Photo: A brand new furnace air filter on the left and a dirty filter on the right. How often to change a furnace filter depends on many factors.
Your furnace filter is an important but often ignored part of your home’s safety system in terms of its ability to clean the air you breathe. While the purpose of a filter is to protect the furnace against damage from debris entering the motor, it also cleanses the air.
Furnace filters are generally changed every 3 months or quarterly. Choose the highest Minimum Efficiency Reporting Value (MERV) rating allowed by your furnace. Reusable washable filters should be washed every 3-4 weeks or more frequently if required but are good at capturing only large particles.
You can easily find furnace filters from $5 – $30 each and in some cases even more expensive brands for specialty high MERV-rated filtration options. You can buy single packs and multi-packs of 10 filters at one time to save more money.
Let’s take a closer look at how frequently to change your furnace air filter and take a deeper dive into MERV ratings to help parents, pet owners and people with allergies choose the right one.
Furnace filter replacement frequency
In general terms, changing your furnace air filter every 3 months or 4 times per year is recommended. Most major furnace air filter manufacturers recommend this schedule on product packaging. But your personal home situation factors in, too.
Why might you want to replace your furnace air filter more frequently?
Home location: If you live in an industrial area, in a construction zone, in a big city, near wildfires, close to a highway, near a factory or in an area with significant pollen and allergens, your filter will certainly work harder than with none of these factors present.
Large families: The more people you have in the home, the more contaminants exist and the more your filter will work to clean the air.
Allergy/asthma sufferers: If you suffer from allergies or asthma or live with someone who does, you might be changing your air filter monthly while also using the highest rated air filter your furnace allows. More on that below.
Pets: Pets like cats and dogs roam around the house and leave pet fur and dander which makes its way into the air filters and wear them out sooner than you might otherwise experience were your pets not present.
Smokers: When you have a smoker in the house, it lowers air quality and the smoke will enter the furnace and get collected by the air filter.
Cooking: Certain types of cooking and foods produce more vapors than others. These too make their way into your furnace air filter to be collected and will degrade it over time.
Weather: The colder your environment, the more your furnace runs in the winter. The warm your summer weather, the more your AC runs. Filters wear out quicker in both cases more so if you operate your system on the Fan setting that keeps it running 24 hours per day constantly.
Furnace filters protect your furnace not you
The actual purpose of the furnace filter is protect your furnace. The fact that it removes particles from the air that you breathe is a bonus.
When debris enters the furnace filter and gets trapped, these are particles that can’t enter the furnace motor, particles that can clog and potentially damage the furnace.
A new furnace may typically cost $2,000 – $6,000 to install and furnace repairs often run $150 – $2,000. High efficiency furnaces are even more expensive ranging from $3,000 – $13,000. Click here to visit my article on costly home repairs including both furnace and AC replacement and repair charges.
So while the better and higher rated air filter you choose does help to filter out smaller and finer particles to make your air cleaner, it’s actually the secondary purpose. The main purpose is to protect your furnace from early replacement or costly repairs from particle damage.
How exactly does one define “higher rated” when it comes to air filters, though?
Minimum Efficiency Reporting Value (MERV)
The Minimum Efficiency Reporting Value (MERV) is a rating system used to measure the effectiveness of an air filter to remove various sizes and types of particles from the air.
The MERV rating scale goes from 1-20 where the higher the number the better the filtering ability to remove finer particles.
NOTE: Even when you may have protection against particles like mold in a lower-rated MERV rating, the higher the rating the better able the filter to actually remove most of that particular contaminant. The higher the MERV rating, the better it filters out smaller particles.
| MERV value | Description | Particles removed |
| MERV 1-4 | Basic filters that block some larger particles but can’t trap smaller ones. May be suitable for window A/C units and some residential uses. Includes washable aluminum filters and disposable fiberglass styles. | Debris, lint, pollen, dust mites, carpet fibers. |
| MERV 5-8 | Used in residential homes, commercial buildings and some industrial workplaces. | MERV 1-4 + mold spores, hair spray, common dust, cement dust. |
| MERV 9-12 | The average air filter in this range is suitable for home use depending on what your furnace manufacturer recommends. A MERV 10-11 is a good filter in a home with kids and/or pets. | MERV 5-8 + humidifier dust, lead dust, auto emissions, pet dander. |
| MERV 13-16 | High efficiency filters that are generally used in commercial environments that require very clean air. You’ll find them used in higher end commercial buildings, smoking rooms and hospital inpatient care. | MERV 9-12 + bacteria, tobacco smoke, debris from coughs and sneezes. |
| MERV 17-20 | The best filters that remove the finest particles. This includes High Efficiency Particulate Air (HEPA) and Ultra Low Particulate Air (ULPA) filters used in environments that require the cleanest air such as hospital operating rooms, pharmaceutical manufacturers and other clean rooms. | MERV 13-16 + viruses, carcinogens, radioactive materials. |
Best air filter for home use
Check your furnace label, instructions and warranty to determine the highest rated MERV filter it will accept. For many home owners a MERV rating of 10 will suffice. Allergy sufferers will benefit from a MERV 12 rating if your furnace will allow it.
When you choose a higher MERV filter, the ability for air to easily pass through the holes in the filter decreases, meaning your furnace has to work harder. It restricts airflow which can cause your furnace to expend more energy, work harder and eventually be damaged if you choose a MERV rating that is too high for it to handle.
How to tell when your furnace filter needs to be replaced
Have a look at it!
Taking a peek at your furnace filter is the quickest way to see if it needs to be changed. That and putting a note in your calendar to change the filter after 3 months or whatever your suggested replacement period is.
A new furnace filter begins clean and white. White is the perfect shade for a furnace filter for the same reason that factory workers wear white and why hotel linens and towels are always white:
White doesn’t hide dirt.
The moment your white furnace filter turns gray or black and looks like it’s wearing a fur coat with all the dirt and debris, it’s beyond time to change it, 3 months or not.
And no, disposable furnace filter can’t be rinsed or vacuumed to extend their life. Their job is to attract the dirt and debris to keep it out of the furnace motor and out of the air. Once 3 months are up, it has done its job and needs to be replaced, period.
Why does my furnace filter get so dirty so quickly?
- The colder the weather, the more your furnace runs and the more air it filters and the dirtier it will get.
- If you’re using a cheap filter, it may show and wear out quicker than a higher quality one.
- Do you leave your “Fan” setting on? If so, your fan runs 24/7 and will constantly be filtering air so the filter will wear out quicker. Using the “Auto” function means the fan only runs when the furnace is on.
- You live in a construction zone or have had recent work done inside the house. The more dust and debris brought into the home the more it will make its way into the filter.
- You have a big house and family. The bigger the home, the more dirt and debris. Same goes with when you have more people living in the house.
- You have air return duct leaks and openings. Heat can escape from these openings which reduces furnace efficiency and can also draw in dirtier air which clogs the air filter.
- You have pets. Cats, dogs and other hairy animals add dander and fur to the air which end up in the air filter at some point.
- If you smoke, light candles or incense, the materials burned will end up in the air filter too.
How to change a furnace filter
Please check out my article called Is Duct Cleaning Necessary which not only covers air duct cleaning and whether or not it’s necessary but also discusses how to choose a furnace filter and what options are available.
When changing your furnace filter:
- Make sure you choose the correct size for your furnace. Check the current size that is in the furnace. Also check the label on the furnace which will typically show the required size.
- Ensure that you insert the furnace filter in the correct way in the direction of airflow. The filter has a directional arrow on it which refers to the direction of airflow. To determine which way air flows in your furnace, take a look at the filter when the furnace is running as it will be pushed hard towards the direction of air flow.
Here’s a short video to visually illustrate how to do it.
What happens if I don’t change my furnace filter?
Air quality in the home will suffer if a dirty air filter is kept in use and not discarded and replaced on schedule. During both summer (AC) and winter (furnace) usage, allergy and asthma sufferers will notice the decreased quality of air.
Your furnace also has to work harder to force air through your ducts when the air filter gets full of contaminants it has pulled out of the air.
In the extreme, the filter will stop filtering particles and may degrade to the point where it starts to allow pollutants into the air and may also back up airflow and overheat the system. Eventually the system can shut down and cycle on and off again in a phenomenon known as short cycling.
Short cycling occurs when not enough (or any) air is allowed back into the furnace due to the dirty filter causing the system to overheat and shut down. Short cycling can damage the furnace and also increases energy consumption.
Is no air filter better than a dirty one?
No. The best furnace filter is a clean one of course. The second best is one that is slightly past replacement date. Having no filter at all is the worst case because the main purpose of a furnace filter is to remove fine particles and particularly ones that could damage the furnace motor.
Modern furnace filters are designed for many uses and in varying styles such as:
- Disposable fiberglass
- Disposable pleated
- Disposable electrostatic
- Permanent washable electrostatic
- High-efficiency pleated
- Allergen, Bacteria and Virus pleated
- and more
Clearly many of the filters available on the market these days are aimed at people not furnaces but their main goal is still to block particles from entering the furnace and damaging the motor. The use-specific filters add a further element to help people who suffer from specific ailments that a better quality filter can help with.
Your best bet is to find the highest MERV filter that your furnace accepts and that meets your needs, use it and replace it as needed.
Are washable air filters any good?
Reusable washable air filters are ones that don’t need to be thrown out and replaced but rather rinsed regularly with water to wash out dirt and debris before placing it back in the furnace. They are typically made from aluminum, contain no paper and may last up to 10 years.
In general terms washable air filters are at the lower end of the air filter quality level and are in the MERV 1-4 range although I’ve seen one MERV 8 model available. The average home needs a MERV 8-12 filter depending on what your furnace recommends and what you personally require.
While a washable filter may suffice, ensure that its MERV rating meets your needs.