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When it is loaded with dust, of course. But, unfortunately, there is more to the story than this simple statement.
First, a short lesson on media air filters. All filters, be they commercial or residential, battle the same three forces of nature; resistance to flow (*see below), amount of dust they will hold, and efficiency or ability to remove particles from the air. Just when you have a filter with high efficiency, it won’t hold much dirt or the resistance to flow is too high. Conversely, when you get low resistance to flow, it usually won’t be very efficient.
Residential heating and air conditioning equipment presents several challenges for the homeowner from the standpoint of air filtration. The first limiting factor involves filter depth. Most older homes have an air handler with a 1 inch slot for an air filter inside the unit. This filter is changed by removing the door to the unit and removing and reinstalling the filter.
Newer homes have a filter grille, located at the return air opening. This grille is hinged and can be opened to expose a track for a 1 inch filter. In short, regardless of where the filter is located in a residential unit, there is usually only 1 inch in depth allocated for the air filter.
The second limiting factor is the blower horsepower of residential units. the standard unit has a 1/4 or 1/3 horsepower blower that allows for a *limited amount of resistance to the flow of air. Anything in the system is considered resistance, but the typical things in a residential system are ductwork (a friction factor to the flow of air), air conditioning coils, grilles and registers, sometimes fire dampers and filter(s). Resistance in a HVAC system is measured in inches of water – pressure forcing water to rise in an enclosed tube. Residential units can normally have about 0.5 inches of added pressure in the system and the typical unit is allocated only about 0.1 inches (w.g.) for filters. And, media filters increase in the resistance to flow as they load with dust. This increase in resistance leads to a decrease in the velocity of airflow in your unit.
Because of the 1-inch restriction combined with a limited allocation for pressure, homeowners are limited as to their choices of filters for their home without retrofitting the system.
The choices in the 1-inch variety normally are:Standard Fiberglass Throwaway Filter – these are filters from the 1950’s that are designed to remove only the larger particles from the air and, in industry-designed testing, do not do that very well. These filters do have a very low resistance to the flow of air and, for this reason, are the filters most often sold for home units.
In today’s marketplace, higher efficiency filters are available at most retail home stores and homeowners need to be aware that some of these filters (usually in the MERV 11 and higher category) may create low airflow problems in their system.
***Reposted from NAFA***