You know you do it. You just grab the first air filter you see off the shelf at your local store, not giving a second thought as to whether you are buying the right filter for your needs.

The kind of air filter you buy can make a huge difference, and you should be aware of a few things when planning on how to maintain and choose the correct filter for your air conditioning system in your home or business.
Air Filter

A discussion on air filters could last forever, but here are three things to be aware of when planning on how to maintain and choose the correct filter for your home or business’s air conditioning system.

Table of Contents

MERV rating

The MERV rating is a measurement scale designed in 1987 by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers to rate the effectiveness of air filters in indoor air quality. MERV stands for Minimum Efficiency Reporting Value, and you can check out a chart in a previous post if you want to know what the numbers mean. In general, filters with a higher MERV rating need to be changed more often since they pick more contaminants out of the air—but that’s why you want them. Filters with a high MERV rating get more out of the air! Another factor to consider is the air filter’s thickness. For example, a MERV 8 pleated 1 ” filter doesn’t filter any better than a MERV 8 pleated 4″ filter. However, the 4″ filter will last longer because of the increased surface area compared to the 1″ filter.


Changing intervals

There is a common myth sold to homeowners concerning the infamous “30-Day Air Filter” vs. “90-day Air Filter.” As filters pull things out of the air, they block your air more and more, making your equipment work harder, even keeping it from cooling the house well! Running your system with a dirty filter can make your costs skyrocket. The reality is that each home has different conditions within it; those conditions will affect how much is in the air for the filter to remove. If you have pets, or if your kids are going in and out of the house all day, you will probably need to change your filter before the box’s recommendation. There are options for those who do want a filter that will last longer than 30 days, but they often aren’t cheap.

The most common option is purchasing a system that uses a larger filter. Go to our indoor air quality page for some information on these types of systems. Lower airflow due to a dirty filter can decrease capacity. Consequently, the system’s energy efficiency ratio or performance factor can increase your heating and cooling cost. A filter should be periodically checked and replaced when needed, not just every 30 days!

Correct Sizing

Using the correct size filter for your return is essential when selecting your air filter. Your filter is meant to protect your air-conditioning equipment, not just you. Under-sizing a filter can result in the air flowing into the return grille. So, it bypasses your filter, dropping dog hair and dust on the expensive, heat-exchanging coil instead of the changeable filter. That would allow your system’s components to suffer the consequences of a lack of protection from your air filter. You essentially waste the money you spent on an air filter. Moreover, poor filter sizing might result in higher energy bills and possibly damaging the equipment.

The most common under-sizing would be in cases where a filter is an inch or less from a more common size, like 15×20 or 20×21½. If you are under-sizing, the dust and debris that you see on your filter every month may only be a part of what is traveling through your air system or even clogging up expensive equipment!

We hope this article helps your selection process. If you have any questions, please visit our homepage,, or call us at 352-243-7088.