This article first appeared in the November/December, 2001, issue of Smart Homeowner magazine. We found this to be the best article on the Internet we could find that does a great job of explaining the differences between furnace filter types void of the hype and false sale's pitche's from Internet sellers who will try to push the MERV 2-3 washable furnace filters on their customers as something they're not.

FURNACE FILTERS

The primary purpose of the filters in a comfort conditioning system is to protect the equipment. Dust collecting on the blades of a blower can throw it out of balance, causing vibration, equipment wear, and noise. Dust on the fins and coils of a heating or cooling system can restrict air flow through the system, and reduce the efficiency of heat transfer. A filter is relatively inexpensive to change or replace. If a cooling coil is blocked with dust and lint, it may have to be removed for cleaning. That can cost $250 or more. 

The air filtering system can also contribute to your good health and well-being. A 1999 Mayo Clinic study showed that mold is the culprit in nearly all chronic sinus infections. 

Most home heating and cooling systems use a filter that is about one inch thick. A visit to a home center store may produce seven or more different filters in that thickness, ranging in price from 49 cents to $15. Their effectiveness varies considerably, as well. 

What Is the Difference? 
Depending upon the design of the filter, it may remove mostly large particles--"dust mice" and pet hair. Others will remove particles as small as fungi and bacteria. The American Society of Heating, Refrigeration, and Air Conditioning Engineers has devised a rating system for filters (ASHRAE 52-1). The MERV (Minimum Efficiency Reporting Value) shown on the packaging of most filters is based on their ability to capture particles larger than one micron (a unit of measure slightly less than 1/25,000 of an inch). This rating system is designed to represent, with a single number, the performance of the filter over a large range of particle sizes. These values range from 1 to 12, with the higher number capturing more of the dust. 

A second rating system, Filter Performance Rating (FPR), is primarily used by the 3M Company, a major manufacturer of filters. It is based on the ability of the filter to capture particles from 0.3 to 1.0 microns. These sub-micron particles are the ones most likely to be inhaled, where they can cause problems in the lungs. The rating of filters in this system range from 300 to 1600. In general, the cost of the filter increases as the efficiency increases. 

 
How Often Should I Change or Wash the Filters? 
Filters are rated when they are new. As they accumulate particles, the spaces between fibers in the filter become smaller, allowing them to trap progressively smaller particles. Filters have been compared to cheese--the older it gets, the better it gets, until it gets so good you have to throw it away. I have found fiberglass filters, which are among the least efficient, plugged with dirt so solidly the system shut down because it could not pull enough air through the filter to keep the furnace from overheating. In one case, the suction from the furnace fan against the filter was so strong the cardboard frame buckled and the filter was sucked into the ductwork, which had to be taken apart to remove it. 

In general, filters should be changed at least every three months. Some types of filters may need washing or replacement more frequently. Factors which affect filter life include: 

• dirty duct work
• construction in the house
• number of house pets
• sanding projects
• presence of smokers
• continuous fan operation 

The filter should be checked monthly until the appropriate interval has been established for your house and lifestyle. 

Which Filter Should I Choose? 
The choice depends on the purpose of the filter. The least expensive (and least effective) filter uses a fiberglass or cellulose pad, usually held in a cardboard frame. It is capable of protecting the equipment, catching most of the larger dust particles which tend to block heating and cooling coils. It will not remove the fine particles which tend to cause allergic reactions. 

The next step up in efficiency is a washable/reusable filter. It also uses an essentially flat pad, sometimes of plastic and sometimes of metal foil shapes. These filters usually have a metal or plastic frame, and can be washed with a hose and reinstalled. Some of the metal filters can be sprayed with a tacky coating material to increase their ability to catch small particles. They should be washed monthly, and may last 3-5 years. 

A pleated polyester filter provides more filtering capacity than a flat filter, and many are made with fibers which have a permanent electrostatic charge which attracts small particles. The life of these filters is about 3 months in most houses. There are a few plastic filters on the market which claim to produce an electrostatic charge from the air flowing through them. These are seldom more effective than standard filters and are significantly more expensive. 

As more and finer fibers are added, the ability to trap sub-micron particles increases. However, the density of the filter causes more air resistance than coarser filters and may require a different or larger fan to pull enough air through them. They can reduce the air flow so much that the furnace will shut down because of overheating, or the overload protection on the fan motor will trip. The density of the filtering material must be balanced with resistance to air flow. The design of energy-efficient heating and cooling systems must balance these factors carefully. 

If you have allergies or other problems related to indoor air quality, even better filtering systems are available. The deep-pleated high-efficiency mechanical air filters are about the same size as the ordinary filter but are 4-6 inches thick. They do not fit in standard filter holders and require a special box to be built into the duct system. The filter element is a pleated paper unit, similar to the one in the air cleaner of an automobile. As much as 80 square feet of filter material is contained in these filters. Again, electrostatically charged fibers can be used in the filtering media. These filters can remove particles as small as tree pollens. They should be effective for at least a year, and possibly as long as two years. Replacement filters are in the $30-40 range. 

The ultimate in residential air filters is the electronic filter. It is about the same size as the box for a deep-pleated filter and requires electricity to operate. The air is directed through a high-voltage grid which applies a positive charge on any particles in the air stream. These particles are then attracted to a negatively charged element. These elements should be washed about monthly, and the elements reinstalled in the filter. This type of filter can remove particles as small as smoke. The time between washing can be extended by using an inexpensive filter ahead of the electronic unit to catch pet hairs, etc. The disadvantages include high initial cost ($in the $500 range), the maintenance required, and the cost of repair if the power unit fails ($150-200). 

The effectiveness of various filter types in removing sub-micron particles is: