In general, most air filters should be changed every 90 days or 3 months. This may vary depending on the location of your home, if you have pets, and the age of your system and equipment. The best way to determine when to replace an old filter is to perform a visual inspection of the filter every month. After a few months, you'll have an idea of how quickly it gets dirty.
You'll need to re-evaluate if you're having a new pet or if the outside air quality has been poor. A general rule for pleated air filters (such as those manufactured by FilterBuy) is to replace the filter every 90 days. As the filter traps more dirt, dust and allergens from the air, its efficiency decreases. There are certain factors to consider when determining when to replace an old filter.
The filter collects dust and dirt only when the system is operating, so the more it works, the faster the filter will pick up dirt. During an extremely cold winter in Minnesota, an oven filter will need to be changed more frequently than during a mild Missouri winter. On the other hand, you'll need to change your filter more often during a hot, humid Missouri summer than during the Minnesota summer. Remember that a filter also gets dirty during AC mode.
If you find pet hair in your oven filter, it means that the hair fibers of dogs, cats, rabbits and other mammals are large compared to the dust and dirt normally trapped by the filter. That means they clog the filter and block the air flow quite quickly. The more important clean air is, the more often the filter must be cleaned or changed. A coarse-fluid filter, MERV 11 or higher, or an electronic air purifier is the best choice when someone in the household has asthma, severe allergies, or respiratory problems for any reason.
The air will be cleaner than if you use a basic fiberglass air filter. You may also want to consider an air purifier for your system. Cheap fiberglass filters need to be changed less frequently than pleated filters. Some sites do it exactly the other way around, stating that thicker filters can last longer between changes.
That goes against the design of the filters. Coarse-media filters trap more and smaller dust particles, so they obviously clog up faster than fine fiberglass filters which don't trap as much dirt and debris. 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 you clean dirt with a brush instead of replacing the fluid filter, be sure to change it after two or three cleanings as cleaning it with a brush won't remove enough deeply embedded dirt and debris. In larger homes, more air flows through the filter than in smaller houses because air transports dust, pet hair, and other debris which makes the filter get dirty more quickly in a system that works in a large house. Some people wonder what continuous fan mode is and why an owner would use it. There must be a setting on the thermostat such as Automatic and Fan options where in automatic mode, the fan works only when the system is heating or conditioning the home while in fan mode it works all the time until you turn it off which makes the filter get dirty faster and you will have to replace it more often.
Continuous fan mode has its pros and cons; it helps clean air by running it through an air filter more often but also consumes electricity which increases your electricity bill as well as brings warm air from inside walls or from the attic to living spaces which makes your air conditioner work longer than it should leading to higher electricity costs. We don't recommend continuous fan mode as standard operating procedure but if you do use it make sure to check your filter regularly and change it when necessary. The only way to be sure how often your air filter needs to be changed is by performing a visual inspection of it every month so you know what noises indicate a problem such as wheezing even when the filter is clean which will get worse when it's dirty blocking airflow in your system.