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Residential Air Filtration
When to Change
your Air Filter
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 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 are normally:
1. 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.
2. Pleated
Filters - these filters achieve more filter surface area by
folding the media into a 1 inch frame and can use a higher
efficiency media without adversely effecting the resistance to flow.
- to a point.
3. Others -
anything that does not fall into the above two categories (metals,
plastics, cellulose, electronics, etc.).
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.
Saving MERV for another
discussion, suffice to say that the higher the MERV number, the
higher the efficiency.
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