Air filter efficiency is defined in MERVs; MERV stands for “Minimum Efficiency Reporting Value.” The MERV scale has a range of 1 (least efficient) to 16 (most efficient), indicating a filter’s ability to remove particles from 3 to 10 microns in size. (For comparison, an average human hair is 100 microns thick.) Filters with higher ratings not only remove more particles from the air, they also remove smaller particles. A typical 1-inch-thick spun fiberglass furnace filter has a rating of 1 to 4 on the MERV scale and has a 60% to 80% efficiency at trapping particles bigger than 10 microns like sawdust, carpet fibers, dust mites, and pollen, but has a less than 20% success at trapping particles in the 3 to 10 micron range. ENERGY STAR Version 3, Rev.6 requires that MERV 6 or higher filters be installed in each ducted mechanical system and each outside air intake. A MERV 6 filter has a 35% to 50% efficiency at capturing 3- to 10-micron particles, such as mold spores, cat and dog dander, and hair spray (ANSI/ASHRAE 2007). The American Lung Association Health House® guidelines require filters to have a minimum rating of MERV 11 (ALA 2012), which equals an 85% efficiency at trapping particles 3 to 10 microns and a 65% to 80% efficiency at trapping particles 1 to 3 microns (Legionella, lead dust, auto emissions, etc.). Higher MERV ratings (from 13 to 16) can remove bacteria and smoke particles. HEPA filters with MERV ratings from 17 to 20 are used in special environments like hospitals and manufacturing because they can filter out particles smaller than 0.3 microns, including viruses (ANSI/ASHRAE 2007).
Builders and homeowners may be tempted to install the highest MERV filters they can find, but filters with very high MERV ratings (MERV 13+) can restrict air flow so much that they use substantially more electricity and can even cause the blower motor to burn out.
Filters were originally intended to protect the HVAC equipment by stopping large particles from clogging the air passages of the coils. The familiar spun fiberglass filters (typically 2 MERV) filter out enough of the large particles to protect the furnace while allowing maximum air flow. Maintaining the furnace manufacturer's specified air flow is important for achieving energy efficiency and maximizing the life of the blower motor and heat exchanger.
Filters should be selected as part of the overall duct design process, as described in the Air Conditioning Contractors of America (ACCA) Manual D Residential Duct Systems (ANSI/ACCA 2009). Air filters cause resistance to air flow, as do other components of the HVAC system, like undersized ducts, duct length, bends in duct, and register grilles. This resistance is called a pressure drop and is measured in inches of water column (in. WC). 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:
- The HVAC system’s overall efficiency (Seasonal Energy Efficiency Ratio or SEER) will go down.
- 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.
- 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.
Accumulation of dirt and particles can greatly increase pressure drop across a filter. Because high MERV filters can trap more particles, they are likely to clog faster than low-MERV filters. One way to increase their capacity is to increase filter area. Filter manufacturers do this by increasing the dimensions of the filter or be increasing the thickness of the pleats of the filter, i.e., a filter that has 4-inch-high pleats has twice the surface area of a filter with 2-inch-high pleats.
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).
It should be noted that pressure drop across 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 in. WC, depending on filter thickness and air flow velocity. For example, a 4-inch-thick MERV 12 filter can have a 0.2 in. WC pressure drop at a velocity of 300 CFM and a 0.35 in. WC pressure drop at a velocity of 500 CFM (which is the maximum desired velocity for residential HVAC; 200 CFM to 400 CFM is preferred). The velocity across the filter depends on filter are; 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. Not all manufacturers give specifications on their filters. Avoid purchasing filters from manufacturers who won’t provide static pressures measurements.
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 furnace filter installation, see Proper Installation of Filter.
How to Select a High MERV Filter
- 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 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.
- 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 w.c.
Figure 1 - ENERGY STAR requires furnace filters that are MERV 6 or higher to both protect the HVAC equipment and improve indoor air quality.
Figure 2 - Furnace filter resistance varies by surface area; deeper pleats add surface area.
As illustrated inn Figure 2 above, furnace 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 in. WC at 300 feet per minute (FPM) air flow velocity
Filter B is a 2-inch thick filter with 0.13 in. WC at 300 FPM
Filter C is a 4-inch thick filter with 0.12 in. WC at 300 FPM