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Optimizing Air Filters Maximum Indoor Air Quality AirCondAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Air conditioning / heating filter suggestions for optimum indoor air quality
• How to select & install a cascade of types of HVAC air filters for maximum air filtration and maximum reduction of indoor air particles
• Air conditioning / heating system filter product sources and recommendations
• Air conditioning / heating system air handlers - Fan Coil Unit Inspection, Diagnosis, Repair, Replacement

Here we discuss How to Select and Install Air Filters on Heating and Air Conditioning Systems to Obtain the Best Indoor Air Quality by superior air filtration. (Naturally if there is an IAQ problem source in the building it is essential to also find and correct that condition.

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.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © 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.

OPTIMUM INDOOR AIR FILTERS - Air Filter Choices & Placement for Optimum Indoor Air Quality

Selecting Air Conditioning System Filter Types & Air Conditioner Filter Cascading for Optimum Indoor Air Quality and maximum reduction in airborne particles involves selection of high efficiency filters and other filtration equipment and placing them in the correct order and in the most effective location within the air conditioning system air handler and duct work.

Air conditioning and heating system filter efficiency

Highly efficient air conditioning or heating air handler or duct air filters which trap smaller particles are available, right down to HEPA filters. We recommend improved filtration installed particularly in buildings where occupants have mold, allergy, or other respiratory concerns.

For the front-end air filter described in the cascaded air filter system outlined below, we selected a Filtrete Filter - the Micro Allergen 1000^TM reduction filter made by the 3M Corporation. This filter is described as also resisting microbial growth.

• Washable air filters such as the stainless steel mesh air filter that came with our air handler are the least efficient at capturing airborne dust particle, with a relative efficiency of roughly 1/20th that of the Micro Allergen 1000^TM air filter.
  
  
• Non-electrostatic pleated air filters have a relative efficiency of about 1/20th that of the Micro Allergen 1000^TM air filter.
  
  
• Non-electrostatic mini pleated air filters have a relative efficiency of about 50% that of the Micro Allergen 1000^TM air filter.
  
  
• The 3M Micro Allergen 1000^TM reduction filter and air filters of similar rating made by many other air filter manufacturers has a performance rating of 1000 by 3M and a MERV of 11 with an air-flow rate of 1.5 meters per second. This filter is rated as capturing 80% of airborne dust particles in the 3 to 10 micron size range (ASHRAE particle class E3).
  This is more than adequate to capture most typical indoor allergen and mold particles, but will be less efficient at capturing the smallest mold spores such as Penicillium sp. and Aspergillus sp. which can produce some spores in the 1-2 micron size range.
  
  
• The 3M Filtrete Ultra Allergen 1250^TM reduction filter is rated by 3M at 1250.

Air filter effectiveness and MERV ratings are described in detail at AIR FILTER EFFECTIVENESS.

Cascaded Air Filter Designs: Optimum Air Filtering Can be Obtained by Air Filter Selection, a Combination of Air Filters, & Air Filter Placement

At our forensic microscopy lab where it is important to keep the dust and airborne debris to a minimum, when we replaced our heating/cooling system we specified the following filters in the order shown, moving from the building air return to the air handler itself.

Cascades of filters will significantly reduce airborne particulate debris, mold, pet allergens, dust mite allergens, and similar particles. But as we discuss below, cascaded air filters may also cause an un-wanted reduction in total air flow unless the heater or air conditioner blower fan is of adequate capacity. It may be possible to regain some of this lost air flow by making sure that the blower assembly is clean and that the duct system is not leaky or blocked anywhere. We discuss the detection of duct system defects such as blockages and leaks at DUCT SYSTEM DEFECTS.

1. 
   Front end air filter: at the central air return we installed a typical inexpensive 1" disposable filter which protects the return ducts from high levels of debris. This filter is changed monthly. Our first photo just above in this article shows the installation of a front-end filter at the return air register in our lab.
   
   Our photo at left shows the first stage of a cascaded air filter system: here we've installed a filter right at the return air register. A dirty filter is shown at lower left and the new clean filter has been set in place.
   
   If your HVAC system has only one air filter, the best location for it is at the return air inlet register such as shown in this picture. This protects the entire duct system and air handler from accumulating un-wanted dust and debris. Often we see that the only filter on an air conditioning or warm air heating system is installed right at the air handler, perhaps at the blower inlet. The disadvantage of this filter location is that the entire return duct system can serve as a collector for dust and allergens, becoming a potential reservoir for growing mold colonies, especially if the duct system is exposed to high moisture or water leaks.

   Effectiveness of the front-end air filter: In our experience, the front-end air filter has been doing a great job of keeping the dirt and debris level very low in our air handler, as you can see in our photo at left where we've removed the filter to show the interior of the sheet metal return air plenum chamber after three years of system operation.

   Installing a good air filter at the return register not only keeps the return duct system itself clean, reducing future duct cleaning costs, it also reduces the workload for the electrostatic air cleaner (so we have to wash it less often), and further reduces the rate of particle clogging on our HEPA air filter installed after the electrostatic air cleaner.

   Cleaning of the electrostatic air cleaner elements may be performed less frequently, perhaps annually, and replacement of the HEPA filter has been necessary only every two or three years with this cascaded filter installation.

   
   
   
2. 
   Washable metal air filter: in our cascaded air filter system, the front end filter shown above is followed by a washable metal air filter that you can just see in this photo looking "up" in the return plenum under our air handler.
   
   This washable air filter is unnecessary in our cascaded air filter installation, but because it is very low in air restriction and because it came with the air handler, we just left it in place. If airflow restriction had been a problem with this cascaded filter system we'd leave this air filter out of the system.
   
   
   
   
   
   
   
3. Electrostatic air cleaner: at the air handler return plenum [See our photograph of an electrostatic air cleaner installed at the air handler, shown at below left.]

   The electrostatic air cleaner causes airborne particles that escaped our front end filter to plate-out and stick to the electrostatic air cleaner's blades. Periodically the electrostatic air cleaner elements are removed and washed. Some manufacturers recommend simply running the unit through the dishwasher.
   
   Be careful not to bend the metal plates or break the wires that area exposed when you remove the electrostatic air cleaner element for cleaning.
   
   

4. 
   
   HEPA air filter: at the air handler return plenum, after the electrostatic air cleaner. This arrangement permits us to use the more costly HEPA filter for many months before it needs to be changed out.
   
   See our photograph of a HEPA filter in the air handler, shown at right where the filter has been removed from its housing for display purposes.
   
   
   
   
   
   
   
5. UV light treatment in the duct system or air handler: is also available for HVAC systems and may be slightly helpful for areas where occupants are at extreme medical risk. I have not see enough supporting data to be convinced that these lights have much effect on air flowing past them as the contact time for particles moving in the duct system will be minimal.
   
   Some installers place these lights in the return or supply plenum on the hypothesis that they are disinfecting the surfaces there. This is an appealing argument except that nothing is disinfecting the other 99% of the surface area of the duct system. I did not install this equipment at my lab.
   
   
6. We also installed an automatic two-speed blower fan in the air handler. The technician selected this blower fan as one capable of providing adequate CFM of air flow in the system with the cascade of filters we describe above.

Effectiveness of the Cascaded Air Filter System in the Air Ducts and Air Handler

Measuring the airborne particle level before and after installing the cascaded filter system we describe here, and obtaining actual counts of airborne dust particles per cubic meter of air, we found at least an order of magnitude reduction in the dust level in the building. When we compared before and after airborne particle counts, airborne particles in the 1-20 micron range were reduced by about 70%. Visual inspection of horizontal surfaces on which dust accumulates showed that dust accumulated at a much slower rate than before we installed the new filter system.The actual dust level reduction that you can expect in a building where these improvements are made depends on these factors (and probably others that some readers may be kind enough to suggest):

1. the initial dust level in the building
   
   
2. the sources of building dust, mold, pollen, mite fecals, or other allergens, and steps that are taken to remove and prevent recurrence of these sources
   
   
3. the efficiency of the air filters installed
   
   
4. air duct and air handler or blower fan maintenance, including changing the front-end air filter on a regular schedule, keeping the duct system clean, etc.
   
   
5. percentage of time that the HVAC blower system is in operation (more hours on means greater reduction in building dust levels)

How to use a Cascaded or other Air Filter System on an HVAC System

1. Select a blower fan capable of continuous duty and with enough power to overcome resistance of the air filters. For maximum IAQ improvement in buildings: for the new blower installation in our forensic microscopy lab we selected a fan unit which had both the capability of delivering adequate CFM of air flow and a blower fan motor duty cycle which permits continuous operation if we wish to run the system that way.
   
   
2. 
   Make certain that the blower fan itself is completely clean. Dust or debris collecting on the blades of a squirrel cage blower fan assembly can cut the airflow by 50% or more.
   
   Even if it costs a few hundred dollars to clean a dirty blower fan, the increased air flow can more than earn this cost back in increased air flow and thus reduced heating or cooling costs.
   
   
   
   
   
3. Run the air conditioning or heating system blower fan continuously or on regular timed cycles. Running the blower continuously at low speed resulted in continual air scrubbing in the building. When the heating or cooling needs of the building require, the fan shifts automatically to high speed.
   
   
4. 
   Change the front end air filter regularly to avoid passing dust and debris into the air handler and the downstream filters."
   
   Because air filters restrict air flow more as they become dirty, 3M recommends (no surprise) that where air flow reduction is a concern, consumers should simply replace the air filter more often.
   
   
   
   
   
   
   
   
5. Remove any unnecessary air filters or duct obstructions such as the washable metal air filter we described above.
   
   
6. 
   
   Tune up the ductwork: make sure the duct system is not leaky, crimped, blocked, wet, moldy, damaged, etc. See DUCT SYSTEM DEFECTS for details of various air duct problems and how to find and correct them.
   
   
   
   
   
   
   
   

Other Steps to Regain Air Flow Lost by Cascaded Air Filters or Other Duct System Problems

In addition to the steps outlined just above at How to use a Cascaded or other Air Filter System, you can take maximum advantage of any air filtration system on a building's heating or air conditioning system if you install and use a blower fan which can operate continuously for maximum reduction of indoor dust levels and improved indoor air quality.

We discuss this suggestion in detail at CONTINUOUS BLOWER FAN OPERATION. However before installing a high-efficiency HEPA filter on your system you need to review the air flow capacity of the blower and the duty-cycle rating of the blower fan motor (can it be run continuously).

Cascades of air filters in the duct system may reduce airborne particulate debris but cascaded air filters may also cause an un-wanted reduction in total air flow unless the heater or air conditioner blower fan is of adequate capacity.

It may be possible to regain some of this lost air flow by making sure that the blower assembly is clean and that the duct system is not leaky or blocked anywhere. We discuss the detection of duct system defects such as blockages and leaks at DUCT SYSTEM DEFECTS.

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 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.

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