Question

What is the best air filter for my 3 year old furnace considering air flow and reasonable dust removal?

Asked by June Stellberg
Beloit, WI

Three bedroom ranch with one occupant in southern Wisconsin.

Answer

Lucas Johnson

Answered by Lucas Johnson

Seattle, WA

Cascadia Consulting Group

June 24, 2013

Most of the time when people inquire about filters, it is due to indoor air quality (IAQ) issues they are experiencing.

While the filter is sometimes to blame, it is even more common that leaky, or otherwise poorly installed, ductwork is the culprit. Filters can actually be part of the problem, rather than the solution, if there is leaky ductwork!

Leaky ducts bring IAQ down

The majority of ductwork in the United States typically has a leakage rate of roughly 30%. While there is much debate about the specific percentage, and even how to calculate the percent leakage, there is very little debate about air leakage in ductwork being too high.

  • Having a poorly sealed duct system will not only waste energy and decrease comfort, but also will likely pull pollutants into your air distribution system.
  • The most common pollutants tend to be insulation fibers, mold spores, and rodent waste.

So, in other words, before changing your filter, one should ask, "Is my ductwork properly installed, tightly air sealed, and not running through areas containing materials that are potentially hazardous to my health or IAQ?"

Further compounding this issue is that air leakage between the attic/crawl/basement and living spaces can also cause major IAQ issues. So, once you have invested in air sealing your ductwork, attic, and floor then it can be worth investing in a good filter.

Choosing an air filter

Filters should be properly fitted into an air tight "filter box". The filter box should be located between the return plenum (where the ductwork connect back to the air handler) and the air handler itself. The filter should have a gasket along the perimeter so the filter fits snuggly into the filter box.

Filters are rated in MERV, with the higher MERV meaning higher levels of filtration.

  • However, please keep in mind that a higher MERV rating will mean more resistance to air moving through the system.
  • Higher levels of resistance to airflow will likely cause balancing issues and will likely result in pulling more pollutants into the system, especially if the ductwork hasn't been air sealed first.

Most people are fine with a MERV 10 max and I have seen up to a MERV 14 filter work well, but only after ductwork and whole-house air sealing is done first!

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