How To Interpret Furnace Filter Ratings

Furnace filter ratings provide very important information that should not be overlooked when trying to improve the indoor air quality of your home. The problem with the information provided by filter manufacturers is that it is generally in terms that we are not familiar with. To interpret these filter specifications correctly we need more information. Most people really don't have a clear understanding of the information that is provided about each filter type. It is also important to remember that sometimes even filter retailers are a bit unclear about some of these filter ratings.

Typically you will see air cleaners, electrostatic filters and electronic filters rated by efficiency only and they will not have a MERV rating. Media type furnace filters such as HEPA, Throw Away (commonly referred to as Panel Filters), Pleated filters, and Filter Pads will generally have a MERV rating and an efficiency rating. The best rated filter available is the HEPA filter, but it is not suitable for all applications.

There are some important terms used to rate furnace filters and air conditioner filters. As you will see below, not all methods of rating furnace filters applies to all filter types and some manufacturers even use their own methods to rate filters.

• MERV
• Pressure Drop
• Arrestance
• Dust Spot Efficiency
• Dust Holding Capacity
• Efficiency
• Initial Efficiency
• Sustained Efficiency

MERV rating - (Minimum Efficiency Reporting Value) is an industry standard used to rate media type air filters. A filter with a higher MERV rating is more efficient than one with a lower MERV rating.

Pressure Drop (sometimes referred to as Filter Resistance) is the effect that the filter has on air flow. Higher pressure drops tend to decrease the operating efficiency of hot air furnaces and air conditioners and a lower pressure drop allows the equipment to operate at maximum efficiency. As a media type filter becomes plugged with dust the pressure drop increases.

Arrestance is the measure of a filters ability to capture dust particles under test conditions. This is not a good method of selecting the best filter for your needs because this rating is obtained under test conditions using larger sized particles.

Dust Spot Efficiency is a measure of a filters ability to remove dust particles from the air under test conditions. This usually applies to larger particles.

Dust Holding Capacity is the amount of dust a filter can hold at a given pressure drop. When comparing dust holding capacity ratings it is important to compare the filters at the same pressure drop.

Efficiency refers to the filters ability to stop particles such as dust, pollen, mold, bacteria and even gasses from passing through the filter. As a filters efficiency increases, the size of the dust particles it will collect becomes smaller.

Initial efficiency is the rating filters have when they are new. This is not necessarily when a filter is at its most efficient.

Sustained Efficiency is the rating that refers to the efficiency that a filter maintains over its life.

Merv Ratings
MERV ratings are determined by adding particles of varying sizes (1-12) into a controlled testing environment. The particles are added upstream of the test filter and a laser particle counter samples the air before it enters the filter and after it leaves the filter.

Selecting An Air Filter For Home Use
When selecting an air filter for home use it is important to determine what your objectives are. There are many different types of home furnace filters to choose from. Some air filters are very efficient and stop the smallest of particles while others are very inefficient and are a total waste of your hard earned money.

Your Air Conditioner Filter Does More Than One Job
What many people don't know is that an air conditioner filter serves more than one important purpose. It prevents the evaporator coil from becoming plugged with dust, which restricts airflow and reduces the efficiency of the air conditioner.