What Are the Highest MERV Ratings for Different Types of Filters?
MERV ratings from 1 to 20 are possible. What is the highest MERV rating for furnace filters? For AC filters (which are the same thing)? What about an air purifier? How is MERV related to HEPA? What about ULPA?
This guide answers these questions for those that want excellent air filtration in their HVAC and air quality equipment. We get right to it.
If you want to learn more about MERV ratings, microns and air filtration, the information follows the Different Filters and their MERV Rating section.
Different Filters and Highest MERV Rating
Here are the most common air filter types available and the range of MERV ratings offered in each filter type.
MERV 1 to 4 – Almost Worthless Fiberglass and Aluminum Mesh
These are made of coarse spun fiberglass (most common by far) or aluminum strands. These low-MERV filters trap insects, human hair, carpet fibers and the largest airborne particles, but not much else. Their best use in the home is in a window air conditioner. In fact, they’re pretty worthless in those units. It’s unclear why window air conditioner manufacturers don’t make better filters for their devices. The best guess, from a pro’s point of view, is that they are concerned that better filters might slow air flow and make their ACs less effective. Fair enough.
Why is MERV 1-4 a bad idea? If one of these MERV 1 to MERV 4 filters were used in an HVAC system, they would only do a fair job of preventing many smaller particles from getting into your system and possibly causing damage over time. In short, in little time, the inside of your furnace or air handler would be dusty and dirty, and that grime would be flying around your home through the air vents.
The cause of poor air filtration: One reason they have such a low MERV rating is because the surface facing the airflow is basically flat, except for the texture of the filling itself. This gives it a smaller “face” area than a better pleated filter. This type of filter is only reliable for trapping particles larger than 10 microns. When rated for particles between 3 and 10 microns, most trap about 20% of them. They will do very little to improve your indoor air quality and, in fact, might make the situation worse for those suffering with asthma, allergies or other breathing issues because dust and other irritants and pollutants get blown into the air by forced air systems.
MERV 5 to 8 – Cheap Disposable Filters
Though relatively inexpensive at less than $12 each, filters of this type are considered to be a medium-quality filter that will do a sufficient job of filtering the air in your home. Usually made of finer spun fiberglass, they can trap some particles that are as small as 3 microns. This would include such things as mold spores, animal dander, and the droppings of dust mites, which are highly troublesome for those with allergies.
They are mostly constructed with a flat surface area, but since the fiberglass is finer and more tightly woven throughout the filter frame, they are more effective than MERV 1-4 filters. They will trap 20% to 85% of the particles from the air measuring 3 to 10 microns, but still less than 20% of those smaller than 3 microns. In other words, if you have no allergies or mild allergies at worst, a MERV 5 to MERV 8 filter might be OK. Those with severe allergies should choose a filter with a higher MERV rating or also have an air purifier in their home with a True HEPA filter. We discuss air purifiers below, so stay tuned, allergy-sufferers!
MERV 9 to 12 – Better Air Handler and Furnace Filters
Pleats please! Now we’re talking about effective air filtration! The U.S. Environmental Protection Agency states that one of the most effective ways to improve your home indoor air quality includes upgrading the filter in your furnace or HVAC system. Stepping up to a MERV 9 to 12 air filter is definitely a step in this direction.
Filters in this category are usually made of polyester with a pleated surface, thus giving them more face area. Plus, the media material is more tightly woven and/or denser. Some of them will be made with cotton fibers integrated into the filter as well. Whatever the specific construction details, they trap more of the bad stuff found in every home.
These filters are often considered to be the best all-around filters for a home for two reasons:
1). They do a very good job of removing smaller particles from the air, and,
2). They are the most restrictive to airflow that many residential HVAC systems will accommodate without overloading. The italicized part of that last sentence is explained below, should you be wondering what it means.
MERV 9-12 filters trap at least 85% of air particles 3 to 10 microns in size, 50 to 90% of those 1 to 3 microns, and more than 20% of those smaller than 1 micron.
MERV 13 to 16 – High Quality Air Filters
Many experts believe that this level of air filtration is more than is normally needed in a residence. Even some who sell air filters classify this type of filter as designed for hospitals and other commercial and industrial applications where superior filtration is required. They are often (but not always) manufactured in 2” or even 4” thickness, which would usually prevent them from being used in a residential HVAC system because most furnaces and air handlers only have enough room for 1” filters unless the slot or housing for the filter is enlarged.
MERV 13-16 air filters are getting close to HEPA effectiveness. But “close” to HEPA isn’t HEPA. In fact, there’s a rating called “near-HEPA” which we think is deceptive, or at least meaningless and confusing to some. HEPA starts at 17, so calling a MERV 14 or MERV 16 filter “near-HEPA” only means it isn’t HEPA! To prevent confusion, manufacturers often label their HEPA filters as “True HEPA,” though technically, if the filter says HEPA without a “near,” then it should be at least 17 MERV.
Filters with a MERV rating of 13-16 trap 90% to 95% of airborne particles 3 to 10 microns in size, 90% of those 1 to 3 microns, and 70% to 90% of those as small as 0.3 microns. That’s pretty small, as is explained below! Tiny particles such as humidifier dust, tobacco smoke and the nasty Legionella bacteria that is deadly and sometimes found in AC systems will be captured in the pleated synthetic fibers of these filters.
Did you know? Legionnaire’s Disease and the bacteria that causes it have an interesting, if also morbid, history. It’s part of air conditioning lore. The outbreak which gave the bacteria and disease its name came at the 1976 convention of the American Legion.
MERV 17 to 20 – HEPA & ULPA Filters
These are a class of air filters not designed for home HVAC system use, but we list them here for comparison.
And there might be a “True HEPA” filter in your vacuum cleaner, shop vac, wood-working dust collection system or air purifier (more on air purifiers later).
HEPA stands for High-Efficiency Particulate Air filtration – some use “Absorption” for the “A.”
ULPA stands for Ultra-Low Particulate Air, meaning the air that passes through an ULPA filter will be quite clean, containing few particles of pollutants, allergens and other stuff you don’t want to breathe.
HEPA and ULPA filters are both made of superfine silicate fibers that cause contaminants to stick to them through electrostatic attraction. They are of a completely different shape and size than filters designed for home HVAC use.
Commonly found in hospital operating rooms, biomedical facilities and electronic laboratories, HEPA and ULPA filters are very expensive as might be expected. HEPA filters can remove 99.97% of airborne particles as small as 0.3 microns, while ULPA filters remove a minimum of 99.9995% of airborne particles as small as 0.1 microns.
A Note of Caution – Air Flow Restriction
OK, let’s explore this phrase – the most restrictive to airflow that many residential HVAC systems will accommodate without overloading.
It’s not uncommon for folks to want to install the “best” filter possible in their home HVAC system. The strategy is to filter out as many impurities from the air as possible. However, every HVAC system has its limitations when it comes to resistance to airflow – much of which will be caused by the filter. It stands to reason that a filter that traps smaller particles might also force your system to work harder to push/pull air through the filter. Try breathing through a mask – we all know what that’s like, right? Well, the thicker the mask, the harder it is to get air through it.
In your air handler or furnace, this could cause heat buildup, and over time, a breakdown of the blower motor. In fact, the effect of poor airflow might extend to the AC or heat pump compressor. Since low airflow means not enough cooled or heated air is getting to your thermostat, the thermostat might keep “telling” the compressor/condensing unit to “keep working!” This is one of the most common causes of low airflow through ductwork and vents.
Important Tip: Be sure to consult your owner’s manual for the correct filter to match the airflow requirements of your system. And if you can’t find your manual, you might be able to locate it online by searching your air handler or furnace model number. If that fails, ask a qualified HVAC technician to inspect your equipment – it might need cleaning and a tune-up anyway – and recommend the right MERV filter for it.
Air Purifier for Even Cleaner Air in Your Home?
You or a family member might be suffering from allergies or have a chronic breathing problem that requires the cleanest air possible in your home. If this is the case, you might want to look at some of the stand-alone air purifiers on the market today. They can do what an HVAC filter can’t.
An air purifier works somewhat differently than a regular air filter. A filter is designed to only remove airborne particles, while air purifiers can both remove and sanitize them, too. Some types use special filters to trap particles as the air passes through them, while others may actually neutralize particles in the air without filtering them first. Still another type is a negative ion emitting air purifier, which helps to attract positive ion particles in the air, so that they are neutralized.
Stand-alone air purifiers can be found for any size room in your house and may be the answer to the breathing problem and hard-hitting allergies. In fact, there is a whole class of air purifiers that do a good job removing allergens from your air.
Air purifier filters should be replaced every 2 to 3 months for best results, and can be found online as well as in some big box stores.
HEPA recommendation: If your air quality is poor or you need extra-good air filtration for another reason, choose an air purifier with a True HEPA filter.
Want to learn more? Our PickHVAC Air Purifier Buying Guide is a great place to begin your research into air purifiers.
What is MERV?
One of the primary differences in types of home HVAC filters is their MERV rating. MERV stands for Minimum Efficiency Reporting Value. It’s a 1 to 20 scale that tells you what size particles the filter will trap as air passes through it. A higher MERV rating means that it traps smaller particles. This is a national, industry standard rating that was created by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). OK – now that we’ve gotten those big names out of the way, let’s talk about this in more practical terms.
As mentioned, the MERV rating is based on the size of the particles in the air that the filter will capture as air passes through it. Airborne particles can be as large as dust that you can see to as small as bacteria and even viruses that you can’t see. These particles are measured in “microns”, which is a measurement of size used in scientific applications, medicine, nuclear laboratories, etc.
What’s a Micron, and How Small is It?
A micron is one millionth of a meter in size. Many of us are familiar with centimeters (one hundredth of a meter) and perhaps even millimeters (one thousandth of a meter), but few of us ever get involved with microns.
For comparison, table salt is about 125 microns in size, while a human hair is around 70 microns thick (depending on the type). The smallest particle that can be seen by the naked eye is 40 microns, so 1 micron is 40 times smaller than anything you can normally see. The best air filters will trap particles much, much smaller – as small as 1/10 of a micron! That’s a super high-efficiency filter unlike anything we use in our homes.
