When was the last time you changed the air filter on your furnace? Did you even know you needed to change it? Despite the critical role air filters play in the function of a furnace, many homeowners aren’t even aware they exist.
A traditional forced-air furnace—which is the variety that uses a filter—works by pulling air from your home, warming it as it passes through a heat exchanger, and then blowing it back throughout your house. These systems have a filter that catches dust, debris, and other air contaminants as fresh air is pulled in to be heated.
Furnace air filters have two primary functions:
-To protect the internals of your furnace: Without a filter, dust, lint and other bits would build up within your furnace, particularly in the blower motor which pushes warmed air throughout your home. This build-up can at the least reduce the efficiency of your unit—and at the worst cause expensive damage or complete unit failure.
–To improve indoor air quality: Preventing all those air contaminants from reaching the inside of your furnace also means that they aren’t blown back into your home. In this way, a clean furnace filter functions as a massive air purifier.
It’s not enough to just have a furnace air filter installed though. You also need to keep it clean. In most homes, this means replacing or cleaning your furnace filter every three to six months. A dirty filter reduces the airflow to your furnace which can likewise affect its performance and even create a fire hazard.
When replacing an existing filter or installing a new filtration system, you have a few different options as far as furnace air filter types. Keep in mind that you’ll need to ensure that the filter you purchase is the right size, but many units allow you to choose the type.
Fiberglass or synthetic air filters are the most affordable option, but they offer minimal filtration only catching around 80 percent of particles 50 microns or larger. This is sufficient to prevent dust building up in your heat exchanger and blower motor but will do very little for indoor air quality.
Filters made of polyester are median-sized meaning they are manufactured to fit as many furnace sizes as possible. Polyester filters are effective in catching 80 to 95 percent of particles as small as 5 microns or larger making them more capable of improving your home’s air quality compared to fiberglass or synthetic options.
This increased effectiveness is unfortunately reflected in the filter’s price, being quadruple that of the fiberglass/synthetic type.
Electrostatic air filters are made of positively charged cotton fibers that attract negatively charged particles similar to the way static cling sometimes causes clothes to stick together in the drier. They are offered in both disposable and reusable formats.
The reusable option will need to be periodically cleaned with warm, soapy water and allowed to dry thoroughly before re-installation. Insufficient drying time can lead to mold growth. Reusable electrostatic filters can last up to eight years with proper care.
Pleated filters vary in material, but their unique design is highly efficient in trapping minuscule particles as small as 0.3 microns. This enables a pleated air filter to catch even bacteria and viruses!
Pleated filters are also more durable than fiberglass and synthetic filters, and often more cost-effective than electrostatic options.
HEPA filters are excellent at trapping particles of 0.3 microns or larger removing 99 percent of germs and other pollutants. However, the disadvantage of HEPA filters is that they don’t optimize airflow. As a result, your system must work harder to circulate air leading to increased electricity bills.
Air filter ratings are based on their minimum efficiency reporting value (MERV). MERV works on a scale of 1 to 16 with sixteen being the highest quality filtering capacity. It is essential to consider a filter’s rating along with materials and price.
1 to 4: Filters with a low MERV rating are aimed at providing basic levels of heating, ventilation, and air conditioning (HVAC) filtration. Larger micron particles that could damage the coils or build up to compromise efficiency are targeted. At this rating, upwards of 20 percent of dust, pollen, and other pollutants of similar size are filtered. These filters are low priced but sacrifice air quality filtration.
5 to 8: Filters with MERV ratings in this category can remove 20 to 35 percent of dust-sized particles along with fungal spores. These filters are well-priced and more efficient.
9 to 12: This MERV range filters all dust-sized and microscopic particles that can cause allergies as well as pet dander and auto emissions. A filter at this level keeps your HVAC system clean for longer, which can extend the life of your furnace. Unfortunately, as MERV ratings increases, so do filter prices. If you don’t have pets, smokers, or anyone with allergies in the home, this cost may not be warranted.
13 to 16: These are both the most effective and most expensive air filters available outside of HEPA filters. A filter with a MERV rating of 13 to 16 will filter out dust, dander, bacteria, viruses, and much more. They are especially vital for those with compromised immune systems.
It is an easy process to replace your current furnace filter with new one of similar size. Simply remove the filter from the furnace and read the dimensions that are printed down one side. The actual size of the filter will be slightly smaller than the given dimensions, so be wary of measuring the filter to determine fit. This is because the size given is actually the size of the furnace and not the filter.
It is important to get the correct filter size for your unit as ill-fitting filters can inhibit HVAC function—if you can install them at all.
As we mentioned at the beginning of this article, changing the air filter in your furnace regularly is important. When airflow is obstructed, it not only stresses the system but raises the cost of running the system and creates a fire danger.
When you purchase your filter, it will have a change-out date included in the packaging, but a general guideline is the before mentioned three to six months. You may find that your filter needs to be changed more or less than this depending on the level of air pollution in your home and how frequently your furnace is actively running. You can generally tell if a filter needs cleaning or replaced by visual inspection. You’ll see discoloration and build upon the surface of the filter.
We hope this guide on air furnace filters has helped you to better understand what your filter is, how it works, and how to maintain it. If you still have questions regarding filter replacement, you can always speak with your HVAC technician during annual maintenance.