How to Choose a Furnace Filter

Are you replacing your HVAC unit’s air filter once a month? If not, you should be! HVAC filters have become increasingly important because they play a significant role in maintaining indoor air quality and energy efficiency. A dirty HVAC filter hurts the unit’s energy efficiency and causes the system to be overworked.

A 2009 study by the American Society of Heating, Refrigeration and Air Conditioning Engineers has shown that dirty HVAC filters can reduce efficiencies by 10 percent or more.

How do you go about choosing the right HVAC filter? You need to be aware of the different HVAC filters available to you as well as their minimum efficiency reporting value (MERV), a measure of the effectiveness of the filter. Inefficient HVAC filters have a lower MERV rating while more efficient ones boast a higher MERV rating.

It’s really not in your best interest to use fiberglass filters for your HVAC unit — they are very thin and feature just a basic flat panel surface for trapping air particles. Fiberglass filters were developed to protect your HVAC unit, not to improve indoor air quality.

These filters typically sell for $1 apiece, and they remove less than 10 percent of the air pollutants from your home. Fiberglass filters usually have a MERV rating between 1 and 4.

Polyester and pleated filters are pretty similar to fiberglass filters, but they tend to have superior dust-trapping ability. These HVAC filters remove up to 45 percent of the air pollutants in your home.

They typically cost around $10 each and have a MERV rating between 8 and 13.

Most HVAC technicians agree that washable air filters are the worst HVAC filters you can purchase. These filters typically sell for about $20 apiece, usually have a very low MERV rating — between 1 and 4 — and require extensive maintenance.

They easily collect fungus and bacteria, which can escape and circulate throughout your home. You should avoid using washable air filters for your HVAC unit.

High-efficiency HVAC filters can remove up to 85 percent of the air pollutants from a home or building space. These filters typically sell for $25 apiece and they boast a MERV rating between 14 and 16 (16 is the highest rating).

Often made from pleated filter paper or synthetic polyester fibers, high-efficiency filters can trap very small particles of dust, pollen, mold and other irritants. These filters, however, are not very practical for residential use — hospitals or other “clean-room” environments use these industrial air filters to even remove bacteria from the air.

Keep in mind that the best HVAC filter for your heating or air conditioning system might not be the one with the highest MERV rating. An overly restrictive air filter can slow the rate of airflow, causing your unit to malfunction. The ideal air filter is one that has the least effect on airflow, but removes the greatest number of air contaminants.

HVAC filters with a MERV rating between 9 and 12 are usually the best choice for homeowners who want the best dust control possible.

Editor's note: This is an updated version of a story originally posted on March 7, 2013.