Image of an air filter in a home

7 Little-Known Facts About HVAC Air Filters

One of the most common HVAC problems is poor maintenance or failure to change HVAC air filters. A dirty air filter means dust, dirt, and other particles build up in your central AC system. Not good! Think you know everything about your HVAC filter? Read on for seven things you may not know about HVAC air filters.

1. The Purpose of HVAC Air Filters is Not About Air Quality

Contrary to popular belief, the purpose of HVAC air filters isn’t to improve air quality in your home. They protect the system’s HVAC equipment from harmful particles and debris. Many filters are marketed to homeowners as being keys to eliminating allergens and improving air quality. While they do that, those features don’t go hand-in-hand with protecting the AC unit.

2. The Higher the MERV Rating, the Lower the Efficiency

What is the MERV rating? The Minimum Efficiency Reporting Value (MERV) measures how effectively an air filter blocks particles and other debris from entering an HVAC system. The higher the MERV rating, the smaller particles it can filter. Thus, the higher the rating, the better the filter, right? Most of the time.

MERV ratings range between 1 and 20. MERV 1 filters are only able to remove the largest dust particles. Whereas a MERV 20, the highest rating available, can filter down to the smallest particle. You’ll usually only find these in medical settings where the air needs to be extremely clean. Facilities like hospitals and other medical facilities have high-power, commercial-grade heating and air conditioning systems that can accommodate those high-efficiency filters.

Most residential central AC systems can’t handle a MERV 20 filter. The system would need to work too hard to push the air through the filter. Typically, a residential system can take as high as MERV 13 without straining too much. Anything higher than that begins to cause problems.

Highest Rated Filters Can Restrict Airflow

Not all air conditioning filters are made for all air conditioners. Most residential systems are not designed for the highest-rated air conditioner filters. Your HVAC system may not have the motor or fan capacity to accommodate higher efficiency filters. In fact, some home AC systems are designed for filters with a MERV rating no higher than 4. The result is restricted airflow throughout your home.

High-Efficiency Filters Can Damage Your Unit

As stated above, high-efficiency air conditioning filters can slow airflow in furnaces and air conditioners alike and reduce your unit’s efficiency. In turn, this raises your electric bill and can cause damage to your unit over time. Your furnace may even overheat! Likewise, the filter can cause your air conditioner’s condensing coils to freeze. Homeowners have long been told that dirty HVAC air filters can cause this. They certainly can, but so can the wrong filter. It’s wise to understand your air conditioner’s requirements when choosing your air conditioner filter.

3. Filters Do Less for Air Quality Than You Think

HVAC air filters do not reduce the number of small particles in the air that you think they do. Particles tend to be present wherever there is human activity, which is not always where your ductwork and registers are to capture them. Most of your home’s air is filtered through a single point in your HVAC system.

In addition, a furnace or central AC unit only runs seasonally. The unit can’t filter air when it is not running. And, an efficient unit won’t run twenty-four hours per day. You’d have to run the fan all year long on a continuous setting to keep your air filtering. 

4. Unadvertised Drawbacks to Washable Air Filters

Many homeowners invest in washable air filters with the intent of saving money over time since they can last up to five years. You would go through 60 or more disposable filters in that amount of time. Rather than replacing the filter every month, simply remove and wash the same filter. Let it dry, and then reinsert.

While a washable filter is more expensive, you are only buying one, which means they are more economical in the long run. But there are definitely drawbacks.

Washable Filters Have a Low MERV Rating

A washable air filter usually rates only a 1 to 4 on the MERV scale. That isn’t effective if you intend to purify the air or reduce allergens. Suppose you’re looking to filter out smaller particles, like smoke, aerosol sprays, and pet dander. In that case, the washable air filter may not be as effective as a disposable filter with a higher MERV rating.

Damp Filters Can Attract Mold

A washable air filter can attract mold. After you wash the filter, it takes a long time to dry. If you reinstall one while it’s still damp, you’re inviting mold. Mold and mildew require a warm, moist environment to survive. So, mold spores are right at home in a damp filter and can quickly spread through your HVAC system. The moving air will carry mold spores throughout your ductwork and into your home. Ironically, the air filter designed to remove pollutants from your home becomes the source of them.

5. Electrostatic Filters Aren’t Equivalent to HEPA Filters

You may have heard a lot of talk about the efficiency of reusable, electrostatic filters. There’s a lot that happens as air moves through these complex filters compared to simpler filters. Here is what you may not know.

How Electrostatic Filters Work

As your HVAC system draws air into the return vents and ductwork, the air passes through the air filter. The filter removes particles and dust from the air before the HVAC system conditions the air.

Electrostatic air filters have layers and layers of filtration that clean the air as it passes through the filter. As particles pass through the filter, the filter material positively charges them. Then the particles attach to the next set of layers of the electrostatic filter. The particles begin to bind together. They become too big to pass through the filter and into the AC system.

How Electrostatic Filters Stack Up Against HEPA Filters

The Environmental Protection Agency (EPA) and Occupational Safety and Health Administration (OSHA) state that HEPA filters are best for removing indoor air pollutants.

HEPA filters filter air at a minute level. They block elements as small as 0.3 microns or larger and catch 99.97 percent of all particles. If you have a health need for a high-quality filter, a HEPA filter is still your best bet.

6. What Happens When You Don’t Change Your Filter

If you don’t change your air conditioner filter often enough, you will spend more money on your utility bill without seeing any benefit. A dirty filter means slow-moving air, which increases the run time of your central AC system, and more run-time usually means cooler air. The temperature will not vary as it should if the air filter is clogging the system.

You Risk Clogging Your System with Debris

When your filters are clogged, particles escape through the filter and get into the mechanical and electrical components of the AC system. The longer you go without changing the AC filter, the greater the buildup. This increases the potential for damage to your unit. The faster you can heat or cool your home, the less you have to run the HVAC system. Thus, the longer your unit will last.

Your Home Heats and Cools Unevenly

If your system does not have adequate airflow, your system cannot effectively move the air to the furthest rooms of your house. As the air filter clogs, less and less air moves through. As a result, the rooms further away are too warm in the summer and too cold in the winter and can even become stagnant. The rooms closest to the unit will be more comfortable because they are receiving the most airflow.

7. Save by Ordering in Bulk

Most of us buy our filters from big-box home improvement stores where filters are packaged individually or in small quantities. This and the marketing and labeling that comes with them drive up the price.

Try buying your filters in bulk and having them shipped to you. Doing so cuts out some of these retail and advertising costs. Shop Discount Filters for the best prices on American-made air filters.

Review Your HVAC Air Filters

Now that you’ve made it this far, it’s time to take a look at your HVAC air filters. Do they have the proper MERV rating for your system? Do you feel you are getting adequate filtration? Is the temperature the same in every room?

If the answer to any of these questions is no, you may have the wrong air filter in your HVAC system. If you’re looking for the right filter, shop our selection of AIRx filters for the perfect one for you and your home!

12 thoughts on “7 Little-Known Facts About HVAC Air Filters”

  1. I have a Trane electronic filter for my AC unit . I would like to improve the dust control efficiency of it by using a pre-filter (one of your merv 11) at the entry of the single return to the AC. Does this sound like this will work for me or a recommendation ?

  2. My first purchase of 12 25x25x1 filters arrived today…. excellent quality…. perfect fit….
    Very happy customer

  3. Hello I have a new American standard AC and it came with a Glasfloss filter and the size is 16x20x1 inches but my AC filter slot across is just 19.5 inches and I can only find the Glasfloss ZL filters with 19.5 inches across. All other 16x20x1 filers in stores are 19.75 inches across and are 2.5mm too big. So have to use the same brand and series filter the new AC came with. So it must be correct.

    • Hi Susan! If your filters are light gray and don’t seem to have a lot of build-up, you can leave them for up to 90 days. We recommend changing your filter at least every 90 days to maintain the efficiency of your HVAC system. Hope this helps!

  4. I have a high eff. furnace and use Merve 8 filters that are 4″ thick. I am getting a new air cond. unit and the tech told me to buy a 1″ cheap filter (less dense) and put that in for the summer. Of course it will sit on an angle… I imagine I could somehow have something to hold it in place. The tech. who did the furnace recommended the same thing but using 3 of these and then changing out the lst one each month or two. Do you agree? What is my best option?

    • Hi Sylvia! We do not recommend doing this. 1″ filters are not designed to be stacked, and three stacked together could severely inhibit airflow. It may sound counterintuitive, but three 1″ filters do not equal a 3″ filter. 1″ filters are designed for 1″ slots with a specific amount of pleats and surface area. 4″ filters have more and deeper pleats than 1″ filters, meaning there’s more media and surface area, but air can still flow through the filter. If your furnace is high efficiency, it can easily handle 4″ filters! We’d recommend checking out our high-flow Air Beast filters made specifically for high-efficiency systems. You can check them out here:

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