The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has had filtration system standards and testing protocols in place for many years. Unfortunately, reporting values have always been poorly understood and the old standards have become outdated. ASHRAE’s primary focus was on protecting HVAC equipment and coils, and for this reason the standards were biased toward large-particle filtration efficiencies.

When the goal is to capture large particles, even standard glass-fiber filters (or as we know them, “see-through furnace filters”) have capture values of 75% or better. Filter test reports under the revised ASRHAE 52.2-1999 now show the particle removal efficiency of filters across a range of dust particle sizes. Contractors can use these new reports to help clients compare the difference between furnace filters, which are very efficient at removing relatively large particles, and people filters, which are designed to capture much smaller particle sizes. These revised ASHRAE standards spawned a new term: MERV (Minimum Efficiency Reporting Value). The MERV rating for a filter is based on composite average particle removal efficiencies through a range of particle sizes from 0.3 to 10 microns. The higher the MERV rating, the better the filter is at removing small particles.

It is important to note a comment attached to ASHRAE’s 52.2 test standard, which points out that the test protocol it employed may not be appropriate for “electronic air cleaners” and “electrostatic filters.” The highly conductive carbon dust used to establish the test standards may interfere with the effectiveness of electrically charged air filters and compromise the accuracy of the MERV ratings. For this reason, some manufacturers of electronic or electrostatic filters choose not to have their products tested.

If your goal is to provide improved air quality, you need to install something more than a standard furnace filter. Selecting a high-efficiency filtration system that will effectively remove most of the “respirable” particles is a good choice. Some systems on the market require their own fan to overcome the inherent resistance of these high-efficiency filter media.

We often see the designation HEPA (High Efficiency Particulate Arresting) on stand-alone units designed for bedrooms and other living spaces. These portable HEPA filters may be effective in removing some amount of respirable particles, but they will not deliver the benefits of a whole-house filtration system.

Adding a HEPA filter to the furnace return air system, using a bypass configuration, can be an effective way of removing respirable particulates from the air inside the entire house. In this case, the goal is to clean the air in the house over time by sampling a percentage of furnace return air through the high-efficiency filtration system. This may be an effective strategy if efforts have already been taken to reduce dust sources.

Airflow Resistance

It is almost always true that the more effective a filter is at removing smaller particles, the more it restricts airflow. This is one reason why electronic air cleaners are so popular. They create little resistance to airflow, yet also have a relatively high particle removal efficiency.