Do dust mites plague your home?

If you read my article about the “Ways to Get Rid of House Dust Mites,” you’re aware of how important choosing the proper ac filter is. In the above article, my husband, Gilles, did a lot of research on just how we should filter our mold-remediated home which had become overrun with dust and therefore dust mites as a direct result of the mold. Recall, we reviewed stand-alone units in the previous article. Based on Gilles’s research, this article reveals how to filter out the dust and dust mites from your HVAC system to keep your home cleaner.

Note: If you get the proper filter for your HVAC system you will not have to work as hard to follow the dust mite termination routine described in the previous article.

Gilles is a mechanical engineer. I say that with an appreciation for his attention to detail, especially in a situation where I’ve been “not up to snuff” due to leaky gut from the dust mites and more. Based on his newfound expertise, he not only cleaned our house meticulously but supplied it with the best ac filter for allergies including dust mites.

As per my request, Gilles wrote over 2,000 words (including a few remarks of my own) which you’ll read below on picking the top ac filter for dust mites. As a reminder, he took into consideration the size of the dust mites when doing his research. Recall info from the previous article; the average dust mite measures between 200-300 microns in length. That’s the only measurement you need to know when picking the pore size of your filter.

The Basics of Installing a filter in your HVAC system

Let’s first discuss how to install a filter into your HVAC system. Since your HVAC system (air conditioning and heating) circulates the air in your house when it is running, it also circulates dust in the process. Therefore, to minimize exposure to dust mites in your home, it is important to install a filter in your system. It’s also essential to then replace that main filter at regular intervals.

Where should I install the filter?

There are two locations where you can install the filter: at the return grills (in the ceiling or walls depending on your system) or at the air handler (just before the air enters the unit).

best ac filter for allergiesIf you’re like me, then you’re already confused. Take a quick look at your HVAC system. According to Gilles, here’s how it works. Air is drawn in through the “return” grills in the ceiling or walls. It is then routed through the HVAC cabinet (called the “air handler”) or furnace where it is cooled/heated. Next, the air is pumped back into the living space through the small grills called “registers.” All this air circulation is created by a powerful fan (blower) located into the cabinet or furnace.

The terminology is a little counter-intuitive, but you have to look at it from the standpoint of the system. The air “returns” to the system and is “supplied” to the living space (even though it is “returning” to the living space). I know, big yawn—but this is important to know!

Filters are always installed on the “return” duct-work side, so dust is captured before it reaches the HVAC cabinet to keep the internal system cleaner. Should you install filters at the return grills or the HVAC cabinet? Each option has pros and cons, but unless your HVAC cabinet is in a location that makes it difficult to access, the recommendation of our expert installer is to locate the filter at the HVAC cabinet, not at the grills. 

HVAC Cabinet Filter 

Recall the powerful blower in the unit? It works like a big vacuum cleaner. The duct-work before the cabinet (return) is under negative pressure (a vacuum). In our imperfect world (particularly in older constructions), duct-work is rarely perfectly air tight and often suffers from a certain amount of leakage. Since there is a vacuum in the return duct, a “leak” means that air is being sucked in, not out. If the filter is installed at the grill, that dust is getting right past the filter. That’s not a good thing.

Note in most houses, duct-work is routed throughout the attic; not a particularly “dust free” area. In most attics, a fair amount of loose fibers from fiberglass insulation are still floating around. For this reason, technicians wear dust masks and respirators when working in attics. Finally, to make matters worse, since filters increase resistance to airflow, a filter at the grill increases the vacuum in the duct which will draw in more dust through the leaks. To understand this concept, just try covering the nozzle of your vacuum cleaner with your hand while leaving some room open between your fingers.

How often should I change the filter?

Changing your filter regularly is very important. As your air filter gets dirty, it becomes more restrictive. If you don’t change the filter, this is what happens:

  • Reduces the life of your system since the blower has to work harder to overcome the extra resistance.
  • Increases your energy bill for the same reason.
  • Reduces the air flow through the system which will affect performance (less cooling or heating).
  • Can lead to icing over the coils (air conditioning); The system then shuts down and the ice melts. Very often the drain overflows leading to a watery mess. This can lead to mold.
  • In the case of a furnace, the reduced airflow causes heat to accumulate in the unit which can cause damage. There are many cases of overheating furnaces developing cracks leading to a dangerously high concentration of carbon monoxide levels in the house.

top ac filter for dust mitesSo, how often should the filter be replaced? The short answer is: when it is dirty. I told Gilles this was not a good answer. He responded that it all depends on the type of filter, the size, how dusty your environment is, etc. The only way to find out is to inspect regularly. If you have no filters (like us—totally unversed in anything HVAC) and decide to install one, you should inspect frequently (once a month at least during the first year).

Once you’re familiar with the system, keep a schedule and follow it. Many forget to replace the filter until it presents a problem. It is much worse to have filters excessively clogged with dust and dust mites rather than no filter at all. That said, not using any filter (or the wrong one) is bad for your equipment and for your health. When in doubt, consult a trusted HVAC specialist.

Picking the best AC filter for Allergies

There are several categories of filters available and each one contains several variations of construction for different purposes. They use different materials as filtration media. Some have carbon (activated charcoal) added to the media to also remove odors and VOC’s, while others only use carbon (but these don’t remove small particles meaning dust mites). Your selection should be based on your particular situation and what you are aiming to achieve.

Filters also use a filtration efficiency rating system. Most manufacturers use the MERV rating, but unfortunately, one of the biggest selling filter products, 3M’s Filtrete™, do NOT use MERV, preferring its “Microparticle Performance Rating” (MPR) instead. Fortunately, with a little internet research, you can find cross-reference tables.

What is a MERV number?

MERV is an acronym for “Minimum Efficiency Reporting Value.” It is a Standard which was developed by the American Society of Heating Refrigerating and Air Conditioning Engineers (ASHRAE) in the late 1980s to determine the overall efficiency of media air filters.

It is a number from 1 to 16 and is used to rate the ability of a filter to remove particles from the air as it passes through the filter. You can easily find tables on the internet showing the different percentage and size of particles captured based on the MERV rating system. Suffice it to say that higher MERV ratings mean fewer and smaller dust particles/contaminants will pass through the filter. However, keep in mind a higher MERV number also means higher resistance to airflow. In other words, your HVAC system must be designed to account for this. So, before choosing a filter, ask your HVAC specialist what is the highest MERV number that will not damage your specific system.

Are HEPA filters the top ac filter for dust mites?

HEPA stands for “High Efficiency Particulate Arrestance.” These units filter the air passing through them at a very fine scale. The acronym HEPA describes a category of filters but does not guarantee a specific level of efficiency. Unfortunately, this term is now often used too liberally. The U.S. Department of Energy (DOE) created the standard STD-3020-97, which requires the filter to capture 99.97 percent of all particles 0.3 microns or larger (considered by many as the standard a filter should meet to be a “true HEPA”). In short, let’s just say most experts consider MERV13 or higher to fit into the general HEPA definition.

Exactly what type of filter should I get?

For residential systems, consider the following 3 categories of filters:

  1. Inexpensive disposable 1″ thick spun fiber panel filters. These are the most common type and least expensive (the type you find at the grocery store). This type of filter offers little air resistance, but will only stop large particles (larger than 10µm). These will not help for allergies. MERV 1-4.
  2.  Disposable high-efficiency pleated filters. These filters have higher resistance to airflow but capture smaller dust particles. They are very common and, because they are pleated (the media is folded in accordion-like shape), they are available in thickness varying from 1″ to 6″. This shape increases the surface area which results in longer filter life since the collected dust is spread over a larger surface. As a general rule, the thicker the filter and the larger the number of pleats, the longer the filter will last and the lower the resistance to airflow will be. Finally, they are available in a wide range of MERV numbers.
  3. Electrostatic filters. Some are disposable, some are washable, and some are even “permanent.” They use self-charging electrostatic media that captures small particles by making them “stick” to the media (much the same way you can make a balloon stick to a wall after rubbing it). They require maintenance (removing, washing, and drying). There seems to be a lot of conflicting opinions about their efficiency (probably because there are so many different designs), with some sources even claiming their MERV rating is no higher than 4.
Gilles’s top AC filter for dust mites and other allergens:

Here is what Gilles said after learning how to clean up the “mess” made by others by researching everything on this topic and consulting with our expert HVAC team.

“There is a lot of talk and many claims out there about this topic. But this is my personal opinion. I recommend the use of one of those disposable high-efficiency pleated filters with a MERV rating between 11 and 13 if your system can take it. Don’t go any lower than MERV 8 (unless you really have no choice). It should be installed in the return duct as close as possible to the air handler (HVAC cabinet or furnace). It should be thicker than 1″, preferably 4″- 5″ to extend the time between replacements and minimize the resistance to airflow. I tend to stay away from electrostatic filters because of the inconvenience of the maintenance and truthfully, the claims about their performance are all over the place. Also, I only buy quality filters from a reputable brand.”

If your environment contains VOC’s, those filters are also available with activated charcoal impregnated in the media. Activated charcoal add-on filters are also available. Again, make sure that this is compatible with your specific HVAC system first.

What if I can only fit a 1″ filter in my system?

Ask your trusted HVAC specialist if a different “filter box” can be installed in your system. These units can be installed in the duct next to the air handler/furnace without having to remove the cabinet. They usually have a removable front panel that allows you to take the old filter out and slide in a new one. This minimizes the extent of the modifications. You might be surprised how affordable this option can be.

Important notes about HVAC filters
  • Never stack filters that are not designed for the specific system. If your filter box is designed for a 2″ filter, don’t stack two 1″ filters; buy a 2″ filter. The only exception is an activated charcoal filter as long as your system is designed for it. As always, consult your HVAC expert first.
  • When replacing a filter, make sure to install it with the proper orientation. One side is usually reinforced (typically with some kind of wire mesh) so it does not collapse. Look for an arrow on the side frame portion of the filter. This will show you the direction of the airflow.

Final thoughts

When purchasing or building a home, don’t forget to look at the HVAC. It’s not just about allergies; it’s about toxins, mold growth, and even bacterial growth. We know dust mites can cause leaky gut via non-allergic mechanisms. Unfortunately, we don’t know what else they can do. Why take a chance with your health when it’s easy to have an inspection or hire a good HVAC team? We paid the price for being ignorant. Use these articles as tools to become more aware. Your environment is a key player in your health. Keep yours’ non-toxic.


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