When installing a filter in furnace ductwork, there are some practical limits as to how much restriction is reasonable. For example, most manufacturers rate the airflow capabilities of their furnaces at 0.5 inch to 0.7 inch external static pressure (ESP)—this typically would include the resistance of the standard filter shipped with the unit.
Considering, however, that reasonable duct design would result in a 0.3-inch pressure drop and the resistance of an air conditioning coil would add another 0.15 inch to 0.25 inch, selection of high-MERV filters will have an immediate impact on airflow performance of the heating and cooling system.
Contractors must become familiar with the resistance values for the various filter types. When this issue is included in the design of the air-handling system, the right size blower can be chosen or, preferably, a furnace with a variable speed. This type of furnace can allow the speed of the blower to be adjusted after the system’s airflow is calculated and set.
Noise, Maintenance, and Cost
Noise is an important consideration when selecting a filtration system, because stand-alone types require a fan and the furnace may require increased flow. High-efficiency systems also must overcome the high static resistance of additional filters. Many of the portable filtration systems are so noisy that occupants will not turn them on. The responsible HVAC contractor should advocate for a separately ducted filtration system and quieter furnace technology, such as two-speed or variable-speed furnace and air conditioning systems when very high efficiency filtration is desired.
Intuitively, the more efficient the filter, the more dust it will capture and the more often it will need to be replaced or cleaned. Filter life may be extended by adding more surface area, such as with a deep pleated filter. Some filtration media also have higher dust-loading characteristics. Simple filters trap particles on the surface of the media (referred to as face loading). Better media designs encourage loading throughout the depth of the media.
Ironically, most media filters get more effective over time, as they begin to plug up with particulates. Others, such as electronic air cleaners, get less efficient as they load up. Contractors need to help their customers understand that a very efficient, high-MERV, 1-inch-thick filter is going to have a very short service life, and will require more frequent replacement based on lifestyle, home location, and flooring type.
It’s actually quite easy to enjoy a high level of indoor air quality. Install hard-surface flooring systems; select HVAC equipment that is efficient, sealed, and capable of moving air across a higher quality filter; and help homeowners understand that most particulate matter is a by-product of lifestyle. Houses are more than the sum of their parts; by putting this knowledge to work, we can build the best homes ever constructed.
Gord Cooke, Building Knowledge Inc., contributed to this article.