MERV Ratings and Furnace Air Filter Efficiency
If your home is heated by a central forced air furnace, it’s important to understand MERV ratings and how they measure the efficiency of your furnace’s air filter. Understanding how your furnace filter’s MERV rating relates to the performance of your furnace and the cleanliness of your indoor air will help you make an informed purchasing decision.
What are MERV Ratings?
MERV stands for “Minimum Efficiency Reporting Value” and is an air filter general efficiency rating based on the testing method created by the international association the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). ASHRAE test Standard 52.2, first published in 1999 and updated periodically as research continues, rates a filter’s performance in the particle size range of 0.3 to 10 microns (aka. micrometers). The higher the MERV rating, the more efficient and more effective the air filter.
Common Residential Airborne Particle Sizes
Based on a number of sources, below is a list of some common particles that are found in varying levels in indoor air. Note that in some cases (viruses and smoke), the 52.2 standard does not fully rate an air filter’s effectiveness capturing those extremely small particles; that’s where home air filter effectiveness drops off. In cases where there are significant concerns about issues such as smoke and viruses, in particular, it makes sense to learn more about combining a high efficiency furnace filter with a central air purifier.
Size ranges of some common indoor airborne particles:
- Household dust: 0.5-100 microns
- Pet dander: .5-10 microns
- Pollens: 5-1,000 microns
- Mold and mold spores: 3-40 microns
- Bacteria: 0.3-60 microns
- Tobacco smoke: 0.01-4 microns
- Viruses: 0.005-.3 microns
As alluded to above, certain indoor air pollutants cannot be fully addressed by even the highest quality, highest efficiency furnace filters, as their particles sizes or physical/chemical nature are simply beyond the scope of those products. Included among those indoor air pollutants that a furnace filter cannot fully arrest are certain virus, bacteria, and smoke particles, and a broad range of microscopic allergens and gases, including a class of gases known as volatile organic compounds (VOCs). More on VOCs in a future blog.
The important takeaway: As the particle size of certain indoor air pollutants gets smaller and smaller, even the best high efficiency furnace filters need some help getting the full job done. That’s where a high quality, proven central air purifier can pick up the slack.
MERV Ratings of Common Furnace Filters
Here’s a quick overview of some of the most prevalent types of furnace filters and their efficiency ratings, along with some tips and advice we’ve picked up in the past four decades. In all cases, making sure that your filter is the exact right size for your system, and installed properly, is an important factor that should not be underplayed.
When considering upgrades to a more efficient product, be aware that your ducting by the furnace must have enough space to accommodate the new filter. That’s not always a given, make sure this is addressed when obtaining high efficiency filter quotes.
1-Inch Thick Disposable and Cleanable Filters
The efficiency rating of 1” thick furnace filters ranges from about MERV 4 to as high as high MERV 14, however, our team does not recommend filters from either the top or bottom end of that efficiency range. Essentially, MERV 4 rated filters don’t do a very good job cleaning your air, and 1” filters in the MERV 10 to MERV 14 range could pose other concerns. More on that in a moment.
When servicing furnaces for homeowners in the Denver-Boulder area, our team typically uses MERV 8 rated disposable filters. This is usually a good middle ground that provides a significant increase in performance, while still allowing the furnace to do its job properly.
Many of our customers ask us about permanent, cleanable 1” filters they see online, at the store, or sitting inside their furnaces. Some of these are known as electrostatic filters, typically with ‘beefy’ aluminum frames and packed with lots of filter material. There are also other, less substantial types of cleanable furnace filters, some of which may have been supplied with the furnace from the manufacturer, originally; plastic and aluminum frames were common. In short, we recommend being very cautious or simply avoiding these filters, due mainly to inefficiency or airflow concerns.
1-Inch Pleated Furnace Filters
The old style, flat-panel, disposable cardboard filters that many of us have used for years, with spun fiberglass or another loose woven material, which usually cost less than two bucks, carry efficiency ratings of approximately MERV 4. Simply said, not good enough by today’s standards. These filters are considered to be inadequate by our service team at Save Home Heat. They can, however, be effective at stopping low-energy mice!
As mentioned above, the furnace filter that our technicians use standard in hundreds of homes every year in the Denver-Boulder area is a MERV 8 rated, disposable, cardboard frame, pleated filter. Pleated furnace filters feature a V-shaped, reinforced filter media within the 1” frame, providing increased surface area for proper airflow while doing a better job of filtering your air.
Pleated 1″ filters are readily available with ratings ranging from MERV 5 up to MERV 14, but we find that the higher rated products are frequently too restrictive to provide good airflow in many residential systems. Hence, our MERV 8 recommendation.
High Efficiency Media Filters
The filter cartridge inside a high efficiency media furnace filter – one of our most popular products – is typically about 5 inches thick, and is an easily replaceable, cardboard frame, highly effective pleated filter. The increased surface area and the thickness of these filters allows for proper airflow and even better filter performance. The MERV ratings of high efficiency media filters commonly used in home applications range from approximately MERV 10 to MERV 16.
One of our most popular media filters are MERV 16 rated filters from Lennox Industries, which remove up to 99.99% of particles down to 3 microns in size, 95% of particles down to 0.3 microns, and also feature an activated carbon-coated media: true hospital-grade performance! It’s important to keep in mind that the higher the MERV rating of the furnace filter you consider, the greater the care that must be taken to make sure it’s the right match for your furnace and your home’s ducting system. That’s where consulting a pro like our team at Save Home Heat makes a lot of sense.
The most popular furnace filter upgrades with our customers currently are high efficiency media filters with either MERV 11 or MERV 16 efficiency ratings. Our team’s recommendation of one versus the other is often made in consideration of how we assess the airflow capabilites of the system. Regardless of the filter selected, a real nice feature in many cases is that the filter cartridge needs to be replaced only once or twice a year, even in homes that have central AC.
“High efficiency particulate air” (HEPA) filters are defined by ASHRAE as being at least 99.97% efficient at capturing particles 0.3 microns in size. True HEPA air filters are used regularly in a broad range of industrial, clean room, and medical applications, and are also available for home use. True HEPA filters offer performance that is actually ‘off the MERV scale,’ so to speak, when compared to other residential filters’ performance on the smallest range of particle sizes. And that’s a good thing when it comes to home air filtration, as long as airflow considerations are properly addressed. Most HEPA filters are ‘technically rated’ at around MERV 17.
When considering HEPA filters that are made for central furnace installation, we’ve found there may occasionally be obstacles to meeting customer desires, among them the ability to properly fit these filters in the ducting, cost considerations, and potentially increased sound levels from the fan contained inside the filter housing. But on the other hand, HEPA filters represent the state the of the art in many ways, and are certainly worth learning more about in an informative, low-pressure fashion.
A variety of high quality central HEPA filters are available, designed to fit different furnace and ducting system sizes, and there are also standalone HEPA filters, as well. [A word of caution when shopping for portable units: ‘HEPA-style’ is not the same as true HEPA!]
Electronic Air Cleaners and Hybrid Products
Central electronic air cleaners, with removable, cleanable filter assemblies and prefilters, are not as popular as a couple of decades ago, but they still bear mentioning. Most of these products have efficiency ratings around MERV 11, and one of the reasons they have fallen out of favor is that their cleaning procedures are not very user-friendly. Without regular cleaning, their effectiveness drops significantly. Of note, electronic air cleaners offer the benefit of very little resistance to airflow.
There are also air cleaners available that have hybrid designs, incorporating some of the benefits of an electronic air cleaner with a replaceable media filter, and boasting efficiency ratings as high as MERV 16. As with all of the above products, for homeowners in the Boulder-Denver area, our comfort advisers are glad to take the time to learn more about your system, answer all your questions, make professional recommendations, and help you make an unpressured, informed purchasing decision. Our only agenda is our customers’ long-term satisfaction with the products and service our team provides.
Airflow Is Critical To Your Furnace Filter Decision
We cannot overstress the importance of using a furnace filter that’s the right match not only for your furnace, but for your home’s ducting system, as well. Over the years, standards for the number of supply registers and return air grilles in homes have increased their number, along with the sizing of the metal ducting, itself. On occasion, we still see homes whose systems are not up to today’s standards, and which may pose comfort issues in the parts of a home located furthest from the furnace. In some cases, our team is able to make modifications that can partially improve these situations (two-stage and variable-capacity furnace upgrades can also help). Regardless, whenever we see these types of systems, our team is extra careful about the filters we recommend and use. There are cases where a less restrictive, MERV 5 rated filter could be a better call than the MERV 8 we typically recommend.
The above rationale also applies when considering high efficiency media filter upgrades. The less restrictive MERV 11 media filter might be a better option than a MERV 16 in some homes, due to the greater airflow it allows. In others, it might make sense to discuss other options entirely.
Enhanced Protection for Your AC System
If you have a central air conditioner as part of your forced air comfort system, it’s important to understand that the indoor cooling coil, located in the ducting by the furnace, is a magnet for dust. A less publicized function of your furnace filter is to protect the cooling coil from getting loaded with dust. If routine replacement (or cleaning) of the filter is neglected this can significantly reduce the effectiveness of your AC system and reduce airflow around the house, as well as increase your utility bills. In a worst case scenario – our apologies for repeatedly beating this important drum! – a clogged furnace filter can result in premature parts failures and shortened equipment life. Regular filter maintenance, and considering upgrading to a more efficient furnace filter, can help you out more than you might realize, even during the cooling season.
Is It Time to Check Your Furnace Filter?
Need we say more? In the Denver-Boulder area, please contact our team if you need assistance with this important routine maintenance task.
Expert Furnace Filter Advice For Denver-Boulder Homeowners
For over 40 years, our home comfort team has been helping Boulder-Denver area homeowners improve the safety, effectiveness, comfort, and efficiency of their heating and cooling systems. Please contact Save Home Heat Company for expert, professional, low-pressure advice and recommendations on the best furnace filter for your home.