Written by Vernon Trollinger
No question is more likely to stir up a hornet’s nest of debate (if not brawl) among heating, cooling, and air conditioner (HVAC) professionals than “Which air filter should I use for my home?”
Opinions vary so widely because over the years technicians have seen everything that the wrong kind of air filter can do to a home’s HVAC system: from coils, motors, and blowers too clogged by weak filters to burnt-out motors and controls because of too-restrictive filters. HVAC air filters are important for removing contaminants from the air to improve your home’s air quality. The better the air quality, the better your HVAC system runs and the better your family’s health.
Air filters are rated according to their Minimum Efficiency Reporting Value (MERV). Created by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), it’s a porousness scale that goes from 1 to 16, with most home air filters ranging from 4 to 13 MERV. The higher the MERV, the more contaminants that are removed from the air.
Air filters are made from different materials in different thicknesses and sizes. These factors effect their MERV.
- Spun fiberglass filters (MERV 1-4): Cheap and disposable, these filters will catch 80% of particles 50 microns and larger, and snag 25 percent of the particles in the 3 to 10 micron range. Many manufacturers recommend these filters as minimum protection just from dust and dirt building on fan motors, heat exchangers, and other surfaces. They filter out large particles to protect the furnace components, provide maximum airflow but don’t filter the tiny harmful contaminants that affect your health.
- Disposable pleated paper or polyester filters (5 to 8 MERV ): These median-sized filters trap 80 to 95 percent of the particles 5 microns and larger. They cost four times more than the spun fiberglass filters but do a better filtering job.
- Electrostatic filters (2 to 10 MERV): These use self-charging fibers to attract particulates out of the air. Disposable pleated versions run about $10 in standard sizes (example: 16″ x 25″ x 1″). Washable versions (rated 4 to 10 MERV, usually not pleated) sound like a great way to save money but quality varies with cost. Better quality ones can last up to 8 years. These filters must dry completely after washing in order to avoid mildew or mold growth so a handy trick is to buy two and rotate them out for cleaning.
- Disposable pleated high MERV filters (11 to 13 MERV): High-efficiency filters can trap 0.3 micron particles like bacteria and some viruses. Two to five inch thick versions of these filters fit in box-like housings mounted onto the air handler and can last up to one year. Periodic changes to the filter design can add to the price.
- High-efficiency particulate arrestance (HEPA) filters: These are the true high end of filtration and are able to filter out 0.3 micron particles. HEPA filters drastically restrict airflow and should only be matched to a compatible system.
Unfortunately, one of the biggest selling filter products, 3M’s Filtrete, do NOT use MERV, preferring its “Microparticle Performance Rating” (MPR) instead. Fortunately, there is a place to compare the numbers. Be aware, too, that some retailers also use their own rating system instead of MERV.
Old Resistance, New Developments
A decade ago, it used to be that if you had an HVAC system that used panel filters rated 6 through 9 MERV, putting in a MERV 11 or higher filter would restrict airflow (known as filter pressure drop). In turn, this would lengthen the system’s run-time, adding to your heating and cooling costs.
This assumption is not entirely the case anymore. Newer pleated filtering media increases the filter’s surface area so that while the filter might be finer, there’s more area for air to pass through. The more pleats per foot, the better. The thicker the filter, the more surface area per pleat.
So, while the filter pressure drop issue still has some impact, a 2009 Home Energyexperimental test concluded that “…if no accommodations are made for the greater pressure drop of high-MERV filters, air flow and energy penalties are not likely to be severe — at least, not until the filter is loaded with dirt.”
Some General Considerations
If you are renting and do not have allergy problems or pets or live in an area with excessive dust, then buying cheap spun fiberglass filters with a cardboard frame every three months should work out fine. They’re not a good investment if you own your home because these filters are flimsy and prone to leak dust into the HVAC system. That dust can build up on coiling coils and motors and threaten to cost more over time in repairs than you might save from using cheaper filters.
If you are going to invest in washable filters, then expect to pay more than $20 each for a 8 MERV filter. Cheaper washable ones will have loose filter media, especially after washing, and thus will perform poorly there after (caveat emptor). Keep in mind that disposable filters are more hygienic because all the dirt gets removed from your home when you toss out the filter.
Higher MERV pleated filters do a much better job now that in the past. While the key to better filtration vs pressure drop lies in getting the most pleating per foot, it’s safe to say that newer filter designs have less air flow issues at higher MERV. More important, they are more effective than the median-grade pleated filters at improving indoor air quality (and potentially better health) for you and your family.
All the same, you still need to replace them every three months so price is a concern. For a standard sized 16″x25″x1″ 11 MERV filter, expect to pay $5 or more per filter but you can save a little when you buy them in packs of 12. Antimicrobial or electrostatic treatments also add to the filter’s cost. Some top brands within this general size range are Filtrete, Purolator, and Nordic Pure.
Replacements for thicker (two to five inches) pleated 11+ MERV filters that require compatible framing to the air handler (see photo) and should follow the manufacturer’s recommendation because a thinner, cheaper filter won’t fit properly and so won’t work. While these filters are expensive – $45-$100 – they last a full year and work very well
When it comes to air filters, all systems, homes, and air quality needs are going to be different. Ask yourself if the current type of filter you’re using is doing what you need it to do: filter dust and irritants from the air to protect your HVAC system and your family’s health. If it isn’t, read through the HVAC manufacturer’s filter recommendations. Many are available through their customer support websites. Remember that while cost is an important factor to consider, you’ll save far more money in the long run by maintaining good air quality in your home with good air filters.