How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manfuactured and modular homes

What does a MERV rating mean on an air filter?

Monday, October 8, 2018

It’s 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. 

   A rating below 4 gives minimal filtration and is typically a fiberglass throw-away filter, and the 5 to 8 range is disposable 1-inch pleated filters with a cardboard frame. As the rating tops 8 and heads towards 16, you are in the range of large box filters—the kind that only have to be replaced every 6-months or so—and are capable of removing mold spores, very small particles, and cigarette smoke.

   Unfortunately, as filters approach the holy grail of HEPA status (High Efficiency Particulate Air), the resistance to air flow also increases; and this can both put an excessive load on the air handler blower and reduce air flow through system. So the higher levels of filtration come at a cost: reduction in system efficiency and lifespan.

   One air conditioning contractor we know recommends using no more than a MERV 4 (minimal residential filtration) filter. As he puts it, “if you can’t see through it, don’t buy it!” We think that number might a little low, especially for homeowners with allergies and asthma problems. Plus, a system can be designed to accommodate higher MERV filters. 

   But we do agree with the philosophy of installing a filter with lowest MERV number that accomplishes the level of air filtration you require. More is not necessarily better.

   And  the filter combo in the picture above is clearly “over the top” as far as we are concerned: a MERV 12 box filter in tandem with a pleated filter that’s unmarked, but probably a MERV 7. The homeowner was not present at our inspection to explain his logic for this combination, but the problem it created was immediately obvious. There was a noticeable lack of air flow at the registers. And, while the MERV 7 filter increased the load on the a/c blower and reduced the air flow through system, it did not do the one thing for which it was likely intended: better filtration. 

   We also occasionally see homes where double filtration has been created by installing a filter at the base of the air handler and also behind one or multiple return air registers. Like the previous example, it creates reduction in air flow without any additional filtration benefit, and we recommend removal of one of the filters.

   We suggest that you talk with your a/c technician next time you have a service call, and ask for a recommendation for the MERV rating right for your particular HVAC system. The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) currently recommends a minimum MERV rating of 6 for a central heating or air conditioning system.

   It’s also a good idea to examine the outside edges of your filter while it is secured in the air handler to make sure that it fits snugly all around, with no gaps. Give the filter a light tug in the direction of the air flow, to simulate the suction action of the blower when operating. If the filter easily pulls away from the the mounting frame, that means unfiltered air is going around the edges, and you should ask your a/c technician to reinforce or replace the securing mechanism for the filter.

   Then again, sometimes just installing a filter with a stiffer frame will solve the problem. One-inch thick filters with a high MERV rating and a lightweight cardboard frame tend to buckle slightly as soon as a little dirt/dust accumulation increases their resistance to air flow.

    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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  To learn more about heating and air conditioning systems, see these other blog posts:

How can I find out the SEER of my air conditioner? 

My air conditioner won't turn on. What's wrong? 

How can I find out the size of my air conditioner? 

How can I find out the age of my air conditioner or furnace?

How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump?  

Does an old air conditioner use more electricity as it ages? 

How did homes stay cool in Florida before air conditioning?

How can I find out the size of my air conditioner? 

How can I find out the age of my air conditioner or furnace? 

•  What is wrong with an air conditioner when the air flow out of the vents is low?

Why has the thermostat screen gone blank? 

Why does it take so long to cool a house when an air conditioner has been off for a while? 

Why is my air conditioner not cooling enough? 

What are the most common problems with wall/window air conditioners?  

Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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