how does a furnace filter work?
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How Air Filters Work

If you’ve been reading through our blog reel, then you’ve heard it a million times over: up-to-date, clean air filters are important for making sure the air you breathe in your home is clean and clear. Some argue that the cold of winter kills off a lot of the bad bacteria, but this simply is not true. In fact, because you rarely leave windows open during winter for ventilation, a good furnace filter is most important this time of year. But the question remains – “How do air and furnace filters work?

What Filters Are Made Of?

If you’ve ever looked at an air filter before putting it in or after you’ve taken it out, you’ll notice that it’s constructed of layers of How does a HVAC filter work?tightly interwoven fibrous materials. These materials can be made of fiberglass, polyester, paper, and, more recently, of advanced nonwoven materials. But beyond what they’re made out of, how do they work?

How HVAC Filters Work

There are four components of an HVAC (heating, ventilation, and air conditioning) filter to remove pollutants from the air: fiber size, tightness of the weave in the media, the number of layers, and an electrostatic charge.

Fiber Size

There are some filters that are just flat layers of fiberglass, which only remove the largest bits of dirt and dust from the air. More efficient filters are made of very thin strands of a synthetic fiber called polypropylene. The small size of the strand means there are more strands in each filter, and there is more surface area to capture impurities as they pass by.

Tightness of the Weave

The fine strands of polypropylene are woven into the filter media. The advantage of these fine strands is that they can be woven very tightly together, with very small holes between the fibers. These holes are nearly microscopic, and the smaller they are, the more they filter out. Generally speaking, a higher MERV rating has smaller strands of polypropylene and a tighter weave, resulting in a more effective filter.

Layers of Media

One layer of media is effective, but multiple layers create additional barriers to dust, dirt, and other pollutants as they pass through. The holes in each layer of media don’t line up perfectly, so the small contaminants in the air have to bounce around to get through the filter. Additional layers of media capture more pollutants than a single layer of media.

Electrostatic Charge

In addition to creating additional barriers to capture pollutants as they pass through, the filter media is electrostatically charged, which attracts the particulates in the air to the media, capturing particulates that may be small enough to fit through the holes in the media. As the filter begins to capture the pollutants in the air, the filter becomes more effective.

What Is MERV?

Simply put, MERV is the minimum efficiency reporting value of an HVAC filter media. It’s a scale designed by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. All HVAC filters are rated on the MERV scale, which ranges from 1 to 20, based on worst-case performance when capturing and detaining particles. Like most scales, the higher the rating, the better the filter. It is set based around the size of particles that it effectively captures, between .03 and 10 micrometers.

What Do the Ratings Mean, Exactly?

A MERV score communicates a filter’s ability to trap pollutants and particulates as they pass through the filter. In most cases, residential air filters will range from 1 to 12, meaning they capture particles between 10 and 1 micrometers. Filters that score higher on the MERV scale are typically found in hospitals and pharmaceutical manufacturing facilities.

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