How Do I Compare Filters?

What's a MERV?
When comparing contaminant removal techniques it is important to compare “apples with apples”. Unfortunately many people are deceived (buyer beware!) with statistics that when presented a certain way, although true, do not tell the whole story. For example, the simplest of filters is the throwaway fiber glass filter that comes included with most furnaces.

This filter removes only 2 percent - 3 percent of all particles in terms of quantity but removes up to 70 percent of the mass of particles. How can this be? Very simple, the majority of mass, approximately 95 percent, is in only 10 percent of the particles. We will compare “apples with apples” and will therefore consider particle removal efficiency in terms of quantity and not mass as this provides the whole story.

To help with this matter, ASHRAE (American Society of Heating Refrigeration and Air Conditioning Engineers) has developed a simple and accurate standard test procedure that evaluates the performance of mechanical air filters as a function of both particle size and efficiency. This new standard has established a Minimum Efficiency Reporting Value, better known as MERV, with 16 being the highest rating and 1 being the lowest (the worst).

MERV stands for Minimum Efficiency Reporting Value—a way of comparing air filter performance. Filters with higher MERV ratings capture more of the potentially harmful airborne particles and allergens. MERV is the only industry-accepted standard methodology.

See the Filter Comparison Chart below for a lineup of filter choices.

1” FIBER GLASS THROWAWAY FILTERS
As these filters are only 2 percent - 3 percent efficient in particle removal, they serve to only protect the furnace blower compartment from large particulates. They do next to nothing for cleaning air. They are not recommended to use. They are inexpensive to purchase for a reason. They have a MERV 1 or 2 rating.

ELECTROSTATIC AIR CLEANERS
Electrostatic air filters are made by a number of companies and these can be purchased in most hardware stores. The typical electrostatic air filter is made from polyester or polypropylene strands that are supposedly charged as the air passes through them. These air filters are often rated as 95 percent plus efficiency on the Arrestance scale. Arrestance is NOT efficiency.

It is the percent of weight, in grams of dust, that the Electrostatic Air filter can hold before it loads up completely. These filters are actually about 18 percent on the ASHRAE scale and may have a MERV 4 rating. They usually have a metal frame and do not fit tightly into the air filter frame. A highly over rated air filter type.

1' PLEATED MEDIA FILTER
They usually have cardboard frames. They are an effective media but really designed to remove the larger particles and keep the HVAC blower/motor clean. MERV 5 rating. They are the best inexpensive furnace filter to use as an alternative to the fiberglass throwaway.

6' DEEP PLEATED AIR FILTERS (Aprilaire/Space Gard Type)
There are number of companies that manufacture this type of furnace air filter. It is usually a 65 percent ASHRAE grade media. I has a fair amount of media so the its initial pressure drop is not too high. It has a fairly good efficiency on larger particles and is rated a MERV 8 or 9 depending upon the manufacturer. Some of the newer designs actually combine this air filter with an electronic grid. This type of furnace air filter doesn’t need to be changed but 1-3 times a year.
MERV 8 or 9.

ELECTRONIC AIR CLEANERS
The MERV ratings really don't apply to the Electronic Air Cleaners, as these are not Mechanical Air Filters. Electronic Air Cleaners use a series of wires in a grid form that are electrically charged. These charged wires (or plates in some units) attract the particles being drawn in by the heating/cooling fan. There is no media for which the particles can be trapped and no additional dust cake for continued air filtration efficiency.

Every time the wires or plates are charged they loosen some of the particles that are attached. And, as the wires become coated the efficiency of the Electronic Air Cleaner system is reduced. In fact, Electronic Air Cleaners are the only air filtering device that becomes less efficient as it loads up. Its efficiency can start out as a MERV 12 but drop down to a MERV 8 rating within one week of operation. As a result of their type of performance, they do not use a MERV rating. Electronic Air Cleaners units need to be cleaned frequently to maintain good efficiency.

HEPA FILTERS
HEPA air filters are not MERV rated as they exceed the ASHRAE test protocol 52.2 used in determining the MERV ratings. In fact, HEPA air filters are the ONLY mechanical air filters that are tested and certified to meet a specific efficiency at a specific particle size. All HEPA air filters must meet a minimum efficiency of 99.97 percent at 0.3 microns. ASHRAE or MERV air filters are tested using the Dust Spot tests that incorporate some fine dust, powdered carbon and some cotton linters. HEPA air filters are at least 50 percent more effective at removing respirable sized airborne particles than any of the ASHRAE air filters on the market.

To expand this chart click on the .pdf file attachment below: