How often you change your furnace filter will actually depend on a variety of factors. What type of furnace you use, as well as what types of filters you use will help determine the frequency of changing. Keep in mind that some manufacturers may void out your furnace warranty if they suspect or can prove that proper maintenance was not followed.
Disposable Fiberglass Filters
Disposable fiberglass filters are the filter of choice for consumers, particularly because they're fairly inexpensive. Also, many consumers are not aware of the true reasons for changing your filter often and may feel their furnace maintenance company is simply lying just to get more money. However, these filters are considered to be the least effective. They block large dust particles and dirt particles, so they protect your furnace from damage. However, if you've got mild to severe mold or pollen allergies, this filters won't help. Generally, these filters should be changed every 1 to 2 months.
Washable Electrostatic Filters
These filters are slightly higher quality than traditional disposable filters. They utilize an electrostatic charge that helps gather dirt, dust and other various particles. However, they still do not attract all necessary pollen, molds, dust or other allergens and are estimated to only block between 15 and 20 percent of particles in the air. These filters should be cleaned out once a month. Some filters will last years while others may be made to last 3 months, 6 months, or 12 months so it's important to check to see how often the actual filter itself has to be replaced, rather than cleaned.
Pleated Allergy Filters
These filters use several pleats to increase the total available surface area for catching dust, allergens and mold. That being said, they're significantly higher in quality compared to electrostatic filters or disposable fiberglass filters. They're estimated to be between 35 and 50 percent efficient. Pleated filters should last for a minimum of 3 months. However, you should check them regularly for buildup, which may signify it's time for a change, especially during high pollen seasons or whenever you're using your air conditioner or forced heat regularly.
HEPA filters, or "High Efficiency Particulate Air" filters, are currently the top of the line when it comes to filters. These filters are used in hospitals, manufacturing plants and research centers in order to maintain the highest quality air possible. Unfortunately, they're not recommended for home furnace use because while they may remove up to 90 percent of all allergens, dust particles and more, they will also significantly block air flow. Blocked air flow means your filter will work harder and is more likely to break down.
The industry standard for any filter is to change once a month for the highest level of maintenance. Of course, this may or may not be necessary for all homes depending on their specific environment and how often homeowners use their furnace.