According to the Environmental Protection Agency (EPA), indoor air can be 2-5 times more polluted than the air outdoors. HVAC Filters are composed of highly porous beds of glass or plastic fibers. The fibers collect airborne dust and particles and they are responsible for cleaning the air in air conditioning systems.
Most HVAC filters are single-use, disposable items although there are a wide array of filters that can improve the air quality in your home. Before we discuss the types of filters, it is important to understand the basis for comparison. Recently the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) and the air filter industry have come up with a standardized rating systemof a MERV rating for each type of air filter.
Minimum Efficiency Reporting Values (MERV) Rating
The MERV rating measures the overall effectiveness of any type of a mechanical air filter on removing airborne particulates. The rating indicates the effectiveness of the filter based on size of the particle and sets a standard for final air pressure after the removal or trapping of the air particulates. (See the chart to the right)
The most basic, inexpensive, standard, throw-away filter. Fiberglass filters typically need to be changed monthly and have a MERV rating between MERV 1 -MERV 4.
Pleated replacement filters are the ideal choice for homes where indoor air quality, heating and air conditioning systems efficiency and economy are priorities. Pleated filters can last anywhere from 1 month up to a year depending on the type. MERV ratings range up MERV 12.
Electronic Air Cleaner
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. 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. Electronic Air Cleaners units need to be cleaned frequently to maintain good efficiency.
Electrostatic Air Filters
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% 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% 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. The only advantage of Electrostatic air filters is that you don’t have to change them, just wash them off.
HEPA Air Filters
HEPA air filters are not MERV rated as they exceed the ASHRAE test protocol 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% at 0.3 microns.