When It Comes To Your Air Filter, MERV's The Word

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Changing your air filter is one of the most important things you can do to keep your HVAC system in great shape. Most experts recommend changing the filter at least every three months. But as you change your HVAC system's air filter, you can't help but wonder how effective your air filter truly is at providing clean air through effective filtration.

How MERV Helps

This is where the Minimum Efficiency Reporting Value (MERV) rating system comes in. The MERV rating system offers a relatively simple and easy-to-follow way for homeowners and HVAC professionals to compare different types of air filters. It also takes much of the guesswork out of choosing the appropriate air filter for your home's HVAC system.

Choosing the Best Air Filter Using MERV

When it comes to HVAC air filters, different materials can have remarkably different performance results. When choosing your next air filter, it's important to keep the following in mind:  

  • Most HVAC systems rely on fiberglass air filters to block most forms of dust and debris. These air filters are usually rated on the low end of the MERV scale, usually between 1 and 4. While fiberglass air filters offer basic protection for your HVAC system, they're not as effective at removing harmful air pollutants as other types of air filters.
  • Cartridge-style and pleated air filters are a step up from fiberglass, offering better filtration against most common airborne pollutants than fiberglass. These filters usually rank 5 to 8 on the MERV scale. Higher-quality air filters designed to trap viruses and bacteria are often rated as high as MERV 16.
  • Washable air filters can be cleaned and reused for years at a time, but they usually offer the same performance as a comparable fiberglass air filter. As a result, most washable air filters are rated at MERV 1 to 4.
  • Air filters that rely on activated carbon and other sorbents to remove odors and gaseous pollutants are not rated under the MERV system.

Most HVAC manufacturers have their own specific recommendations in regards to air filters and MERV ratings. However, a relatively low MERV recommendation presents an opportunity for you to upgrade to an air filter with more effective filtration properties.

The Sweet Spot

According to the U.S. Environmental Protection Agency, an air filter rated between MERV 7 and 14 performs just as well as a HEPA filter under most normal circumstances. Nevertheless, many people assume that an air filter with the highest possible MERV rating offers the best possible performance. However, this isn't exactly as true as you'd think it would be.

While air filters rated at MERV 4 and below are not as effective as those rated at MERV 5 and above, an air filter rated at MERV 15 may not be as effective as one rated at MERV 8. That's because having increased filtration capability comes at the cost of decreased air flow.

In order to capture smaller particles, the filtration media has to become denser and smaller. This in turn reduces the amount of air that can freely flow through the filtration media. The higher the MERV rating, the smaller and denser the filtration media becomes. That's one reason why HEPA filters need modified blower fans in order to work effectively in household HVAC systems.

Choosing the most effective air filter for your HVAC system involves striking a balance between good air flow and effective filtration capability. This is why, in most cases, air filters rated at MERV 7 through 14 offer the "sweet spot" for most homes and their HVAC systems.

What About HEPA?

Air filters used for HEPA filtration are rated on a scale of 17 to 20, highlighting their specialized capabilities in comparison with ordinary air filters. These filters aren't normally used for household applications and when they are, they often require extensive modifications to the HVAC system's blower fan as well as its air filter enclosure.

HEPA filters are also commonly found in air purifiers, making them a better option for keeping allergens and bacteria down to a minimum, especially when used in conjunction with a high-quality HVAC air filter rated between MERV 7 and 14.