Recently AnswerLine received a call about furnace filters. The caller wanted to know the tradeoffs of the various kinds of filters with regard to efficiency and air quality. And further, how often did an air filter really need to be changed. Great questions! However, I didn’t have all the answers and requested time to do some research and get back to the caller. In doing so, I learned a lot.
In by-gone days, there was only one thing to know when purchasing a filter for your heating/AC system (HVAC)—the dimensions. It certainly is a different story today. The wide range of filter options, numbers, and prices can be overwhelming and confusing.
The original purpose of the HVAC filter was to do one thing: protect the equipment. Since then, a second function has been added as an option: reduce indoor air pollution. Therefore, the filter’s real job is to reduce the amount of dust, dirt, and debris that is in the air so that it does not accumulate on components inside the HVAC system; and as an option, prevent those inside the home or building from inhaling dust, allergens, and other irritants.
Changing or cleaning the air filter when appropriate increases the efficiency and life span of the system and decreases energy costs. Dirty, clogged filters block airflow and cause the system to run longer; further, they also allow the dirty air to get past the filter and make its way into the fan motor, coils, and other parts of the system along with the home environment and into the lungs of humans and pets.
Air filters are either mechanical or electrical. Mechanical filters capture airborne pollutants on a filter medium; their effectiveness is dependent on both media and design. Information provided by University of Illinois Extension shows that fiberglass filters remove up to 2%, washable/reusable filters remove up to 6%, thin pleated filters remove up to 11%, deep pleated filters remove up to 25%, and pleated electrostatic filters remove up to 49% of sub-micron particles. Electronic filters use an electric field to capture debris like a magnet and remove up to 94% of sub-micron particles; ultraviolet or HEPA filters may be built into or added onto an electronic filter. Most homes use a mechanical filter due to cost unless additional filtration is needed for health reasons.
Filter rating systems devised by individual companies make it hard to compare filters and should be avoided as there is no standard. Rather consumers should look for the Minimum Efficiency Reporting Value (MERV) which sets a standard for rating the overall effectiveness of air filters by measuring the filter’s ability to trap particles ranging in size from 3.0 microns to 10.0 microns. Every filter has a MERV rating. (If it is not on the packaging, contact the manufacturer.) A residential filter commonly has a MERV rating of 1 to 12. A higher MERV rating means finer filtration with fewer dust particles and other airborne contaminates passing through the filter. Here in lies the caveat. A filter with a higher MERV rating will also restrict airflow more than a filter with a lower MERV rating and will likely increase your systems running time and energy use. Remember, the filter only needs to keep the coils and blower free of dust and debris. All HVAC systems are designed for a specific CFM (cubic feet per minute) airflow rating. If the airflow is restricted by say 10%, that could equate to a 10% loss in efficiency, and 10% higher energy consumption along with longer heat up or cool down cycles; or in worst case, if the system is struggling with the CFM due to restriction, the blower motor may eventually over-spin and burn out.
Most home systems require modifications for filters greater than MERV 8. Studies show that medium-efficiency filters strike the best balance between allergen removal and filter cost. Flat, disposable, spun fiberglass filters (MERV 1-4) protect the HVAC system from large particles but cannot block the microscopic particles that are most irritating for allergy and asthma prone humans and pets. Medium efficiency pleated filters made of polyester or cotton paper offer a MERV rating of 5-12 due to the larger and denser surface area thereby allowing them to capture smaller and greater numbers of particles without impeding air flow to the unit. High efficiency pleated filters have MERV ratings of 13-16. Electronic filters do not have a MERV rating.
Most experts recommend that consumers use a filter recommended by the furnace manufacturer. If a manual for the unit does not exist, call the manufacturer or the HVAC dealer. Should additional filtration be needed beyond that designed for the unit, consult the manufacturer or dealer for technical assistance. A flat, fiberglass filter will serve the basic need of protecting the system. If additional air filtration is needed, a flat, pleated filter that fits into the HVAC systems’ slot with a MERV rating of 7 or less is an acceptable filter for most systems; the pleats provide a greater surface area to trap particles thereby protecting the system and improving air quality.
Lastly, no manufacturer can predict how long its filters will last because none of them know the dust conditions in individual homes. Checking the filter often is advised with the rule of thumb being, “if it looks dirty, it is dirty.” A general guideline is to change filters at least every three months to maintain maximum efficiency but monthly checks are encouraged.
For more in-depth information consult:
EPA technical document, “Residential Air Cleaners: A Summary of Available Information”
American Lung Association, “Health House Furnace Filters: Tips about Your Furnace Filter”
University of Illinois Extension, “Healthy Indoor Air”