Air Filtration | Library builder

The addition of filters to ventilation and forced-air heating and cooling systems allows for greater control of air quality. As discussed above, indoor air is often too polluted to properly nourish occupants. The first line of defense against such pollutants is to provide an ample supply of fresh outdoor air through ventilation. Unfortunately, “fresh” air, although considerably cleaner in most cases than indoor air, often contains allergens in the form of molds, pollens, and manufactured pollutants,

Type	Purpose	How it works	Advantages	Disadvantages	Comments
Natural ventilation	To bring fresh air into the home and exhaust stale air	Takes advantage of natural air patterns. Strategically placed openings encourage fresh air to move diagonally through the space, entering low and exiting high.	• Quiet • Free • Maintenance-free • Does not require energy to operate	• Can be drafty and create greater heating/cooling load • Air cannot be filtered • Allows for only minimal control • Requires high level of occupant participation • Suitable as sole means of ventilation throughout the year only in very temperate climates • Outdoor air must be clean	This strategy works best in mild climates and can be enhanced through various roof-ventilation techniques.
Exhaust fans	To remove localized pollution at the point of generation, primarily in kitchens and baths	Stale and moistureladen air is sucked out of the house at the point of generation, using a powerful fan.	• Pollution is quickly removed before the rest of the home is affected	• Can depressurize home, causing infiltration and possible back — drafting	It is important to supply replacement air when fans are in operation. Exhaust fans are required by code in bathrooms and laundry rooms without operable windows.
Supply fans	To provide fresh air	A fan blows fresh outside air into the home, creating positive pressurization that forces stale air out.	• Inexpensive • Pressurization of home prevents contaminants from infiltrating from outdoors	• Cold drafts around fan in winter • Pressurization can cause hidden moisture problems as humid air is forced through wall openings and then condenses	Adequate and strategically placed vents are required to exhaust air. This is a good strategy for venting a basement. Supply fans are not suitable for dispelling kitchen — and bath-generated pollution.

Type	Purpose	How it works	Advantages	Disadvantages	Comments
Balanced mechanical ventilation	To provide fresh air and exhaust stale air while controlling pressurization	A set of fans brings in fresh air through intake and distributes it, then exhausts stale air to the exterior.	• Provides balanced pressurization • Comes equipped with, or can be adapted for, various filtration strategies	• Does not moderate temperature or humidity of incoming air • Can be noisy • Relatively small fans are standard and are insufficient to handle large amounts of gas filtration
Air-to-air heat exchange or HRV (heat recovery ventilator)	To supply fresh outdoor air into the home while exhausting stale air and maintaining indoor temperatures	Incoming fresh air passes through a series of chambers adjacent to outgoing exhaust air. Heat, but not air, is transferred.	• Reduces heating and cooling costs by recovering 60 to 80% of heat	• Chambers can be made of paper, which collects dirt, or plastic, which can outgas; choose one with metal chambers • More costly initially than other balanced ventilators • Causes condensation; must be maintained to remain mold-free	Most effective for tight homes in cold climates. Energy recovery ventilators (ERVs) also recover humidity VenMar Ventilation, Inc.
Fresh-air intake incorporated into forced-air system	To provide fresh air when the central forced-air system is operating	A 3" to 6" metal pipe with damper valve provides fresh air into the furnace supply stream.	• Inexpensive to retrofit •Ventilation supply air is preheated or precooled • Makes use of existing ductwork for distribution • Creates slight positive pressurization and can compensate for air lost through leaky ducts	• Operates only during heating or cooling season • Depends on a well-maintained heating and cooling system to deliver good — quality air	Screen all intake pipes to prevent rodent infestation.

including exhaust fumes, smoke, and pesticides. When your immediate surroundings are less than perfect, you may wish to incorporate some form of filtration into your home.

Home ventilation systems can easily be adapted to filter large particles like pollen and mold spores. However, most home ventilation systems are not equipped with very powerful fans and therefore cannot handle the air resistance created by some of the more efficient filtration methods, especially those designed to remove gases. Consequently, whole-house filtration is often more successfully combined with the forced-air distribution system. Standard filters used with most forced-air systems are designed primarily to prevent large particles from harming the motor, and are insufficient to effectively filter out small particles injurious to human health. Most forced-air equipment must be adapted to receive additional filtration systems. When equipped with good filters, a forced-air system will not only clean fresh intake air but also continue to clean air as it recirculates.

Filter efficiency rating systems such as the “dust spot” and “arrestance” systems have been developed by filter manufacturers to provide information about the ability of a filter to remove large particles from the airstream. This information is of limited use when evaluating filter effectiveness for removing small particles of less than 5 microns, which can be the most damaging to health. In 1999, the American Society of Heating, Refrigeration and Air Conditioning Engineers published a new filter rating system that goes by the acronym MERV. This stands for Minimum Efficiency Reporting Value and is a much better way of rating the ability of filters to remove small particles. The system currently rates filters on a scale of 1 to 16, with the higher number ratings representing filters that can remove smaller particles. Although they are currently not rated by the MERV system, a HEPA filter would likely rate a 17 or higher.

Adequate MERV filters are currently available that will fit into a standard furnace filter slot. For example, a MERV 8 filter is rated to remove greater than 70 percent of particles in the 3- to 10-micron range. This will remove most, but not all, mold spores and dust and should be considered as a much better alternative to the standard fibrous furnace filter. The Filtrete Ultra Allergen Reduction Filter #1250 is a MERV 11 filter (removes 65 to 80 percent of 1- to 3-micron particles) manufactured by 3M. It is commonly available at Lowes and Home Depot and will also fit in standard і-inch furnace-filter slots.

While higher MERV-rated filters as well as HEPA systems are available, these more efficient filters tend to create more resistance, requiring a much larger filter to allow air to flow through them. They do not fit into or work well with standard HVAC systems. Note that when changing from a standard furnace filter to a more efficient filter it is important to replace it after one months use before going to the regular three-month maintenance schedule recommended by the manufacturers because the filter will be overworked when first installed in a dirty environment. If the new type of filter continues to clog too quickly, there maybe a larger than normal dust load in the building. John has observed this when an air path to the attic or crawlspace exists that pulls dirt or insulation fibers into the system.

Most mechanical filters that remove particles by filtration actually become more efficient as air flows through them. They remove particles as a sieve does. As the filter loads, the airflow slows and smaller particles can then be removed. The filter must be replaced before airflow is impeded. Electrostatic filters don’t do as good a job of removing particles when they get dirty and must be cleaned often to achieve the manufacturer’s stated MERV ratings.

Further Reading

Bower, John. Understanding Ventilation: How to Design, Select and Install Residential Ventilation Systems. The Healthy House Institute, 1995.

Mazria, Edward. The Passive Solar Energy Book. Rodale Press, 1979.

Chart 15.3: MERV Ratings Compared to Other Filter Ratings

MERV	Particle size range	Test	Particle size range, pm	Applications
3 to 10 pm	1 to 3 pm	.3 to 1 pm	Arrestance	Dust spot
1	<20%	—	—	<65%	<20%	>10	• Typical residential filters • Pollen, dust mites
2	<20%	—	—	65-70%	<20%
3	<20%	—	—	70-75%	<20%
4	<20%	—	—	>75%	<20%
5	20-35%	—	—	80-85%	<20%	3.0-10	■ 1" residential pleated filter • Pollen, dust, dust mites, most molds, most spores
6	35-50%	—	—	>90%	<20%
7	50-70%	—	—	>90%	20-25%
8	>70%	—	—	>95%	25-30%
9	>85%	<50%	—	>95%	40-45%	1.0-3.0	• Moderate efficiency residential pleated filter (some may need furnace modification for installation) • All of above and Legionella
10	>85%	50-65%	—	>95%	50-55%
11	>85%	65-80%	—	>98%	60-65%
12	>90%	>80%	—	>98%	70-75%
13	>90%	>90%	<75%	>98%	80-90%	0.3-1.0	• Specialty filter (usually need furnace modification for installation) • All of above and smoke, bacteria
14	>90%	>90%	75-85%	>98%	90-95%
15	>90%	>90%	85-95%	>98%	-95%
16	>95%	>95%	>95%	>98%	>95%

Filter type	Purpose	How it works	Efficiency	Advantages	Disadvantages	Comments
Standard furnace filter, MERV 1-4	Filters out large particulate matter to safeguard the motor, not the inhabitants	A coarse, 1 "-thick filter traps large particles.	Removes less than 20% of particulate matter; does not significantly remove small particles	* Inexpensive • Easy to change	* Indoor air quality not significantly improved * Does not remove significant levels of mold spores	Can easily be replaced with 1" pleated panel filter, which will raise efficiency to MERV 6-11
Medium efficiency extended surface (pleated panel) filter, MERV 6-11	Particulate filter	Air is strained through a pleated (extended surface area) filter that maintains airflow.	Removes 2070% of particulate matter	• Relatively inexpensive • Sufficient for most general filtration • Airflow resistance can be low enough to use with most HVAC ventilation systems • Removes many mold spores	• Filtration is inadequate for very polluted environments and/or very sensitive people • Does not filter out gaseous pollution	Many are now available that will work with standard HVAC systems. Media filters become more efficient with time as pores become smaller, but air resistance increases.
HERA (high efficiency particulate air) filter	Particulate filter	Polyester or fiberglass fibers are bound with synthetic resins, creating a medium with extremely small pores.	Removes over 99.97% of particulate matter at 0.3 microns	• Can remove minute particles for extremely clean air — Can remove cigarette smoke and almost all mold spores	• High airflow resistance requires powerful fan • Expensive • May require custom design • Does not filter out gaseous pollution such as VOCs	Not commonly used in residential filtration. A carbon postfilter will help eliminate odor generated by the HEPA filter. An inexpensive, frequently changed prefilter will extend the life of the HEPA filter.

Filter type	Purpose	How it works	Efficiency	Advantages	Disadvantages	Comments
Electro static precipitator (ionizer)	Particulate precipitator	Mechanism is mounted to ductwork, which statically charges dust. Dust is collected at oppositely charged plates in a filter.	Removes 90% of particulate matter when clean	* No resistance to airflow * Efficient when clean * Does not require replacement	• Must be adapted for residential use • Ozone is produced as byproduct of high voltage • Relatively expensive • Does not filter out gaseous pollution • Efficient only when clean • Generates EMFs	Plates must be cleaned frequently.
Electrostatic air filter (passive)	Particulate filter	Electrostatic charge is generated by friction as air moves through special media.	Removes 10 to 15% of particulate matter	• Good for large mold spores and pollen • No customization required on some filters used with HVAC • Inexpensive (washable/ reusable)	• Not efficient for capturing small particles • Limited efficiency • Does not filter out gaseous pollution • Requires frequent cleaning to maintain filter efficiency	May be substituted for standard furnace filters. An inexpensive way to relieve pollen and mold allergies. Medium efficiency extended surface (pleated panel) filters are making these obsolete.
TFP (turbulent flow precipitator)	Particulate precipitator	Turbulent airstream "drops" particles into collection space, where there is no airflow.	Manufacturer claims 100% removal of particulate matter	• No resistance to airflow • Can be used with ventilator •Very low maintenance.	• Does not filter gaseous pollution	Actual performance varies widely.

Filter type	Purpose	How it works	Efficiency	Advantages	Disadvantages	Comments
Partial bypass filter	Absorption of gaseous pollutants	Granules of absorptive material are held in place and separated by a metallic grid. Some air passes through the medium and some flows past unrestricted.	Efficiency varies widely based on amount of air that bypasses the filter	• Allows some air to flow through, thereby cutting down air resistance and requiring less powerful fan	• Not suitable where air is highly polluted • Not suitable in ventilator	Works in conjunction with HVAC, where the same air is repeatedly run through the filter.
Activated carbon filter	Adsorption of gaseous pollutants (not for particles)	Gases cling to many-faceted carbon granules.	Varies	• Effectively removes gases with high molecular weight • Offered in standard furnace sizes for low-pollution situations	• Does not remove certain lightweight pollutants such as formaldehyde or carbon monoxide • Filters become contaminated with use and can release pollutants if not changed	Can be treated to remove more gases. Must be changed regularly per manufacturer’s recommendations.
Activated alumina	Adsorption and transformation of gaseous pollutants (not for particles)	Activated alumina is impregnated with potassium permanganate. It acts as a catalyst in changing the chemical composition of harmful gases and also acts through adsorption.	Varies	• Will remove gases not removed by carbon, including formaldehyde ■ Lasts longer than carbon	• Not as adsorptive as carbon • More expensive than carbon	Activated alumina changes color when depleted.

Air Filtration

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