The main purpose of an HVAC system is to maintain a comfortable environment indoors. Temperature plays a large part of this equation, but air cleanliness, or Indoor Air Quality (IAQ), also plays a part. On average, Americans spend approximately 90 percent of their time indoors, where the concentrations of some pollutants can be 2 to 5 times higher than typical outdoor concentrations. Some of the problems created by poor indoor air quality include allergy and asthma symptoms, irritated skin, headaches, lung irritation, furniture damage and higher energy costs.
HVAC air filters trap pet dander, dust and other contaminants, which is one of the easiest ways to improve IAQ. But the wrong air filter can do more harm than good. Weak, loosely woven filters can allow dust to buildup and clog coils, motors and blowers, while overly restrictive HVAC filters can result in overheated motors and damaged controls. Either situation can decrease the life of a heating and cooling system and negatively affect indoor air quality.
MERV Air Filter Ratings
Most air filters are rated on the Minimum Efficiency Reporting Value (MERV) scale. The MERV rating system was created by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The 1 to 16 scale gauges the porousness of filters, with the largest number of residential filters falling in the MERV 4 to 13 range. A higher MERV score indicates a tighter (less porous) filter weave which will trap more dust, particles and contaminants.
HVAC air filters are constructed using various materials in a range of thicknesses. These factors combine to affect their MERV rating. The following list describes various filters including their rating:
MERV 1 - 4
Disposable Spun Fiberglass Filters
These economical filters trap 80% of contaminants 50 microns and larger and 25% of contaminant measuring 3- to 10-microns. These need to be changed regularly. Furnace companies often specify these low-end filters to prevent debris accumulation on fans, heat exchangers and other surfaces. They stop large particles and allow free airflow, but they don’t trap tiny irritants that trigger allergies and other health complaints.
MERV 5 - 8
Disposable Polyester/Paper Filters
These median-duty furnace filters capture 80 to 95&percnt: of contaminants as small as 5 microns. They are more effective than spun fiberglass filters, but they are also 4x more expensive.
MERV 2 - 10
Self-charging fibers are woven into these filters to attract contaminates and pull them out of the air. Standard-size, disposable versions cost approximately $10. Washable versions (rated MERV 4 to 10) can save money in the long run, but they cost more up front. That said, expensive versions can last as many as 8 years. Washable filters are easy to clean, but they need to dry completely before use, so it makes sense to purchase more than one and switch them out for cleaning.
MERV 11 - 13
Disposable High MERV Filters
These increased-efficiency filters capture 0.3-micron contaminants including some virus strains and bacteria. 2- to 5-inch thick versions fit inside units attached to a furnace.
MERV 13 - 16
The term, HEPA stands for High-Efficiency Particulate Arrestance. These high-end of filters trap contaminating particulates as small as 0.3 micron. While HEPA filters capture the most contaminates, they also dramatically restrict how freely air can flow. HEPA filters should only be used in compatible systems.
Best Bang for Your Buck
Will more efficient filters always save money? The answer depends on how much the system is used and where it’s located. Cost savings primarily come from the reduction in fuel consumed by a HVAC system that is working efficiently and reduced repair costs.
All the air that heats or cools an indoor structure will pass through an air filter, which is reason enough to make sure you are using clean heating/ac filters. For best results, air filters should be checked regularly and changed when dirty. If you can’t see the light through the filter when you hold it in front of a light source, it should be changed.
U.S. Environmental Protection Agency. 1989. Report to Congress on indoor air quality: Volume 2. EPA/400/1-89/001C. Washington, DC.
U.S. Environmental Protection Agency. 1987. The total exposure assessment methodology (TEAM) study: Summary and analysis. EPA/600/6-87/002a. Washington, DC.