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An Analysis of the Effectiveness of Air Filters on HVAC Systems AirCondAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• How to Assess the Effectiveness or Efficiency of Air Filters for Heating and Air Conditioning Systems
• Definition of MERV, Definition of HEPA, MERV ratings explained for HVAC filters
• How air filters work
• Particle size vs. air filter characteristics vs. indoor air quality
• Benefits of gasketed air filters on HVAC systems
• Significance of dirt and debris in the return air plenum or ducts
• Washable air filters for HVAC systems
• Air conditioning / heating system filter product sources and recommendations

Here we discuss and explain various measurements of the effectiveness of different types of air filters for HVAC systems This website answers almost any question you might ask about air filters for heating or air conditioning systems. We explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects. We include photographs to assist readers in recognizing cooling system defects. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. [The page top photograph is of a low-MERV HVAC filter in an air handler.] © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Measuring air filtration effectiveness:

The air filter cascade system installation in our lab provided an opportunity for careful testing of its effect on indoor air quality in the lab, since unlike most building owners, we have an extensive set of indoor air quality monitoring equipment at hand for our IAQ and environmental work. The airborne dust levels of all particle sizes dropped dramatically when the new system was installed (and the existing ducts cleaned as part of the project). Airborne particles in the 1-20 micron range were reduced by about 70%. Simple subjective visual inspection of lab surfaces showed that dust accumulated at a much slower rate.

Air Filter Design Basics: MERV, HEPA, How Air Filters Work

The Definition of Air Filter MERV and what MERV means when buying an air filter

MERV is an acronym for "Minimum Efficiency Reporting Value". The MERV rating on an air filter describes its efficiency as a means of reducing the level of 3 to 10 micron-sized particles in air which passes through the filter. Higher "MERV" means higher filter efficiency. The purpose of the MERV standard is to permit an "apples to apples" comparison of the filtering efficiency of various air filters.

Air filter efficiency refers to the relative ability of a filter to remove particles of a given size or size range from air passing through the filter. If a filter were 100% efficient, none of the particles in a given size range would escape the filter and air which has passed through such a filter would contain zero particles.

The MREV Efficiency Rating Scale ranges from 1 to 16, with 1 being the lowest efficiency and 16 describing the highest efficiency. The particle size range addressed by the MERV scale is 3 to 10 microns. A logical inference is that if an air filter is removing particles down to 3-10 microns, it is certainly also at least that efficient at removing larger sized particles.

What are typical air filter MERV values?

A very low-cost disposable fiberglass furnace filter may have a MERV rating of 1 to 4 - pretty low.

We provide examples of specific air filters and their efficiency at Air Filter Efficiency, a section of our discussion of OPTIMUM INDOOR AIR FILTERS and Cascaded Air Filters

A high efficiency high MERV pleated air filter in the MERV 13-16 range would be expected to remove smaller particles between 0.3u and 1.0u in size at about a 75% efficiency. But before getting too excited about 75% looking not as good at 99.97% (see HEPA filtration discussed below) remember that in a heating or air conditioning system air is being recirculated through the filter system. So if each pass is removing 75% of the particles we want to capture, over time the heating or air conditioning filter system will do a good job of removing a very large amount of airborne dust. This is exactly what we saw when we tested some filtering approaches in our forensic laboratory. The longer we ran the air handler fan the more dramatically we saw the airborne dust level drop in the test area.

The Definition of A "HEPA" rated air filter (High Efficiency Particulate Air [Filter] or better put, High Efficiency Particulate Arresting [Filter]) has to meet more demanding US DOE standards than a high MERV air filter. A HEPA filter is required to remove at least 99.97% of airborne particles down to 0.3u in size (in diameter or in their longest dimension). Any filter, high MERV or HEPA, can be expected to filter out larger particles with an efficiency above its rating.

So do we want to use a high MERV air filter or do we need to install a HEPA filter on our air conditioner or heating system air handler? A high MERV air filter is much less costly than a HEPA filter and can be more effective over time in a residential or office air conditioning or heating system, but there are two warnings that have to be considered:

1. Air filter bypass leakage if the air filter does not fit properly and tightly, no matter how efficient it is, it may be leaking a significant of air around itself at the mounting, reducing the overall efficiency of the air filtration system. You can see this effect by noticing how much debris collects in the return air plenum behind the filter. If at each filter change you see that the ductwork or return plenum is pretty clean, the filter is not bypassing much air. Some air filter manufacturers specialize in providing air filters that include a gasket to seal against air bypass leaks.
   
   
2. Air filter density and reduction in airflow: some air filters may be so dense that the cubic feet per minute of air that can pass through them is reduced drastically. Check with your air conditioner or heating system installer or service technician to review the intended air flow rate (in cubic feet per minute) of your system and on the effects of installing a better, more efficient but slower-air-passing filter. An HVAC technician and some home inspectors have equipment that can easily measure the air flow rate at a given point in the system. In our lab experiments we made certain that the new air blower assembly we were installing and testing was able to move enough cfm of building air with the improved cascaded filter scheme we were testing.

Smaller airborne particles than those covered by the HEPA or MERV rating might be filtered out with still higher efficiency. Their smaller mass means they may "stick" to the filter surface well but some filters, such as the budget lowest-cost fiberglass furnace filters, will pass these small filters right through when they are new, but filter out more of them as the filter surface becomes clogged and dirty with debris (leaving smaller and smaller openings through which air must pass.

To continue reading this air conditioning inspection guide, use links to the document chapters at left. Links shown in green font indicate where you are in this document.

How do air filters actually remove particles from the airstream?

This photograph shows a soiled surface of a conventional air filter up close. The fibers are clearly visible but not the openings through which air has to pass. If we look at a high efficiency, high MERV or HEPA air filter under a microscope we'll see a mat of randomly crisscrossed fibers of filter material (fiberglass, polypropylene, paper, or other materials). The space between the filter fibers will be larger than the smallest particle size which the filter is asserted to remove. So how do these filters stop the small particles? Let's look at three air filtration mechanisms in order of decreasing particle size:

1. First, some airborne particles smack directly into a filter fiber, either because they happen to be on that unlucky (for them) trajectory or more likely because the particle is so large that it can't pass through the opening, or part of the particle smacks into the filter fiber. Air filter scientists call this method "impaction". Impaction captures larger particles which in a HEPA system are those larger than about 0.4u.
   
   
2. Second, some airborne particles moving in the airstream past the filter happen to be close enough to a filter fiber that they stick to it. How close? the particle has to be closer to the filter fiber surface than one radius or diameter of the particle itself. In other words, a 2u particle which is only 1u away from a fiber will probably stick to the fiber. Air filter scientists call this method "interception". Interception captures particles mostly in the 0.3u size to 0.1u size.
   
   
3. Third, very small particles, say below 1u in size, collide with larger gas molecules swirling around in the air turbulence caused by air moving by the filter fiber. This collision, called "diffusion" in filter science, causes the particle to zig zag out of the airstream passing through the filter opening and to stick to the filter surface.

To continue reading this air conditioning inspection guide, use links to the document chapters at left. Links shown in green font indicate where you are in this document.

Before scrolling down in this article, be sure to read Particle Sizes & IAQ for a description of the sizes and behaviors of some of the common particles that are an indoor air quality concern.

What airborne particle sizes are an IAQ concern?

What mold, house dust dust, allergen fragment, mite fecal, cat dander, or other airborne particle sizes are a concern for indoor air quality? In the photograph shown here the large black Stachybotrys chartarum mold spores can be seen against our eyepiece micrometer which, after calibration, shows that these particular spores were about 7u x 15u in size. The brownish tubular structures are fungal hyphae. Another, smaller fungal spore is in the background. What's not addressed by some of the science in the air filtration and IAQ field is just what particle sizes are a worry. In general, larger particles, say 30u or 50u or long fibers, say 200u, are so big that they tend to be filtered in the nose of a human breathing that air. (1u here means 1 micron in size).

A more complete discussion about the size and behavior of problematic indoor air particles which form an indoor air quality concern can be read at Particle Sizes & IAQ. You should read that before continuing below.

The Benefits of Gasketed Air Filters on HVAC Systems

A gasketed heating or air conditioning system air filter improves the filter effectiveness. The benefits of a high efficiency air filter material when used in an air handler and duct system are not fully realized if air bypasses the filter around its installation frame due to poor fit or poor installation. [The filter shown here is not a high quality gasket-type air filter.] That's why some manufacturers emphasize the value of air-sealing gaskets on their products. Simple observations made in the field will make for agreement with the manufacturers that filter bypass is an issue at some heating or air conditioning installations. When changing an air filter at a return air inlet or at an air handler, when the old filter has been removed, take a look inside the return duct at the return register, or inside the return plenum at the air handler, to see just how much debris is passing the filter. [The photograph above shows how a badly-fit HVAC air filter can not only leak bypass air into the blower compartment but also it can collapse right into the blower fan - this is a potential fire hazard!]

Implications of dust and debris in the return air plenum

On an old heating or air conditioning system where filters have been ineffective, not properly installed, leaky, or not maintained and changed on schedule, we often find quite a bit of debris entering the air handler and duct system. This is in fact so common that most HVAC installers and technicians consider dirty ducts to be "normal". And they're quite correct: typical house dust that has collected inside of a duct system is not normally a toxic substance. But 1) a high level of dust and debris inside of an air handler or duct system indicates that system filtration has not been effective, 2) the dust level indicates that the system has been delivering dust and debris to the occupants providing poorly-filtered air, and 3) there is an increased risk of mold in the duct or air handler system if moisture enters the system from a leak or from air conditioning condensate mishandling.

Washable air filters for heating and air conditioning systems

Washable re-usable air filters are very appealing from a cost viewpoint. I would look for data to indicate whether or not the washable filter runs at a lower MERV or filtering efficiency. In our cascaded filter proposal (discussed at this website) we included and continued to use a washable filter that came with the equipment since it was not reducing the airflow CFM, but we preceded it with other filters for different purposes. The photograph shows a typical washable metal air filter. Other types of washable air filters are available as well.

To continue reading this air conditioning inspection guide, use links to the document chapters at left. Links shown in green font indicate where you are in this document.

Be sure to read Particle Sizes & IAQ for a description of the sizes and behaviors of some of the common particles that are an indoor air quality concern.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR CONDITIONING SYSTEMS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
  Air Filter Location
  Dirty Air Filter Problems
  Missing Air Filters
 OPTIMUM INDOOR AIR FILTERS
  Air Filter Efficiency
  Optimum Air Filter Design / Location
  How to Cascade Air Filters
 CONTINUOUS BLOWER FAN OPERATION
 AIR FILTER EFFECTIVENESS
  MERV, HEPA Definitions
  MERV Air Filter Ratings
  How Air Filters Work
  Particle Sizes & IAQ
  Gasketed air filters
  Debris in Return Air Plenum
  Washable air filters
 FIBERGLASS & AIR FILTERS
  Air Filters Shed Fibers?
  Fiberglass & Test Lab Accuracy
  Variations in Fiber Size
 SOURCES FOR AIR FILTERS
  OTHER AIR CLEANERS
AIR HANDLER UNIT
  DIRTY A/C BLOWERS
    Leaks, Rodents In Air Handlers
    Mold Growth in Air Handlers
  DAMAGED COOLING COIL
  DIRTY COOLING COIL
  DIRTY COIL CLEANING PROCEDURES
  FROST BUILD-UP
  BLOWER LEAKS, RUST & MOLD
  ADDING A/C: RETROFIT SIZING
CLEANING & Legionella BACTERIA
COMPRESSOR CONDENSER
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
FAN AUTO ON Thermostat Switch
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS
Air Conditioning "How To" Books
FURNACES WARM AIR HEATING SYSTEMS

Technical Reviewers

• Daniel Friedman - principal author/editor of the InspectAPedia ^TM Website
• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Wikipedia provided background information about the definition of HEPA and airborne particle interception.

More Information on Building Diagnostic Inspections and Repairs

InspectAPedia ^TM Home & Site Map - Building Inspection, Diagnosis, & Repair, Environmental Inspection & Testing - Research Website

The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Home Inspection Construction Consulting Services & advice for home buyers

Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.

Contact Daniel Friedman for website content suggestions or for fee-paid consulting

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