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The Top 5 HVAC Air Filter Myths

August 14, 2016 |

Air Fitler Myths

Good myths stick around because they have some amount of logical, believable, and easily understood content. With HVAC filters being a relatively simple product it is no surprise that some decades old myths are still circulated in our industry – even by so-called experts! It’s time to set the record straight and explain the truth behind these top 5 filter myths:

#5) Low Initial Static Pressure Equals Energy Savings
A few years ago the EPA came out with a statistic that “a 30% increase in static pressure across a coil results in an increase cost of $200 per 10,000 cfm of air movement” and couple that with Energy Management Systems and Variable frequency/Speed Drives on HVAC fans and the race to produce the lowest static pressure products began. This low static is accompanied with claims of energy savings, but rarely deliver. Most of the “low-static-pressure” options use medias that allow a lot of airflow when clean and tested in a lab, but have poor dust holding capacities which means when they are in-use the static pressure over the life of the filter is higher, causing the HVAC equipment work harder and use more energy when compared to filters that may have a higher initial static pressure, but a lower sustained static pressure.

Static pressure changes over time

#4) Anti-Microbial Treatment Makes The Air Safer
Anti-microbial treatments on media became common in the 1990’s and there are a few manufacturers that continue to offer  media treated with anti-microbial, but most have abandoned it. Independent industry group ASHRAE published a journal article in December 2000 after they tested the three most widely used anti-microbial treatments applied to HVAC filters and there were two meaningful results. First, there wasn’t enough exposure time of microbes to the filter media to kill them as they passed through the filter, so small microbes that weren’t caught by the filter weren’t killed. Second, media is naturally hostile to microbial growth and particulate that is caught by the filter is what produces the nutrients for microbes to survive. Because the captured particulate isn’t treated with the anti-microbial the growth of microbes on filters was found to have no difference between treated and untreated filters.

Learn How To Audit Your HVAC Filters

#3) High-Efficiency Filters Use More Energy than Medium Efficiency Filters
It makes sense that a filter that removes half of all airborne particles would need a higher powered fan than one that catches about 1% of all airborne particles. However, high efficiency filters have significantly more surface area than medium efficiency filters which allows air to more easily pass through the filter – lowering energy requirements. Also, high efficiency filters have benefited from extensive research and development with major design changes taking place while medium efficiency filters have remained mostly unchanged. A high efficiency filter from 1980 looks much different than its modern counterpart, while a pleated or panel filter from 1980 looks very similar to pleated and panel filters made today. Energy testing has shown MERV-13 filters using modern media and design can require under 1,000kWh per year, while MERV-8 pleats can require over 4,000 kWh per year.

Air filter Particle removal comparison

#2) MERV-11 is High-Efficiency
The line had to be drawn somewhere to separate “medium” and “high” (particulate removal) efficiency, and it is unfortunate that it was drawn at MERV-11. This distinction seems to be because of a combination of a hold-over in interpretation of old air filter test methods and that filters that are MERV-11 were commonly 12” deep at the time the MERV system was adopted. MERV-11 catches only a few percent more particles than MERV-8, which is why there is currently pleated style filters that are being called everything from MERV-8 to MERV-10 when there is no difference between the filters. If the conditions are right there is little stopping a MERV-8 filter from testing out as MERV-10 in a lab. When we do in-situ tests it isn’t uncommon for a MERV-8 to reach MERV-11 levels. When mating particulate capture rates versus ambient air particulate counts, you wouldn’t consider anything under MERV-13 as high efficiency.

#1) Filters Get More Efficient As They Load
This one makes too much logical sense to argue with, so there are still filter salespeople telling people this myth. When filters are tested in a lab ASHRAE dust (also once known as Arizona Road Dust) was used as the challenge aerosol. The problem with it is the average particle size of ASHRAE Dust is over 8 microns in size, while more than 99% of ambient air is under 1 micron in size. These submicron particles are too small to act as part of the filter media, while the larger 8 micron particles can layer on each other to help aid in removing additional particulates. In addition, the current widespread use of synthetic coarse fiber filter medias are so well documented to lose efficiency while in use that ASHRAE Standard 52.2 (which is the current standard of testing HVAC air filters in the United States) states on page 3 that the lab testing in the standard can produce a higher efficiency than achieved in actual use.

Ambient Air vs ASHRAE dust

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Written by Ben Klawitter