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How to choose the right air filter size

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You probably already know the importance of changing your HVAC system’s air filter regularly. From preventing airborne particles from causing things like allergies and asthma to helping your system to continue to cool your home effectively, a clean air filter is instrumental in keeping your system humming along, doing its job.

Most homeowners are more than capable of changing the air filter themselves. It’s an easy job that does not require the assistance of an HVAC technician. If you’ve never done it before, a simple internet search should be all you need to teach yourself how to do it. What’s a bit more complicated, however, is figuring out what kind of filter you should buy. Not all filters are created equally, and it’s very important to choose the right one for your system.

The first thing to do is to look at the existing filter in your system. It should have a size on the label. Remember that there is a differential between the actual air conditioner filter size and the nominal size, usually of about a half inch on each side (one inch total). When you head to the store to buy a replacement filter, keep this difference in mind so you are sure to buy the right size. Better still, bring the label from your existing filter with you, and look for a new filter with matching specs.

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But what do you do when you can’t see the information on your label? It may be that the information has worn off. In some cases, the filter may not have ever had the label in the first place. If either of those things happens, you can still determine size of the filter yourself. Simply measure the sides of the filter and write those measurements down. To save yourself time later, it’s also a good idea to write down you measurements on your a/c unit itself, so you can refer to that information next time you need to replace the filter.

OK, so what happens if the old filter isn’t available? Let’s say you’ve just purchased a new home and there’s no filter in the unit, or you accidentally disposed of the old filter before taking note of the size. No problem. Just measure the area where the filter is supposed to go, and deduct about an inch (see above). You can also call the internet into service on this task. You can look up size tables online for your air filter sizes that will give you the nominal size based on your measurements.

The reality is, there’s only one right air conditioner filter size for your unit. Your system will only perform optimally if you use the right filter. Using a smaller one will lead to issues like:


  • The evaporator will get dirty and clogged, which can lead to a seizing of the system.

  • The blower can get bogged down with dirt, which can lead to a loud noise from the system.

  • There will be an increase in dust and other airborne particles/allergens in your indoor environment.


All these issues can lead to serious problems with your system, which could require a visit from a technician. Check this out for more information.