the air handler. The air follows the duct work in a U-turn to enter the air handler from below. The first component the air contacts is the filter (notice the horizontal filter panel at the bottom of the unit). The heat pump coils are located above the filter, as indicated by the refrigerant lines (upper left) and condensate drainpipe (lower right). This configuration allows the filter to protect the rest of the system from debris and dust.

Filters must be installed in the correct orientation to work properly. Most commercial filters have an arrow on the edge of the filter indicating the direction of airflow when properly installed (Figure 2). If the filter in Figure 2 were to be installed in an upflow air handler (like the one shown in Figure 1), then the arrow should be facing upwards (even though the writing would be upside down in this case).

Filters are available in a range of styles, materials, and sizes. They’re generally one to four inches thick, made of polyester and/or fiberglass, and styled in a flat or pleated pattern (Figure 3).

Filters use either mechanical filtration or electrostatic filtration to remove particulates from the air. Mechanical (i.e., surface media) filtration is the capture of particulates through a dense fiber medium. Typically, the filter media are pleated, which allows for more surface area to capture debris. Electrostatic filtration uses electrostatic precipitation to remove particulates. Some filter models on the market combine mechanical and electrostatic filtration. Filters can be replaceable or washable and reusable. Replacement filters are typically made of synthetic media or fiberglass. They should be replaced every three months or sooner if needed, especially if the HVAC equipment is used continually. Filters loaded with particulates should be thrown away. Washable filters typically use electrostatic filtration and are made of aluminum mesh or foam rubber. They should be removed for cleaning once every one to three months, rinsed with water or cleaning solution, air dried, and then re-installed. If washable air filters are not dried properly, they have the potential to harbor mold.

There are two elements of HVAC filter performance: (1) the effectiveness at removing particles from the air and (2) resistance to airflow (i.e., pressure drop) across the filter. The Minimum Efficiency Reporting Value (MERV) rating is one measure of a filter’s ability to capture particles sized from 0.3 to 10 micrometers (μm) from the air stream. It is measured by the test method in the ASHRAE 52.2 Standard. MERV rating corresponds to a level of performance ranging from 1 to 16 - the higher the MERV rating, the more effective a filter is at capturing particles passing through it. ENERGY STAR requires filters that are MERV 6 or higher, and DOE’s Zero Energy Ready Home program and EPA’s Indoor airPlus program require MERV 8 or higher. The EPA and the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) recommend a minimum filter rating of MERV-13 for managing COVID-19 and other virus concentrations in a home (EPA 2023, ASHRAE 2023).

Another measure of a filter’s effectiveness at removing particles is particle size efficiency, which is the fraction (or percentage) of particles captured on a filter, as determined by the test method in AHRI Standard 680. Particle size efficiency is measured across three particle‐size bins: 0.3 to 1.0 μm, 1.0 to 3.0 μm, and 3.0 to 10.0 μm. The percentages correspond to MERV rating as shown in Table 1, which is based on the National Air Filtration Association’s Understanding MERV Guide and the U.S. Environmental Protection Agency’s (EPA) Residential Air Cleaners (Second Edition).

The second aspect of HVAC filter performance is pressure drop or resistance to airflow. As the air stream passes through the filter, it decreases its velocity due to the resistance of the filter. This resistance is measured in inches of water column (IWC, or in. w.c.) at either a specific face velocity or airflow rate. The resistance to airflow of a new filter is called the “initial pressure drop,” whereas the resistance when the filter is loaded with particulates is called the “final pressure drop.” The contribution of the filter to the total system pressure drop is typically 20%-50%, depending on the system configuration, filter efficiency, and loading condition. HVAC system engineers and designers are supposed to take the initial pressure drop of the filter into account when determining how to size HVAC equipment and related ductwork for residential or commercial buildings.

The resistance to airflow in a high-static-pressure system causes the controls of brushless permanent magnet (BPM) blower motors to increase speed and power draw to maintain system airflow, resulting in an increase in energy consumption. Permanent split capacitor (PSC) blower motors do not have airflow controls like BPM blower motors and thus will not increase power and speed to maintain system airflow. Instead, since PSC blower motors cannot adjust speed or torque, airflow rates slow down in response to increasing system pressures. This results in a drop in power consumption as well. This is known as “fall off,” when the system pressure forces reduce airflow and power consumption. As a result, the run time necessary to cool or heat the ambient air to the thermostat’s set‐point temperature is extended, which can lead to an overall increase in energy use. In addition, excessive pressure drop can damage furnaces due to overheating, can freeze condensing coils in air conditioning units, and can burn out blower motors. It’s important for homeowners to purchase filters with a pressure drop performance that meets their HVAC system specifications to run their equipment efficiently and prevent damage. The Air Conditioning Contractors of America (ACCA) Manual D Residential Duct Systems offers guidance for sizing residential ducting systems, including sizing HVAC filters for pressure drop in the system.

Having a high MERV rating does not necessarily mean that a filter will have a higher pressure drop than one with a lower MERV rating. Pressure drop can be greatly reduced by the design of the filter, regardless of MERV rating. Filters with deeper pleats or an increased number of pleats tend to have lower pressure drop. Having a high number of pleats and/or deeper pleats increases the overall surface area of the filter media, which in turn lowers pressure drop without changing the MERV rating. For example, a filter that has 4-inch-deep pleats has twice as much surface area as a filter with 2-inch pleats. If a homeowner is requesting a very high MERV filter, it may require the alteration or replacement of ducting if the pressure drop of the filter is greater than the pressure drop allotted to the filter in the system design. Another option is to advise the homeowner to purchase separate air filtration equipment that can clean air without impacting the performance of the HVAC equipment.

One option for reducing pressure drop involves modifying the return ductwork near the air handler or furnace to increase filter surface area. If the furnace is in a location where it can be accessed by the homeowner for filter replacement and where the furnace location permits an increase in the width of the return duct, the following approach may enable the installation of higher MERV filters.

1. Calculate pressure drop and airflow requirements for the desired filter MERV rating and dimensions in accord with Manual D (ANSI/ACCA 2009).
2. Construct a box to hold two filters side-by-side (or one larger filter) in the return duct above the 90-degree turn into the return air plenum.
3. Increase the duct width to accommodate the filter dimensions above and below this filter box.
4. If possible, install the filter box at shoulder height for easier servicing. See Figure 4.

Most residential HVAC systems operate efficiently when the total pressure drop across the system (including all the components) is no higher than 0.4 or 0.5 in. WC static pressure. The standard air handler with a permanent split capacitor (PSC) motor is capable of moving the required air flow at about 0.5 in. WC of pressure drop. A variable speed air handler with an electrically commutated (ECM) motor is most efficient when operating at low static pressures (0.3 to 0.5 in. WC).

If a filter with a pressure drop of 0.25 is used in an air handler with a PSC motor, the filter alone can use half of the system’s available static pressure. If a high MERV filter is installed without designing for it and the static pressure is increased to an amount much higher than the optimum, several negative consequences can result:

1. The HVAC system’s overall efficiency (Seasonal Energy Efficiency Ratio or SEER) will go down.
2. The equipment could malfunction or burnout. Motor burnout is more likely to occur with an ECM motor than with a PSC motor. A PSC blower motor will work up to about 0.5 in. WC, then it will “fall off,” i.e., the motor will stop pushing, although the fan will continue to turn. With an ECM, although it is most efficient at 0.3 inches of static pressure, it will continue to work if the static pressure goes up, even as high as 1 in. WC. The ECM will not fall off like a PSC motor; it will continue trying to meet the target cubic feet per minute (CFM) of air flow, but the fan motor will work harder and harder until it burns itself out.
3. In hot, humid climates, if the pressure drop is too high across the filter (i.e., if the filter offers too much resistance), it slows down the air coming through the fan and decreases supply air speed so much that the air in the supply ducts gets too cold. If the ducts are located in a vented, hot, humid attic, the temperature differential is enough to drop the outside surface of the ducts below dewpoint, which can cause sweating on the ducts and eventually can cause the evaporator coil to ice up.

It should be noted that pressure drop through a high-MERV filter varies depending on the velocity of the air flow. Air filters with MERV ratings of 7 to 14+ can have pressure drops anywhere from 0.05 to 0.3 inches WC, depending on filter thickness and air flow velocity. For example, a 4-inch-thick MERV 12 filter can have a 0.2-inch WC pressure drop at a velocity of 300 feet per minute (FPM) and a 0.35-inch WC pressure drop at a velocity of 500 FPM (which is the maximum desired velocity for residential HVAC; 200 FPM to 400 FPM is preferred). The velocity across the filter depends on filter area; the velocity is an input in the Manual D calculation. Filter area can be increased by selecting filters with higher pleats or larger overall dimensions. This may be limited by the HVAC equipment purchased or may require in-field fabrication of a larger filter port on the air handler unit.

As illustrated in Figure 5 below, filter resistance varies by surface area. Deeper pleats or bigger filter dimensions add surface area. Three MERV 7 filters made by the same manufacturer can vary significantly in rated resistance:

• Filter A is a 1-inch-thick filter with 0.20-inch WC at 300 feet per minute (FPM) air flow velocity
• Filter B is a 2-inch-thick filter with 0.13-inch WC at 300 FPM
• Filter C is a 4-inch-thick filter with 0.12-inch WC at 300 FPM

Not all manufacturers give specifications on their filters. Avoid purchasing filters from manufacturers who won’t provide static pressure measurements. Beginning April 1, 2019, the State of California required manufacturers to label HVAC filters with the MERV rating, particle size efficiency (across three particle size bins), and pressure drop (across five airflow rates, as applicable). The label either resides on the edge of the filter or on the pleats so consumers can readily access the information to purchase an appropriate replacement (Figure 6). Looking at Figure 6, the MERV rating of the filter is shown on the very left (MERV 13). Next to this is a table of particle capture efficiencies (e.g. in the particle size range of 0.30-1.0 μm, this filter has a 62% capture efficiency). On the right hand side is a table of pressure drops and associated airflow rates (e.g. this filter has a pressure drop of 0.08 IWC at and airflow rate of 410 CFM. To the very right is the maximum rated airflow for this filter (1,390 CFM). The highly pleated design of this filter allows it to achieve reasonable pressure drop at lower airflows given it’s high MERV rating and narrow profile (1 inch).

The accumulation of dirt and particles can greatly increase pressure drop across a filter. Making filters accessible for easy replacement and providing controls that tell homeowners when replacement is due will help to eliminate problems such as clogging and filter collapse, which are more likely to occur with higher MERV filters (Springer 2009). If exceptionally high filtration is desired (above MERV 13), some sources suggest using separate air filtration equipment with a HEPA filter that can clean the air without impacting furnace performance, although their functionality is localized as opposed to whole house (EPA 2009).

For more on HVAC filter installation, see the guide “Proper Installation of Furnace and Air-Handler Filters.”

How to Select a High MERV Filter

1. Design (or require the HVAC contractor to design) the HVAC duct system using ACCA Manual D to determine the maximum static pressure that the filter can have and select a MERV 6 or higher filter within that limit. Adjust the duct size, duct length, and/or filter surface area as necessary to ensure that the total pressure drop across the system does not exceed the blower fan motor’s limit, given the size of the unit.
2. After the HVAC equipment and filter are installed, measure the total static pressure of the system. With a manometer, measure the supply side with respect to ambient house pressure, measure the return side with respect to ambient house pressure, then add both numbers together. The result should equal no more than 0.4 or 0.5 inches w.c.