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How Do I Know What Size Air Filter I Need for My HVAC System?

How Do I Know What Size Air Filter I Need for My HVAC System?

Many people know that HVAC systems have filters and that these filters need to be replaced from time to time. However, many people don’t realize that filters come in various shapes and sizes. This is important since each HVAC system corresponds to a specific filter size. 

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If you want to learn how to find the HVAC air filter size for your system, this article will explain exactly how to do that! 

Why Does the HVAC Air Filter Size Matter?

An HVAC filter is an integral part of an HVAC system. It catches dirt, dust, and other debris and prevents it from entering the air. 

HVAC systems are designed to work with a specific filter size. Filters that do not conform to that size simply will not work. A smaller filter will leave space for air to travel unfiltered. Meanwhile, a larger filter may not fit at all. 

Think of it like shoe size. If you wear a shoe that is too big, your feet will slip around, making it hard to walk. If you wear a shoe that is too small, your feet will be cramped or unable to fit entirely. You wouldn’t wear the wrong shoe size, so you shouldn’t use the wrong air filter size. 

How Do HVAC Air Filter Sizes Work?

Understanding that HVAC systems have different filter sizes is just one part of air filter maintenance. You also need to understand what goes into determining these sizes. 

First, you should understand the dimensions of air filter sizes. All air filters are measured in inches. These measurements are expressed as length by width by depth. So, an air filter measuring 25x30x1 is 25 inches long, 30 inches wide, and 1 inch deep. 

Most filters will have the width as the widest measure. However, the width and length are the same in many filters. 

Nearly all filters will have the depth as the smallest number. This is because almost all filters are either 1 to 2 inches deep or 3 to 6 inches deep. The smaller end of this spectrum is usually found in residential air filters, while the larger end is usually reserved for larger HVAC systems, like those found in commercial or industrial buildings.

Difference Between Nominal and Actual Size in Air Filter Measurements

While on the topic of the filter size, it is important to note the difference between nominal and actual filter size. This difference will be important when you start looking for new filters, as they will inform the size you are looking for. 

Nominal size is a list of the general dimensions of the filter. Usually, this is rounded to the nearest whole number or half number. This makes it less exact but also makes it easier to look for and keep track of. 

The actual size is more precise. It uses less rounding and, in doing so, gets closer to the actual size of the filter. This gives a number that is a little more cumbersome to use but is more accurate. 

Most filters have a long series of numbers listed on the side of their packaging that identifies the size. In most cases, this is the nominal size. You may have to check the manufacturer’s website to find the actual size. 

4 Ways to Find the Right Air Filter Size for Your HVAC System

If you want to replace your air filter, you need to determine what size you need. You can do this by checking the size of your current filter or by checking the system itself.  

1. Check the Old Air Filter

Check the Old Air Filter for Its Size

The easiest way to determine what size filter you need is to check the size of the old air filter. As previously mentioned, the size of your old air filter is often printed on the side of its frame. There, you can find the dimensions you need listed. These dimensions are typically listed in inches. 

Just make sure that the air filter you have access to actually fits into your HVAC system. Check to make sure that the filter fits snugly into the system. It should fit but shouldn’t be too loose. Once you’ve determined that you have the filter you need, you can use the numbers off of it to go out and find a replacement. 

2. Check Your HVAC Owner’s Manual

You can check your HVAC owner’s manual to find out what kind of filters your HVAC system needs. Here, you will find a lot of information about your system as a whole. One piece of information is the exact filter size your system needs. The advantage of using this method is that some owners’ manuals give actual sizes rather than nominal sizes. 

If you don’t have the owner’s manual, you may be able to get access to it online. Find the manufacturer’s website and check if they list it. If they don’t, you can contact them for more information. 

3. Measure It Yourself

If you’re unable to determine the size of the filters you need through the previously listed methods, you can measure the filters yourself. The easiest way to do this is to measure the old filter. Simply take a tape measure and use it to measure the filter. As you do so, make sure to take a measurement of all three dimensions and write these measurements down so you don’t forget them. 

4. Measure Air Register or Air Filter Slot

Air Filter Slot

If you don’t have access to an old filter, you might think you’re out of luck, as several of the methods listed here require it. However, you can measure the filter slot instead. This lets you know exactly how big the slot is, which tells you how big of a filter you need. 

When doing this, just be aware that the slot will always be slightly bigger than the filter itself. You’ll need to knock a little off the size to find a filter that will fit. So, you won’t get as exact of a measurement this way as you would with other methods. However, it should be close enough, and you can always contact your HVAC service provider for help.

Call Eanes for Professional HVAC Maintenance

If you need to replace your air filter or need other HVAC maintenance, make sure to contact us at Eanes. We can help you address any confusion or solve the problem for you entirely. With our fifty years of experience behind us, you can trust the professionals at Eanes. Schedule a service online or give us a call!