Home owner 101: HVAC Filters

How to choose the right HVAC filter for your home

HVAC filters, furnace filters, and AC filters are all interchangeable terms that refer to the same thing: the filter that slides into your interior unit. This filter exists mainly to protect your HVAC equipment from dust and particulate matter, not to improve indoor air quality as most people believe. Foreign matter like dust can get into your system and wreak havoc on the coils before freezing or destroying the unit. This is why choosing the right filter is just as important as frequent changing. There are different types of materials used, ranging from low-cost fiberglass to high efficiency particulate air (HEPA) pleated filters.

Sizes

There is no standard size for HVAC or furnace filters. Some brands carry over 1500 sizes! In addition to the width and length, there is also thickness to worry about. Most smaller units in older homes are simply 1 inch thick. Newer units and those cooling very large homes could be 3 inches or more in thickness. Thicker does not mean better, and your unit needs to be able to push air through the filter without struggling. Never attempt to double up on filters or get a thicker one than the unit calls for. A thicker filter will not help you if your unit freezes up. The simplest way to determine which size you need is simply look at the old filter for the size or measure it yourself. If your size is unique, you can purchase a kit to make the size you need. The most important thing is to achieve total coverage in the opening where the filter goes.

Pro Tip: Once you determine your size, write it in permanent marker on your unit for easy reference.

Ratings Explained

Minimum Efficiency Reporting Value, or MERV, is an industry standard rating on a scale of 1 to 20 used to measure how many large particulates get captured. The higher the number, the higher the percentage of airborne particles that get filtered. For residential purposes, MERV ratings max out at 16, with 11 or 12 being a fantastic option. Higher MERV ratings are usually reserved for hospitals or large facilities with powerful HVAC units. Particles are measured in microns and are categorized as small (less than 3 microns) and large (over 3 microns). For reference, a human hair is 90 microns wide, a red blood cell is 8 microns wide, and common bacteria are about 1 micron in size. According to the indoor air quality charts found on epa.gov, a MERV rating of 3 would filter less than 20% of large particles (between 3 and 10 microns in size). By contrast, a MERV rating of 12 would filter over 90% of large particles and over 80% of smaller particles as well (less than 3 microns in size). If you live in an area with good air quality, don’t smoke or have pets, or if you maintain additional air filters in your home, you can get away with a low MERV rating. But if you suffer from allergies, have pets, live in a small space with multiple people, or live in a city with low air quality, you’ll want to invest in a higher MERV rating and change the filter more frequently. In humid areas where mold growth is common, choose a filter with a MERV rating of at least 6 to maximize the amount of mold spores that get trapped in your filter.

Frequency of Changing

Regular air filter replacement can lower your energy bill as well as protect your HVAC system from unfiltered air, making it last longer. But how regularly does that mean? Most filter companies recommend you change a 1” filter out every 3 months, a 3” filter every 6 months, and a 5” filter every year. It is a good idea to add “Check AC Filter” to your monthly home maintenance lists. Even if hasn’t been the recommended amount of time, change the filter if it is visibly dirty or has changed colors. If your heater fan is part of your system, you will notice that the winter months require more frequent changing since cold, dry air holds more dust and particles. If you notice a change of smell when the AC or heater turns on, change your filters. If you suffer from allergies, own pets that shed, or smoke indoors, check and change your filters more often than recommended. On the other hand, if your AC only runs for a few weeks in the summer, you might only need to replace the filter seasonally.

Pay attention to the local air quality alerts or major atmospheric events that could impact your air quality. For example, every summer in Texas we get huge dust clouds that have traveled from the Sahara Desert. Last summer we happened to need to change out two exterior doors during this dust storm. All the surfaces inside the home were covered with fine yellow-orange dust. Two days later, I opened the HVAC filters to see it clogged and orange. Even though the filters were only 6 weeks old, I installed new ones.

Reusable Filters

There are several washable filter options available on the market, including metal or plastic frames and DIY ones. These filters cost more upfront (about $23 for a MERV 8) but can be re-used for up to 10 years. An $8 disposable filter purchase for those ten years adds up to $320 (plus tax or shipping). For a homeowner with a mortgage, this is a worthwhile investment. They should be rinsed once a month and prior to re-installation, should be allowed to thoroughly dry to avoid mold and mildew. Alternatively, you can buy two of them to swap one out for another while you wash and dry. Some even include carbon filtration for odor removal. Electrostatic filters are most effective when new and clean, so they should be washed frequently. In addition to saving money, you will avoid sending over 40 disposable filters to the landfill over the life of a reusable one. Unfortunately, HVAC filters are not recyclable.