In general, most air filters and HVAC companies recommend changing your filter 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 only way to determine how often you need to change your air filter is to perform a visual inspection of the filter every month. After a few months, you'll get an idea of how quickly it gets dirty and if you need to re-evaluate due to a new pet or poor outdoor air quality.
The ideal is to change your air filter every three months, however, there are certain factors to consider when determining when to replace an old filter. Depending on your lifestyle and the type of air filter you choose, you may want to change it more often. A number of factors need to be considered when deciding how often a furnace filter should be changed, and many filter manufacturers recommend how often their product should be replaced. A general rule of thumb 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, the efficiency of the filter decreases. The most important factor is how often the system is running. The filter collects dust and debris only when the system is operating, so the more it works, the faster the filter picks up dirt. During a cold winter or hot summer, a furnace filter will need to be changed more often than during milder seasons.
Additionally, pet hair can clog up a filter quickly, so if you have pets in your home you may want to consider changing your filter more frequently. If someone in your household has asthma, severe allergies, or difficulty breathing for any reason, a coarse-media filter (MERV 11 or higher) or an electronic air filter is the best option for cleaner air than if a basic fiberglass air filter is used. You may also want to consider an air cleaner for your system. Cheap fiberglass filters require less frequent replacement than pleated filters as they don't trap as much dirt and debris and don't clog up as quickly.
However, some of these 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 and if cleaning with a brush instead of replacing the media filter, be sure to change the filter after two or three cleanings as it won't remove enough deeply embedded dirt and debris. In larger homes, more air flows through the filter than in smaller houses so it will get dirty more quickly in a system that serves a large house. Some thermostats have an Auto and Fan option - in automatic mode, the fan works only when the system is heating or conditioning the air in your home while in fan mode it works all the time until you turn it off.
Running the fan requires electricity so you'll see an increase in your electricity bill but it also helps balance temperatures in your home and draw cool air from basements to help cool upstairs floors. In summary, we do not recommend continuous fan mode as a standard operating procedure as it increases electricity costs and humidity levels in your home. To find out when to change your air filter, perform a visual inspection every month and listen for wheezing noises which indicate that it's time for a change.