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Not all air filters are created equal

September 15, 2017

Clogged air filterForced air heating, ventilating, and air-conditioning (HVAC) systems require effective air filtration for optimum energy efficiency and maintaining clean(ish) ductwork and good indoor air quality. Air filters should be changed regularly. How often they need changing depends upon a number of factors including, but not limited to:

  • Presence of pets that shed
  • Amount of carpeted versus hardwood floors
  • Where you live — amounts of dust, pollen, etc.
  • Use of wood-burning supplemental heat sources
  • Presence of cigarette smoke

The air inside our homes is full of particles originating from inside and outside sources. As the name “forced air” implies, conditioned air is blown into the house through ductwork. In order to operate efficiently, the air supplied is returned to the system for reconditioning, taking with it all the particles in the air and the occasional “tumbleweed” of pet hair that many of us are familiar with.

This junk-laden air flows through a filter before encountering the HVAC equipment. For cooling, there is usually an A-frame arrangement of what look like car radiators. For heating, it is generally a combustion chamber. Without a filter, the cooling coils would get clogged, and the heating side would burn off whatever was in the air. Ah, the smell of burnt hair in the morning…

Air filters trap a lot of debris that otherwise would end up back in the house, stuck in ductwork, clogging HVAC equipment — or in our lungs. Filters have more choices than you can shake a stick at. Fortunately, they can be broken down into two nicely defined categories, making the selection process manageable. The two are:

  • Permanent or disposable
  • Flat or pleated media (with a handy MERV rating)

Disposable are the most prevalent. Some in the flat media group look like they will stop only particles larger than a golf ball. They have flimsy cardboard frames and a thin, flat mesh you can easily see through. While they are cheap, don’t waste your money. Your HVAC system and lungs deserve better.

Pleated filters perform better using media you cannot see through. While they look impervious, air can move through under pressure, leaving its airborne cargo trapped. Pleated filters are better, in my experience.

Remember MERV? It’s a rating system that tells you how effective a filter is at trapping particles. Standing for Minimum Efficiency Reporting Value, it’s a measure of efficiency. The scale runs from one to 16 (higher is better) and is based on trapping particles 3-10 microns in diameter. Research shows that residential filters with a MERV rating between seven and 13 are likely to be as effective as true HEPA (high-efficiency particulate arrestance) filters. This class of filter is used in clean room manufacturing, and at the extreme end, it can trap particles much smaller than the diameter of a human hair — as small as 1 micron.

So, should you jump in and grab a supply of high MERV filters? Not without some research. All filters increase resistance to air flow. HVAC systems are designed to operate at a particular pressure and should support MERV ratings of one to four. A higher MERV value increases resistance, making the system work harder. It loses efficiency and increases wear on operating components.

How do you decide which level of filter to use? If you have your system’s operating manual or can grab it online, check for recommendations. Otherwise, my advice is to go with a decent (MERV three to five) pleated filter and check it once a month to see how it is performing. Also check to see if the dust inside abates.

Spend a little more and breathe a lot easier with a regular schedule of air filter replacement. It’s a simple change that pays big dividends.