We at Crucial talk a lot about how important it is to change your filters on a timely and regular basis. Whether it’s your household furnace or central air, your vacuum cleaner, or your humidifier or air purifier, taking care of your filters is important. This is especially true if you suffer from allergies or any other sort of breathing malady. To understand why it’s so important to change your filters, you need to understand how filters work. It’s also important to understand how filters are rated; after all, not all filters are created equally.
In general, filters in your vacuum cleaner and other household fall into the realm of mechanical filters. This means that the filter physically traps particles in the filter materials. This trapping makes them very good at removing larger allergens from the air. This includes things like animal dander, dust, mold, and pollen. As long as the air is blowing them around, mechanical filters can remove them from the air. Better filters remove smaller particles from the air, with the best filters able to remove particles that are as small as 0.3 picometers in diameter.
Most of us know about HEPA filters, whether from ads or from the little sticker on your vacuum cleaner. HEPA is an acronym for High Efficiency Particulate Arrestance. The HEPA standard is regulated by the United States Department of Energy. To qualify for a HEPA rating, the filter has to remove 99.7% or more of particles that are 0.3 microns in diameter. This is because 0.3 microns is where a HEPA filter is most inefficient, so if it’s least efficient there, it will be much better at trapping larger and some smaller particles.
A HEPA filter relies on three mechanisms to trap particles to filter fibers: interception, impaction, and diffusion. Interception is the mechanism of direct contact between the filter and the particle. This is where the filter does act like a sieve or colander. Impaction relies on the weave and thickness of the filter. Air is forced to follow a meandering path through the filter because of this weave. As the particles flow through the filter, they impact the fibers and are trapped. The last mechanism is diffusion. Diffusion is a more technical way for particles to be trapped. Particles that are smaller than 0.1 microns are slowed and delayed by impact with gas molecules. This slows these tiny particles until they can be trapped by interception or impaction.
To put it simply, a HEPA filter doesn’t just remove pollutants by acting like a single sieve and trapping particles in a tight mesh. That’s part of it, but the overall weave of the filter is made up of multiple layers with each layer oriented in a different direction. Think of it as hundreds of sieves, all stacked on top of one another. If you dump dirty water through this stack of sieves, the water won’t just pour right through, it’s going to slowly trickle through and be cleaned as it passes through. As it trickles, more stuff gets trapped. But it’s this trickling that highlights one of a HEPA filter’s biggest drawbacks. HEPA filters can drastically slow down the air passing through them. So vacuum cleaner motors have to be more powerful to compensate. It also makes a true HEPA filter unsuitable for use in a forced air central heat or air conditioning system. So filters for these systems have to be designed differently and thus, use a different rating.
MERV’ing on up
The filter for your central air conditioner or furnace system must be much less dense to avoid needing a motor the size of your garage to push air through it. Luckily, with a proper vent and draw setup, the air in your home will circulate through your furnace filter several times per day. So whatever debris and particles aren’t caught the first time can be caught on another pass through until the air is satisfactorily cleaned. This leads to filters having a MERV rating, or Minimum Efficiency Reported Value.
The higher a filter’s MERV rating, the better it is at eventually removing particles from the air. These filters are much less dense and range from a rating of 1 to 20, with higher numbers indicating a better filter. As an example, the permanent filter from a window air conditioning unit has a MERV rating between 1 and 4. This filter isn’t going to remove any small particles; its main purpose is to keep large particles from gumming up the interior moving parts. For your home heating and cooling system, the large box filters will have a MERV rating between 5 and 12 and are able to remove almost every particle that would give an allergy sufferer pause. Ratings above 12 are for filtration systems used in hospitals and industrial clean rooms, where delicate electronics are made and are rarely used in household air systems.
Essentially, where a HEPA filter has to do all of its filtration and particle removal in one pass, your central air filters get to do it in multiple passes. This allows the filter to be less dense, which prevents a loss of air pressure.
Wrapping It Up
So why change your filters? Well, essentially, your filters eventually get full. As they filter more and more crud from your air, the paths through the filter get clogged. For a HEPA filter, air can no longer get through its normal path, so it starts forcing its way, dislodging previously trapped bits. This removes two of the three removal methods, drastically reducing the effectiveness of your filter. For furnace filters, the filters will get coated with dirt, hair and other large bits, which again, reduces their effectiveness.
How long it takes to get to this point varies, depending on the construction. For HEPA-filters found in air purifiers and vacuums, a good rule of thumb is to replace your filter every four to six months. If you live in a dusty environment or one with lots of airborne particles (multiple pets, for example), you need to swap your filter out sooner.
For your furnace, the time between changing your filter depends on the thickness. A 1-inch thick filter will usually last one to three months, while 3-inch filters will last three to six months. Again, the longevity of your filter is going to depend on how much work you put them through.
So now you have a basic understanding of how filters work and why you need to change them, not just dust them off and put the originals back in. We at Crucial specialize in helping you find the right filter for your needs. Whether you’re looking for enough furnace filters to keep you going for the year or a small filter for your hand-held Dustbuster, we’re here to help with any questions you might have.