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Understanding Furnace Filters: Function, Differences, & Maintenance

understanding furnace filters

You may know it’s important to clean or replace your furnace filter as the manufacturer recommends, but what else do you know about them? Do you know about the different types, sizes, and efficiency ratings, or what exactly these things mean? If not, don’t worry about doing hours of research! We put together this handy guide to understanding furnace filters to help you learn about the available options and select the right one for you.

What a Furnace Filter Does

While air filters do help maintain indoor air quality, their primary purpose is to protect your furnace’s blower by collecting and, you guessed it, filtering out dust and debris pulled in by return ducts. Doing so helps keep the system working with peak performance and efficiency, and prevents potential issues and costly repairs. That’s why it’s so important – and worth mentioning again – to follow the manufacturer’s guidance for cleaning or changing the filter!

Key Differences Among Furnace Filters

Not all filters are built the same, and no single option will work best for every system. Here are the key differences to consider when selecting a furnace filter.

Efficiency Ratings

For measuring filtration efficiency, Mechanical Efficiency Reporting Value (MERV) ratings represent the standard system accepted by The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The values indicate how well a given filter captures larger particles between 0.3 and 10 microns. Essentially, the higher the MERV rating, the better the filter collects specific particle types/sizes.

However, it ultimately depends on the capabilities of the system, as higher filtration typically leads to reduced air flow and/or decreased energy efficiency due to the increased workload on the fan.


  • Disposable fiberglass/synthetic filters are typically the most common and inexpensive type of filter. However, they are also the least effective and don’t do much more than block large particles of dust and debris.
  • Disposable pleated filters are made of either cotton paper or polyester and are also very popular due to their eco-friendly materials, affordability, and ability to capture mites, spores, and some other small particles. But keep in mind that collecting more debris means these filters become dirty/clogged more quickly and will need more frequent changing.
  • Electrostatic filters feature self-charging electrostatic cotton or paper fibers capable of trapping small, potentially harmful particles. While affordably priced in general, they can get more expensive if you choose options with higher efficiency ratings or custom sizes or that are washable/reusable.
  • High Efficiency Particulate Air (HEPA) filters represent the gold standard of air filtration as they filter out 99.97 percent of airborne particles that are 0.3 microns in size. These are commonly used in hospital settings due to their efficiency, but this also makes them expensive and not always compatible for many HVAC systems.

Size & Thickness

Filters range in thickness from one to five inches, and the most common sizes are 16” x 20”, 16” x 25”, and 20” x 25”. Filters must fit snugly, so refer to manufacturer guidelines for your particular make and model. These specifications can usually be found on the door to your system’s filter cabinet.

Size and thickness (as well as filter type) will impact the recommended replacement/cleaning schedule. For example, some one-inch filters are designed to last three months while some four-inch filters can last six months to a year. Be sure to clean or replace your filter per manufacturer instructions.

Furnace Filter Replacement/Maintenance

Start by referring to the manufacturer guidelines of the specific filter being used for maintenance and replacement instructions. Doing so will help promote smooth, clean airflow and a healthy, efficient system. Additionally, if you have pets or people with asthma, allergies, or other respiratory conditions living in the home, you’ll want to check and clean or change the filter more often.

Understanding furnace filters and the differences among them helps you maintain your system’s performance, efficiency, and management of indoor air quality. Still not sure which to choose? Not to worry! The expert HVAC technicians at Heating & Cooling Two are here to help find the ideal filter for your system, preferences, and budget. Contact us today to learn more.