Answers to the Most Common Questions about AC/Furnace Filters

September 03, 2021

There is so much information online about indoor air filters located in so many different places, we know it can be easy to get more confused as you search for answers to make sure you get the right air filter for your home.  We wanted to help by picking out some of the most common questions and answering them all in one place for you.  We also give you additional information you can read as well if you are looking for something more in-depth.  Without further adieu, lets begin…

Do indoor air filters actually work?

In short, the answer is yes, they do work.  Determining how well they work….that’s a different story as there are several variables to keep in mind.  For starters, there are multiple types of filter media that an air filter can be constructed with.  Depending on the type of media (fiberglass vs. lofted polyfiber vs. pleated media) your air filter uses, it may or may not be meeting all of the needs for you and your family.  You can find out more about types of air filters and filter medias HERE.  Secondly, other than fiberglass air filters, all others are tested and rated in some type to determine the overall quality of the air filter.  This rating will also help relate to you how much of each size and type of airborne pollutants and contaminants the air filter can help reduce in your home.  You can find out more about MERV, MPR, and FPR air filter ratings HERE.  One of the great things about most type of indoor air filters is that you can perform a visual inspection and see dust and debris on the filter itself to have visual proof that your air filter is working.  This also helps lead into our next most frequently asked question.

How Often Should I Change my Air Filter?

As we mentioned above, there are a few different types of air filters that you can have in your home.  The type of air filter you have is one of the factors that determines how frequently you should change your air filters.  It is recommended to change fiberglass air filters every 30 days, polyfiber air filters typically every 60 days, and standard pleated air filters every 90 days.  These numbers are based on normal conditions.  Some things to keep in mind, is that if you own pets, are doing home renovations, or live in certain extreme climates, you may have to change your air filters more frequently.  In these cases, it is generally recommended at a minimum to use pleated filters and change them every month as opposed to every 90 days.  An outlier here that we should mention as well is washable filters.  While not as popular as the options listed above, they do need regular maintenance.  While you may not have to change them as often (most filters last at least 1 year), you do need to remove the filter media and wash it monthly.  For washable air filters, it is generally recommended to have at least 2 filters at all times.  That way you have one to use while you are washing the one that was just used and it is drying out.

What Size Air Filter Do I Need?

In the quest to find the correct size air filter that you will need for your home, we must first give you a clear understanding on the different types of sizes.  The sizes are actual size vs. nominal size.  Let’s start with actual size, because this is the easiest to explain.  Actual size is the exact measurements of your air filter for the length, width, and thickness.  For example, 16.75”L x 25.75”W x .75”D would be what an actual size measurement would look like.  Nominal size is a measurement of the actual size rounded up to the nearest whole number.  Using our actual size measurement above, the nominal size would be 17”L x 26”W x 1”D.  The nominal size is the size that you see printed in large bold print on your air filter.  Not to worry though, as most filters will also have the actual size printed in smaller numbering somewhere on the cardboard or insert as well.  We would be doing you an injustice if we didn’t give you a disclaimer that while this actual size vs nominal size breakdown does hold true in most cases, it doesn’t hold true in every case.  Some branded replacement air cleaner filters can have a larger actual size than nominal size.  Thankfully, you do have one very important resource in trying to determine the correct size filter to buy.  The one you are currently using!  Just take the measurements off it with you to your favorite store or online retailer to make sure you are getting the correct size replacement.

Another question that often gets asked when looking at filter size, is how thick should the filter be.  Air filters typically come in 1”, 4”, and 5” thickness.  In comparing 1” thick filters against 4”/5” thick filters, the best answer would be to use the thickest filter you can to achieve maximum efficiency of your HVAC unit.  However, you should never force the filter to fit in a smaller space than it is designed for.  For instance, if you try to use a 4” or 5” thick filter in a system designed for a 1” filter, your system efficiency could actually be worse as that thick of filter is not designed for that system.  On the flip side, trying to use a 1” filter in a system designed for a 4” or 5” thick filter, would result in a loose fitment and unfiltered air able to pass by the filter and into the home.  So in short, the best practice to use when it comes to determining which filter thickness you should go with, would be to use whichever your system is designed to work with to achieve maximum efficiency.

What do Carbon Air Filters Do and How do they Work?

When looking at indoor air filters, the primary function of a carbon air filter is to help eliminate odors.  So, on top of helping reduce the airborne contaminants in your home, it will also help trap odors so your home will stay smelling fresh.  Carbon activated air filters are generally ideal for those that have pets in the home, smokers in the home, or even for temporary use like around holidays when lots of cooking will be done in the home or during a home renovation.


Hopefully this has helped answer some of the most common questions you may have regarding your indoor air filter.  We know that a lot of thought and effort goes into the buying process for something relatively inexpensive that is designed to be thrown away or recycled at the end of its short life.  For more answers to your questions about indoor air filters or anything else related to indoor air, check out some of our other blog entries.

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