Air filters and oven filters should be checked at least once a month. If they are dirty, they should be replaced with a clean, clean air filter. To determine if the filter is dirty or clogged, hold it close to a light source and if you can't see the light through the filter, it's time to change it. The ideal is to change your air filter every three months, however, there are certain factors to consider when determining when to change an old filter.
The filter collects dust and debris only when the system is operating. The more it works, the faster the filter picks up dirt. During a cold winter or hot and humid summer, a furnace filter will need to be changed more often than during milder weather. Additionally, a filter also gets dirty during AC mode.
Pet hair fibers from dogs, cats, rabbits and other mammals are large compared to dust and dirt that normally trap the filter, so they clog the filter and block the airflow quickly. A coarse-media filter, MERV 11 or higher, or an electronic air filter is the best option when someone in the household has asthma, severe allergies, or difficulty breathing for any reason. Cheap fiberglass filters require less frequent replacement than pleated filters. Some sites get this exactly the other way around, claiming that thicker filters can last longer between changes; however, this goes against the design of the filters. Slim fiberglass filters don't trap as much dirt and debris, so they don't clog up as quickly. The good news is that some of those filters can be cleaned with a nylon brush and placed back in the oven or air controller instead of replacing them.
Cleaning should be done outside or in a garage to keep dust and dirt out of your home. If removing dirt with a brush instead of replacing the media filter, be sure to change the filter after two or three cleanings. In larger homes, more air flows through the filter than in smaller houses. Because the air carries dust, pet hair, and other debris, the filter will get dirty more quickly in a system that serves a large house. Some thermostats have a setting such as Auto and Fan options; in fan mode it works all the time until you turn it off which will cause the filter to get dirty more quickly. Follow these steps to find out when to change your air filter: check it at least once a month; if you can't see light through it when held close to a light source; if you have a new pet or if outdoor air quality has been poor; if someone in your household has asthma, severe allergies or difficulty breathing; if you have a large house; or if you use continuous fan mode on your thermostat. You may hear a little wheezing even when the filter is clean; it will worsen when the filter is dirty.
Know your system and the sounds it emits to know what noises indicate a problem. In summary, changing your air filter every three months is ideal but there are certain factors that can affect how often you need to change it.