HEPA Filters in Forced-air Furnaces and Air Conditioning Systems

Reader Contribution by Troy Griepentrog

Our recent article about energy-recovery ventilators (Ventilate Your Home with Minimal Energy Loss) stated that using a “true high-efficiency particulate air (HEPA) filter” in your forced-air heating or air conditioning system would reduce allergens more effectively than regular filters. But reader John Alderman pointed out that this could be dangerous. So we went to the experts for their advice.

Mel Mossman, Chief Mechanical Engineer at RS Means, helps us explain:

For existing furnaces the manufacturer should be consulted as some residential systems may not have enough fan or motor capacity to accommodate higher efficiency filters. Specially designed and built HVAC systems may be equipped with true HEPA filters and the design may include pre- and/or post-filter pressure sensors to signal the need for filter replacement/maintenance. An alternative method sometimes employed, is to install a bypass system. This system is ducted into the return air plenum of the air handler or furnace and is equipped with an integral blower unit so that the furnace or air handler is not required to overcome the airflow resistance of the HEPA filter.

We asked the Environmental Protection Agency, too. Here’s an official statement from them:

True HEPA filters normally are not installed in residential HVAC systems; installing a HEPA filter in an existing HVAC system would probably require professional modification of the system. A typical residential air-handling unit and the associated ductwork would not be able to accommodate such filters because of their size and increased airflow resistance. Some residential HVAC systems may not have enough fan or motor capacity to accommodate higher efficiency filters.

Therefore, the HVAC manufacturer’s information should be checked prior to upgrading filters to determine whether it is feasible to use more efficient filters. Installing a higher efficiency and HEPA filter would probably require sheet metal modifications to the existing ductwork to permit the installation of the thicker air cleaner. In addition, a more powerful fan often must be installed to overcome the higher pressure drop. Specially built high performance homes may occasionally be equipped with true HEPA filters installed in a properly designed HVAC system.

True HEPA filters with a MERV between 17 and 19 are defined by the IEST test method as having a minimum efficiency between 99.97 percent and 99.999 percent in removing 0.3 ìm particles. A MERV of 20 is rated for 0.1 to 0.2 ìm particles. HEPA filters have higher efficiencies for removing both larger and smaller airborne particles.

Manufacturers market HEPA filters to allergy and asthma patients. Experimental data and theoretical predictions indicate that medium-efficiency air filters, MERV between 7 and 13, are likely to be almost as effective as true HEPA filters in reducing the concentrations of most indoor particles linked to health effects. Available data indicate that even for very small particles, HEPA filters are not necessarily the preferred option.

For these small particles, relatively large decreases in indoor concentrations (around 80 percent) are attainable with medium filter efficiency (such as a MERV of 13). Increasing filter efficiency above a MERV of 13 results in only modest predicted decreases in indoor concentrations of these particles. Predicted reductions in indoor concentrations of cat and dust mite allergens carried on small particles vary from 20 percent with a MERV 7 filter to 60 percent using a HEPA filter. Increasing filter efficiency above a MERV of 11 does not significantly reduce predicted indoor concentrations of animal dander.

Medium-efficiency air filters are generally less expensive than HEPA filters and allow quieter HVAC fan operation and higher airflow rates than HEPA filters because they have less airflow resistance. Pleated filters 1 to 2 inches thick that have a MERV of 12 are available for use in homes and may often be installed without modifying residential HVAC systems; however, manufacturer’s information should be checked prior to installation.

Higher efficiency filters with a MERV of 14 to 16 have a higher average resistance to airflow than medium-efficiency filters. Higher efficiency pleated filters, sometimes inaccurately called “high efficiency,” “HEPA,” or “HEPA-type” filters, are similar in appearance to true HEPA filters.

Medium and higher efficiency filters should be properly installed in HVAC systems so that leakage of air bypassing the filter is minimized. The higher a filter’s efficiency, the more attention must be paid to its sealed installation because increased airflow resistance is more likely to create leaks. Air filter effectiveness may be substantially reduced if air leaks through a poorly installed filter frame and its holding system. Effectiveness may be decreased if air exiting an exhaust grille of the HVAC system is not well mixed with room air before re-entering the system. This situation can occur if air return and intake vents are too close together.

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