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Selecting an Air Filter

April 15, 2016

When you go to a hardware or big box store to purchase air filters you will find a bewildering array of products. 3M, for example, offers 9 different types of filters each with different ratings and at a different price, and then each of those comes in different sizes. It can be confusing, to say the least! We’ll help you make sense of it all, so you can choose the best air filter for your needs.

Size Matters

Size is perhaps the easiest decision: get the size filter that fits the filter holder built into your HVAC system. It should fit snugly, with no air gaps around the filter. Be aware that filters come in thicknesses of 1-inch to 6- inches, with the thicker filters having greater capacity to catch dirt without increasing resistance to air flow and increasing the time between required filter changes.  However, you cannot put a thick filter (4 or 6 inches thick) into your HVAC system if it’s designed to accommodate only a 1-inch thick filter.

Rating Matters

In our previous post, we talked about the MERV ratings of filters.  MERV is an industry standard measurement of filter efficiency.  Individual manufacturers often have their own rating system as well.  3M’s rating is called MPR, which standard for Microparticle Performance Rating.  As with the MERV rating, higher MPR numbers mean a greater percentage of particles and smaller particle sizes will be captured by the filter.

Price vs Performance

How do you decide which MERV rating to get?  Generally, the higher the MERV (or MPR) rating, the more expensive the filter.  For example, the price of a MERV 13 filter is about twice the price of a MERV 8 filter of the same size. 3M offers multiple filters having a MERV 12 rating, but with different MPR ratings, and so even among filters of the same size and MERV rating, the prices can vary.  Which filter to buy depends on how much cleaning you want your filter to do. The lowest price filters at MERV 5 will remove dust, debris and lint. This is the minimum needed to maintain the health of your HVAC system’s heat exchanger and coils.  A MERV 11 filter removes smaller particles such as pollen, dust mites, mold spores, and pet dander. A MERV 12 or 13 filter can filter some bacteria, viruses, and odors.  Since the prices jump up dramatically at MERV ratings higher than 11, and the increased efficiency is only minimally incremental, using filters with more than a MERV 11 rating is probably not cost effective, unless there is some particular medical need.  In the chart, those shown in the “green zone” in the cost column represent the best value on a price vs. performance scale.