What is the right MERV number for my air conditioning filter?
Monday, August 6, 2018
MERV is an acronym for Minimum Efficiency Reporting Value, and the standard was created by ASHRAE (American Society of Heating Refrigeration and Air-Conditioning Engineers) in 1987. MERV rates the size of particles that the filter will trap, with higher numbers indicating smaller particles are trapped by the filter.
Finding the right MERV rating for the your air filter is a balancing act. As the number goes up, the level of air filtration in your home improves. Smaller and smaller particles, down to a couple of microns in size, get trapped in the filter. But the cost for the filters goes up, and the increased energy needed to pull the air through the fine mesh of a higher MERV filter raises your electric bill. Also, the extra load on the blower shortens the air handler lifespan. So the right MERV number is somewhere on a sliding scale between high filtration/cost and low filtration/cost.
There are essentially three filtration levels: basic (around 3), better (5 to 8) and best (9 to 12). While there are filters with higher MERV ratings than 12, they are for industrial and hospital air requirements, and don’t provide any additional benefits for a homeowner—other than bragging rights for the highest MERV filter in the neighborhood.
- BASIC basic filters are spun fiberglass and inexpensive. They protect your air handler from larger particles than would otherwise collect on the evaporator coil fins and interior of the unit, but do not provide any air quality improvement. One HVAC contractor we know swears that a MERV 3 filter is all you need. “If you can’t see through it, don’t buy it!” is what he likes to tell his customers.
- BETTER filters catch pet dander, smaller dust particles, mold spores and other allergens. These are typically the the pleated filters and, as the MERV number heads upwards from 5 towards 8, the filter depth increases to 2 or 3 inches.
- BEST filters have a pleated depth of 4 or 5 inches and catch the smallest air pollutants. They also cost the most and have the highest resistance to air flow.
Changing the filter every one to three months is as important as selecting the right filter. Don’t wait until you see gray fuzzies all over the leading surface of the filter (like in the photo at the top of the page) to switch out the filter. When you see evidence of dirt buildup, it’s time.
We suggest putting a note in your phone with filter size so you don’t have to check it each time you are ready for refills. Also, many of the thicker filters have a “nominal” size which is slightly different than the actual dimensions of the filter. Some filters state the actual size on the side, while others just list the nominal size. Unless you are replacing with exactly the same manufacturer’s nominal size filter, It’s best to confirm that any replacement filter is also the correct actual size.
Also see our blog post Where is the air filter for my central air conditioner and furnace? I can’t find it?
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