What is a Furnace Filter MERV Rating?

The furnace in your home may not be the first thing on your mind when it comes to home maintenance, since it’s likely functioning properly the vast majority of the time. In fact, as long as you’re comfortably cool in the summer and warm in the winter, you may completely forget about it. However, it’s important to understand how your furnace works and know how to properly maintain it so you don’t end up with large expenses for repair and replacement when you could have taken preventative measures. This is especially true since things like furnaces tend to fail at the worst possible time – like during a snowstorm in the winter.

When you look at it that way, you’ll start to see how the furnace is the most central part of your home, especially when it comes to comfort and climate control. Without it, you could be in a world of trouble. Therefore, you should make sure that, if nothing else, you are checking and cleaning or replacing your furnace filter on a regular basis. 


First, though, you should understand how to choose the right filter for your furnace, and that’s based on size, type, and MERV rating. Let’s take a look at how all of this weigh into your regular furnace maintenance schedule.

How a Furnace Works

Most furnaces work on forced air dynamics. This means the unit draws in air through return ducts and processes it by warming it over a heat exchanger. Then, it uses a fan to force the heated air out a series of other ducts that feed into rooms of your home.

It continues to run until the temperature in the home – at the measuring point – reaches that which you set on your thermostat. The same thing works in the summer for air conditioning, assuming you have a whole house cooling unit, with the air being cooled by a compressor located outside and a series of coils that live inside your furnace, rather than heated.

Furnaces may run on gas or electricity, depending on how your home is fueled. There are similar systems called boilers that also heat the home, but these work with water rather than air and have a completely different means of heating the home.

Traditionally, most homes use furnaces, while some commercial applications utilize a boiler. Therefore, you’re likely to need to understand the maintenance and repair needs of a furnace. Consider that one of the main aspects of caring for the system is that the furnace has a filter that is necessary to function. What does this filter do?

How a Furnace Filter is Used

By nature, the air we breathe is not always clean. Even our own natural shedding of hair and skin can affect what we breathe in. This is also true of the furnace, since it has an intake of the same air we’re breathing in our home.

When the return ducts draw air into your furnace, they also draw in all the other components, including dirt, dust, hair, dead skin, allergens, and other particles. If these reach the blower fan, they can cake it up and make it difficult for the fan to turn effectively, which will at best lower efficiency and at worst damage the fan beyond repair so that you lose your system. Replacing the fan and other parts of your furnace system can be quite costly.

By contrast, preventative measures taken by the filter reduce potential damage to the fan and the rest of the furnace. Having the filter means that your furnace and the interior fan will last far longer because there will be less of an issue with the fan itself. It captures the particles that would otherwise blow through the system, keeping it safe from harm.

As a bonus, though it’s not the purpose of the filter, this also keeps the air you breathe cleaner. If it weren’t for the furnace filter, a great deal of the allergens and particles drawn into the furnace would be blown right back into our environment, and because they were blowing rather than settled, they could raise greater problems with allergies and irritation.

Of course, there are things to consider regarding maintaining the health of your furnace system. It also means that the filet can get very dirty and clogged over time. This is why you have to assure that you’re maintaining a schedule to check, clean, and replace your furnace filter at regular intervals.

 Types of Furnace Filters

While there are some types of furnace filters that are more common than others, there are several types to choose from, each with their own benefits. The most commonly used type of filter is a pleated disposable kind that comes in a wide range of sizes that are considered standard to fit various furnaces. They also come in several different MERV ratings, which can adjust not only the efficiency of the filter but also the price. The pleated filter is made of paper combined with polyester, creating a formula that does a nice job of taking particles and allergens out of the air that gets fed into your furnace.

Disposable filters are relatively inexpensive, though a few do range in the $30-40 region. You have to consider not only the size but also the brand and the rating of the filter when looking at the overall price. It’s important to check this sort of filter at least monthly for any blockage that may have occurred so that you get the best performance out of it, and you need to replace the filter on a 90 day schedule.

You can also find disposable fiberglass filters, which are even cheaper and utilize a spider web configuration of fibrous threads that are typically colored blue to trap the particles and allergens. While these come in a lot of standard sizes and are easy to find, as well as the cheapest option, you get what you pay for in many cases. They can be less sturdy than the pleated paper and polyester filters, to the point of feeling flimsy. They likely also have lower MERV ratings than their pleated counterparts. You’ll probably need to check and replace a fiberglass furnace filter more frequently, which pretty much negates the initial savings based on lower cost.


You may decide to use a permanent filter that is reusable, also known as a washable filter. These are made of solid aluminum in some cases or with a plastic frame in others, and they are obviously more efficient that disposable filters of any kind in terms of maintenance costs. It’s simple to clean these filters with nothing more than a vacuum to remove debris from the outside, followed by water to thoroughly clean them out.

As with other furnace filter types, they come in a range of sizes, as well as carrying multiple options for MERV ratings. They tend to cost more than disposable filters, but when you consider they have an average lifespan of about 5 years, which would be the equivalent of 20 or more disposable filters, this isn’t something you should worry about. It’s definitely cheaper all the way around than having to replace your disposable filter every 90 days. Instead, simply take the filter out and clean it with this same frequency.

What About Electrostatic Filters?

As you shop, you’ll find that both disposable and reusable furnace filters come in electrostatic versions. Electrostatic furnace filters are self-charging with the air that passes through them, which means that they hold an electric charge that adds to the proficiency in catch particles and allergens. This is especially useful if you have pets in your home, smoke or live with someone who smokes inside the home. You should, however, check the manual that came with your furnace to determine if you can use one of these electrostatic filters with your system safely before deciding to make the purchase.

Size of Filter

As with most items on the market, not all furnaces are made exactly the same way. Thus, you find that different models of furnace require a different filter size. When determining the right size for your filter, you need to measure all three dimensions – length, width and height. Note that width, or thickness, is mostly commonly 1 inch, though 4-inch thickness is also popular among many furnace manufacturers, especially on larger systems.

When it comes to length and height, you’ll find any number of combinations, ranging from 10 x 10 to 30 x 30. Again, though, there are several common sizes, including 16 x 20, 20 x 25, 14 x 25, 25 x 25, and 16 x 25. If you aren’t sure what size filter your furnace requires, you can easily check the old filter by removing it and reading what is written on the side, measuring it yourself, or even consulting the furnace manual.

Some furnaces do require custom sized filters. In this case, you’ll have to speak to the manufacturer of your particular furnace about where and how to purchase the proper filter for your system. This isn’t a common problem, but you never know what you’ll run into at the wrong time. Be sure to know in advance if you’ll have to seek out special providers for your furnace filter.

Replacing a Furnace Filter

When you’ve determined the size and MERV rating you require for your furnace and you’ve purchased the appropriate filter, you have the task of actually installing it. Thankfully, this isn’t a difficult procedure under most circumstances.

1.       Make sure your furnace doesn’t kick in during your work in replacing the filter by turning it off until you’re done making the switch.

2.       Open the compartment door where the filter is contained, somewhere between the air intake and the furnace. Slide the old filter out easily. This shouldn’t require any tools, since the filter typically just sits perfectly in the slot.


3.       Take the time to properly dispose of the old filter, placing it in a plastic bag quickly so that the heavy amounts of dust and allergens don’t escape back into your environment.

4.       If you have a permanent filter instead of a disposable one, instead of throwing it out, you’ll want to first vacuum – perhaps using the brush head to gather the most debris – and then rinse the filter thoroughly, using water. Don’t use soap, since this can potentially clog the filter, and don’t use solvents of any kind since they can cause damage to the aluminum. Simply use clean, warm water. You have to let it dry entirely prior to putting it back in the filter.


5.       If it’s been a long time since you’ve done this, you may notice a great deal of hair or dust lurking around the opening for the filter. It’s important to keep not just the filter but also the environment where the filter lives clean. You should take a vacuum – again preferably with the brush attachment to the hose – around this area and just inside the filter opening prior to installing or reinstalling your filter.

6.       The filter you’re getting ready to install – whether it’s the one you just washed – has an arrow to show which direction airflow happens through the device. That arrow should point toward the furnace side of the compartment where you slide the filter in. If you aren’t careful and install the filter backward, you’re going to have some major trouble with your furnace very soon.

7.       With the filter slid into place, it’s time to turn the furnace back on so you can make sure everything is in good working order.

8.       Don’t forget to check the filter once a month so that you can take note of any excess debris or problems that could point to another issue with the furnace unit. You should also put together a calendar with a regimen of ‘every 90 days’ – whether it’s to shop for a new filter or to thoroughly clean the permanent one.

The process is actually quick and easy to complete, with even the cleansing of a reusable filter not taking more than 10 to 15 minutes. Aside from water and a vacuum, no tools are required in either scenario.

Filter Ratings – What is MERV?

We’ve mentioned here a few times the term ‘MERV Rating’, but you may still be extremely unclear as to what this actually means. ‘MERV’ stands for minimum efficiency reporting value, and the values range from 1 to 16. The MERV rating refers to certain standards set for efficiency and ability to truly ‘filter’ the environment.

The higher the MERV rating, the more particles and allergens the filter is capable of removing from the air. The rating ranges from 1 to 16, with a higher number indicating that there is less admission of particles into the air that’s processed through the furnace. However, choosing a filter that has too high of a rating can be detrimental to the functionality of the entire furnace, making it work harder to produce less results.

The MERV rating is set up to help consumers understand the capability of the furnace filter, putting a number that is easy to understand to the number of particles that are removed from the air. So, the higher the number, the greater number of particles you can expect it to capture. At the high end of the spectrum, however, you may end up with a flow through problem when it comes to getting the air to your furnace. But if you or your family suffer from breathing issues such as asthma, or skin irritations from airborne allergens, then the higher the MERV rating the better.


For most applications, a MERV rating of between 8 and 11 is sufficient, getting you a great level of effective particle removal and filtration while still allowing air to flow properly through your system. However, it’s essential to check with your furnace manufacturer, whether over the phone or in the manual, to see if there is a maximum MERV rating allowable for your furnace to continue to perform properly. Again, every furnace is different, so you’ll want to check individual specifications to assure you don’t cause more harm than good when changing out your filter.

So, what is the best merv rating for filter? As mentioned, the performance of furnace filters improves with higher MERV ratings, up to a certain point. Note that, in addition, the price also goes up. Therefore, while you might be drawn in by the idea of a MERV rating of 14 or 15, you’ll find the price isn’t so nice, and you won’t get the efficiency you would with a rating that is more midrange.

Other Rating Systems


You may find that some of the larger home improvement store chains have created their own rating systems for furnace filters, manufacturing their own product for comparison. In most cases, these systems follow closely to the standard MERV rating for comparison. Still, there can be variance, so if you’re going to purchase one of the brands created by these chain stores, make sure to research and compare carefully so you don’t opt for a rating you think is appropriate but turns out to be incompatible with your furnace system.

Conclusion

While there are plenty of things to consider when you purchase a furnace filter, don’t forget to find out the best MERV rating for your particular system. The MERV rating can significantly impact everything from the price of the filter to the way it functions in your home. You want to make sure you have a high enough rating to know that the harmful particles that could damage your system – as well as affect your breathing – are being pulled out of the air that the ducts take in, while also ensuring you don’t go too high and end up having a problem with air flow.

Regardless of the MERV rating, you need to assure that you’re properly checking and either replacing or cleaning the filter on a regular basis. If you have a permanent or reusable furnace filter, make sure you’re cleaning it every 90 days, and be sure to replace your disposable filter at least this often, checking it once a month.

If you care regularly for your furnace, you’ll find that it lasts a lot longer and requires fewer repairs, saving you a bundle of money, and taking care of the filter is the number one priority to extend your furnace’s lifespan.

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