Furnace Filter MERV Rating: What Is MERV, Best MERV? (Explained)

Choosing the best MERV rating for furnace filters is the #1 thing you need to do. Selecting between MERV 4, MERV 6, MERV 8, MERV 11, MERV 13, and so on, is not all that easy. To help you out, we are going to look at what the MERV rating on furnace filters actually is, how to determine MERV rating, and we will use the furnace filter MERV rating chart to answer questions like is MERV 8 is good enough and if your furnace can handle MERV 13.

Before we start with all this, we need to understand the following thing:

Higher MERV-rated furnace filters are not necessarily better. In many cases, MERV 8 filters (best for furnaces) are a better option than MERV 13 (best for humans / clean indoor air).

Furnace filters with higher MERV rating also need to be changed more frequently in order not to restrict the already restricted furnace airflow due to tightly packed filter fibers in MERV 11 and MERV 13 filters, for example. Clogged air filters is one of the most common culprits for a furnace breakdown.

The best MERV rating for furnace filters balances two vital things:

• Indoor air quality. Higher MERV-rated filters like MERV 11 and MERV 13 will indeed capture more air pollutants than lower MERV 6 or MERV 8 furnace filters.
• What MERV rating can your furnace handle? Using a MERV 13 filter makes no sense if your furnace cannot handle that (leading to breakdowns and shorter furnace lifespan). Higher MERV-rated filters make it more difficult for the furnace to produce adequate airflow, resulting in restricted airflow, higher fuel costs, and, very importantly, a higher probability of a furnace breakdown.

  

Many homeowners will immediately go for the highest MERV furnace filter; breathing the cleanest air possible is a no-brainer, right? However, many of us forget that furnaces will rarely be able to handle a MERV 13 filter, for example. So, when choosing the best MERV rating for furnace filters, let’s keep this in mind.

We are going to take a structured way of explaining everything about MERV and furnace filters. Here is the step-by-step approach that will address the most common questions regarding MERV-rated furnace filters:

• What is MERV rating on furnace filters? First, we need to start with the definitions so we know what we are talking about. What does MERV mean on furnace filters is key to understanding which MERV filter to choose. We will also look at the furnace filter MERV rating list for MERV 6, 8, 11, and 13.
• How to determine MERV rating for furnaces? The key is to find the balance between indoor air quality (IAQ) and furnace performance; ie. what MERV rating can a furnace handle.
• Is MERV 8 good enough? Case #1 for MERV 8 rating furnace filters.
• Can your furnace handle a MERV 13 filter? Case #2 for MERV 13 rating furnace filters.

Let’s start by putting in perspective what does MERV mean on furnace filters:

What Is MERV Rating On Furnace Filters?

MERV stands for Minimum Efficiency Reporting Value. It is a metric that tells you how well can a filter capture indoor air particles between 0.3 and 10 microns. Higher MERV filters are also tested on how well they capture smaller 1 to 3 micron particles and even 0.3 to 1.0 micron particles. The MERV rating itself for different furnace filters is measured by ASHRAE.

Here is the ANSI/ASHRAE Standard 52.2-2017 by ASHRAE document titled ‘Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size’ that explains the methodology behind MERV rating.

In short, depending on how well does a filter capture these air pollutants, we have filters ranging from MERV 1 to MERV 16. The higher the MERV rating, the better the pollutant capture rate. That’s because higher MERV filters consist of more tightly packed fibers that capture pollutants.

For furnaces, we predominantly use MERV 6, MERV 8, MERV 11, and MERV 13 filters. Here is what MERV rating means on these furnace filters:

• MERV 6 means that a furnace filter can capture up to 35% of big 3-10 micron particles.
• MERV 8 means that a furnace filter can capture up to 70% of big 3-10 micron particles and up to 20% of medium-sized 1-3 micron particles.
• MERV 11 means that a furnace filter can capture up to 85% of big 3-10 micron particles, up to 65% of medium-sized 1-3 micron particles, and up to 20% of the smallest 0.3-1.0 micron particles.
• MERV 13 means that a furnace filter can capture up to 90% of big 3-10 micron particles, up to 85% of medium-sized 1-3 micron particles, and up to 50% of the smallest 0.3-1.0 micron particles.

Source: US Environmental Protection Agency (EPA)

If you just check the ability to capture larger 3-10 micron air pollutants, you can see there is quite a big difference between MERV 6 vs MERV 8 vs MERV 11 vs MERV 13 furnace filters. Namely, these filters can remove 35% vs 70% vs 85% vs 90% of these particles, respectively.

Example: By choosing MERV 8 furnace filter over MERV 6 filter, you are effectively capturing 70% vs 35% of larger 3-10 micron particles. That means that MERV 8 is twice as effective as MERV 6. However, a whole 30% of the big particles will not be captured by MERV 8 filter. In comparison, MERV 11 filter will capture up to 85% of these particles and the MERV 13 furnace filter will capture up to 90% of the larger particles, leaving only 10% of the bigger particles to get through.

Now, from this example, it is obvious that higher MERV furnace filters are better, right? Why don’t we always go for MERV 13 filters? Why do we even use MERV 6 furnace filters? Let’s look at these filters from the viewpoint of the furnace:

How To Determine MERV Rating For Furnace?

Here is how you should think about which MERV filters are the best:

• Higher MERV filter is the best for humans. We get cleaner air and increase indoor air quality (IAQ).
• Lower MERV filter is the best for furnaces. With loosely packed fibers in the low MERV filter, the furnace will have an easier job blowing the hot air through them.

In short, you want MERV 13, and your furnace wants MERV 6 filters. The best MERV rating for furnace filters is usually the balance between these two (you can also opt for MERV 6 or MERV 13 furnace filters, of course).

How to know what is the right balance and which MERV filters are the best for your furnace?

There is no standard procedure that would tell you exactly what MERV furnace filter to use. Even if you open up the furnace specification sheet, you will rarely see “Use 8 MERV furnace filter” on the sheet, for example.

Filtration engineers point out that for adequate MERV rating choice, you have to measure the external static pressure of one-stage or two-stage furnaces to determine if your furnace can handle a higher MERV rating filter. In testing procedures, they usually use 0.50 in. WC (inches of water column) external static pressure. This is connected with the expected average pressure drop across the filter; however, all these calculations are predominantly theoretical and don’t give us a clear answer to which MERV filter is the best.

Even without a clean way of telling which furnace filters are the best for your furnace, we can think about furnace filter MERV rating like this:

• HVAC experts mostly recommend MERV 6 and especially MERV 8 furnace filters. These filters will reduce the pressure your furnace has to work against, increase furnace efficiency and prolong its lifespan. MERV 8 filters are also cheaper and don’t have to be replaced as often as higher MERV furnace filters.
• Indoor air quality experts recommend MERV 11 or MERV 13 filters. If you are suffering from allergies such as asthma, increased indoor air quality is essential. It, however, needs to be said the primary job of furnace filters is not to clean the air, and air-cleaning MERV 11 and especially MERV 13 filters will exert a big pressure resistance to furnaces.

Because most of the dilemmas for furnace filters are MERV 8 vs MERV 13, let’s make a case of MERV 8 and MERV 13 separately to see which filters are better:

Is MERV 8 Good Enough For A Furnace? (Case #1)

In the MERV rating chart, MERV 8 filters are less effective than MERV 13 filters. For homeowners thinking about choosing MERV 8 furnace filters, this usually prompts the question “Is MERV 8 good enough?”.

In short, yes. MERV 8 filters are the most commonly used and recommended furnace filters. Most HVAC experts agree that MERV 8 is the sweet spot between what the furnace can handle and how clean air we get.

MERV 8 filters will capture up to 70% of the larger 3-10 micron particles vs up to 90% MERV 13 filters can achieve. That’s a 20% difference; however, it just may be the cost you need to pay in indoor air quality to prevent the furnace from breaking down.

Of course, MERV 13 filters are very good at capturing smaller 1-3 micron and 0.3-1.0 micron particles as well. However, it needs to be pointed out that it’s not the primary job of MERV furnace filters to take care of these small particles.

In fact, a much better option for an easy-working furnace AND superb indoor air quality is a combination of MERV 8 furnace filters and a separate air purifier. Air purifiers use HEPA filters instead of MERV filters, and these HEPA filters are capable of capturing more than 99.97% of particles with a diameter of 0.3 microns.

That means that MERV 8 furnace filter  + air purifier combo is a much better option than MERV 13 filter.

MERV 8 filters are also less expensive than MERV 13 filters and don’t need to be changed as frequently as MERV 13 filters (usually every 3 months or even less, you can check what a dirty furnace filter looks like here).

The bottom line is this:

MERV 8 are the best furnace filters for most furnaces. Using an additional air purifier to increase indoor air quality is also recommended.

Can Your Furnace Handle MERV 13 Filters? (Case #2)

MERV 13 filters have a tightly packed fiber and present a big restriction to airflow. However, they can increase indoor air quality better than MERV 8 filters, with up to 90% of 3-10 micron particles removed. The real question, obviously, is can your furnace handle MERV 13 filters.

Most furnaces will run with MERV 13 filters. However, you are running a risk of reduced furnace lifespan. If you don’t regularly change the MERV 13 filters, you will inevitably run a risk of furnace breakdown.

Arguably, the indoor air quality is increased, but you will have to pay that increase in IAQ by putting your furnace in danger. That’s why most HVAC experts advise against using MERV 13 furnace filters.

MERV 13 filters are also expensive and need to be frequently replaced.

On top of that, a MERV 13 filter will not increase the quality of indoor air as much as a simple HEPA air purifier (it’s not even close).

That’s why using MERV 8 filters that are easy on the furnace is a much better choice than using MERV 13 filters that most furnaces will not be able to handle in the long run.

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All in all, the choice of which MERV furnace filters are the best depends:

• Primarily on what your furnace can run. MERV 6 filters are OK, MERV 8 are a better choice.
• Secondarily on the indoor air quality. MERV 11 and MERV 13 will increase the IAQ but will also damage the furnace in the long run. On top of that, even higher MERV filters cannot complete with simple HEPA air purifiers as far as increasing the quality of the air we breathe is concerned.

That means that MERV 8 filters are recommended for most furnaces and households. This is a safe choice of filters that your furnace will easily handle.