How frequently should you replace your furnace filter?

Changing or cleaning furnace filters regularly helps your heating and air conditioning system function more effectively, saves energy, and extends the lifespan of your units. Plus, it enhances your indoor air quality.

But how often should furnace filters be replaced, and what should you do if they appear clean when it's time for replacement? What exactly do these filters do? How can you tell if they are functioning properly?

Why Do We Need Furnace Filters?

Your heating and air-conditioning system works by drawing air from a room, passing it over coils to either heat or cool the air, and then circulating the adjusted air through ducts to other areas of your home. The furnace filter, or air filter, is located where the air enters the system.

It captures airborne particles that come with the air, preventing them from obstructing the blower or clogging the coils. If the coils become clogged, they can no longer effectively heat or cool the air, and this may damage the system.

Thus, the furnace filter aids your heating and cooling system in functioning properly, keeps it running smoothly, and protects the system, ensuring it lasts longer.

A well-sized, correctly installed, and fully functional heating and air system circulates air throughout your home every hour. During this process, it draws air through the filter. The extent to which the filter cleans the air depends on its MERV rating.

What Are MERV Ratings?

MERV, or Minimum Efficiency Reporting Value, is a measurement system used to indicate the size of particles a filter can remove from the air passing through it. MERV values range from 1 to 16, with higher numbers signifying greater filtration efficiency and the ability to trap smaller particles.

Filters typically display their MERV rating on the packaging, showing the size of particles, measured in microns, that the filter can capture. The higher the MERV number, the smaller the particles it can filter. To provide perspective on particle sizes [sources: INDA, Howarth and Reid]:

Inexpensive, disposable filters that are about 1 inch (2.54 centimeters) thick and made of disorganized fiberglass or natural fiber strands usually have a MERV rating of 1, though they can go as high as 4.

Particle Filtration Percentage

The MERV rating also reveals the percentage of airborne particles a filter will capture as the air flows through it.

Generally, a MERV rating of 6 means the filter will capture up to 50% of airborne particles, while a filter with a rating of 8 will trap between 70 and 85 percent of the particles it encounters.

A filter with a rating of 11 or higher ensures that the air exiting the filter is up to 95% cleaner than the air entering it.

Pleated Air Filters

Pleated filters made from disposable, nonwoven fabric have smaller pores, and the pleats increase the surface area, enabling the filter to trap more particles compared to a flat surface.

These filters have MERV ratings of 3 and above, depending on the fabric's density and the number of pleats [source: INDA]. Some filters are also electrically charged to attract and hold airborne allergens.

Filter Effectiveness and HVAC Performance

The filter's ability to capture particles actually increases as it becomes dirtier; the buildup on the fibers narrows the openings, enabling the filter to trap more particles.

However, this only holds true up to a point. As the filter gets dirtier, its ability to remove particles negatively affects the energy efficiency of your heating and cooling system. The air restriction caused by the buildup can reduce airflow in the system.

Think about the times you've worn a dust mask while working on a project. It's much more difficult to breathe through the mask than it is without one.

If you switch to a HEPA (high-efficiency particulate air) mask that filters particles as small as 0.3 microns with up to 97% efficiency, it becomes even harder to breathe in enough air to keep going [source: Howarth and Reid].

The same concept applies to your heating and air system: The denser the filter, the more effort the system has to exert to pull in air. A clogged filter may cause the system to run constantly, leading to higher heating or cooling bills.

This is why it's essential to check your filters at least once a month and replace them when they become dirty.

What If My Furnace Filter Looks Clean When It's Time for a Change?

Freddie Williams, an instructor of air-conditioning technology at Lanier Technical College in Oakwood, Ga., explains that high-quality filters are the most effective way to purify the air in your home. But what should they look like once they've done their job?

When it's time to replace your filter — typically one to three months after installing a new one — it should appear dirty.

"Dust build-up is usually visible," said Williams. "You should see gray, ashy material on the side of the filter facing the duct."

If your filter looks clean after the recommended period, here are some things to check:

The cleanliness of your furnace filter can also be influenced by the overall condition of your home and how frequently you use your heating or air conditioning.

If your home is tightly sealed, has no pets, lacks dust-collecting furniture like carpets or fabric-covered chairs, and you clean daily, your furnace filters will have fewer airborne particles to trap. Additionally, the filter only works when the system is in use.

If you replace the filter but don't turn on your heating or cooling system for a month or two, the filter should remain relatively clean since no air has been forced through it.