Air Filters: How These Flimsy Parts Make a Big Difference to HVAC Efficiency

Published on: October 7, 2014
Clean air filter | AC Southeast®

Lightweight and inexpensive, HVAC air filters may not seem like they would have much effect on your heating and cooling system. Because they’re one of the first components air hits as it enters the system, however, these seemingly flimsy parts actually have a lot of influence.

Here in the Southeast, where air conditioners get a good workout every year, choosing the right air filter and maintaining it is particularly important for keeping cooling bills down.

How Air Filters Influence HVAC Efficiency

The heating and cooling system was designed to work with a specific amount of airflow. A clean filter of the right efficiency lets air pass through easily so the fan motor can work at top efficiency.

When something about the filter causes it to interfere with airflow, however, it forces the motor to work harder to pull in enough air. Naturally, the motor uses more energy for this, reducing the system’s overall efficiency and raising energy bills.

To keep the system efficient, there are two issues you’ll need to consider where the filter is concerned.

  • Efficiency rating – HVAC air filters are given minimum efficiency reporting value (MERV) ratings to indicate their efficiency at trapping particles of different sizes. In general, the higher the MERV, the more efficient the filter.If you’re concerned about maintaining high indoor air quality, it may seem like a good idea to choose the most efficient filter, but it’s not that simple. The more efficient a filter, the denser it is and the more it interferes with airflow. That means you’ll need to find a happy medium between efficiency and good airflow. Understanding the MERV system will help.
  • Maintenance issues – Even if the filter has an optimal MERV rating for your needs, it can still interfere with system efficiency if you let it get dirty. Over time, a layer of dust and debris builds up on the filter. This makes the air filter denser and causes it to interfere with airflow, overworking the motor.The solution is as simple as inspecting the air filter every month and replacing it when you notice an accumulation of debris. Choosing a higher-efficiency filter also helps. These filters stay cleaner for longer than the cheap fiberglass types.

    Efficiency isn’t the only thing affected by problems with the air filter. When a filter impairs airflow, it places strain on the components, including the fan motor and evaporator coil. Using the wrong filter or leaving in a dirty filter can eventually cause the components to fail early.

For Higher Efficiency, Know MERVs

To give homeowners an easy way to find air filters in the efficiencies they want, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) created the MERV system. While a higher MERV rating indicates higher efficiency, there’s more to consider than just the number.

  • MERV 1 to 4 – These are the lowest-efficiency filters available and their MERV matters so little, it may not even be listed on the packaging. These filters are typically the 1-inch fiberglass filters found in HVAC systems when the homeowner hasn’t bothered to upgrade to a more efficient type.Larger particles of dust, pollen, lint and hair are the limit of what they can remove from the air. While this is enough to keep the heating and cooling system clean, it doesn’t improve air quality. The particles these filters trap are too big for you to breathe in anyway.
  • MERV 5 to 8 – In this efficiency range are the filters that can improve air quality as well as protect the system. They’re capable of trapping the finer contaminant particles that could enter your lungs and threaten your health.Because they’re not much more expensive than lower-efficiency models, they offer a good combination of affordability and efficiency. Consider choosing a filter at the higher end of this range. A MERV 8 filter is around 70 percent more efficient than a MERV 6 filter, but it doesn’t cost much more.
  • MERV 9 to 12 – If someone in your home has allergies or another respiratory concern, this is the efficiency range to choose from. Filters in this range are nearly as efficient as true HEPA filters and, at the higher end, some can even trap certain types of bacteria and viruses.
  • MERV 13 and above – Filters in this range may be highly efficient, but their high density means they interfere with airflow so much, they aren’t suitable for most residential systems. Installing such a filter could severely impair system efficiency or even burn out the fan motor. Before buying this type of filter, consult with an HVAC technician to make sure your system’s motor is powerful enough to handle it.

Which Type of Filter Is Right for You?

Within each MERV range, you’ll find certain types of filters. It’s easy to assume the type of filter you’ve been using in your system is the only one that will work. In reality, there are several types you could be using and your current choice may not be ideal.

  • Flat air filters – Most of these filters are made of a 1-inch layer of fiberglass framed in cardboard. Polyester models are also available and offer somewhat higher efficiency. They’re the cheapest filters you can buy, but they’re also the least efficient, usually with MERV ratings no higher than 5.While these filters are too thin to interfere with airflow when first installed, they’re still a threat to your system’s efficiency. Their limited particle-trapping capacity means they get dirty fast.

    If you have air quality problems and run the air conditioner or furnace daily, you’ll probably need to chance these filters monthly. Forget, and the filter will quickly get dirty enough to limit airflow, impair your system’s efficiency, and leave you with higher energy bills.

  • Washable air filters – Although typically made of foam, these filters are no more efficient than fiberglass models. The difference is that instead of throwing out a dirty filter, you simply wash this one, let it dry entirely and reinstall it.Because they get dirty just as quickly as fiberglass filters, you’ll need to wash them frequently. Even with all that washing, they’re prone to developing mildew and bacteria that can be blown into your home’s air.
  • Pleated air filters – The surface of a pleated filter is bent into a row of V shapes that give it an appearance similar to a pleated skirt. This formation allows a greater surface area to fit into the same space as a standard flat filter. More surface area means more particle-trapping fibers. What’s more, they’re made of polyester or another synthetic material.These improvements allow pleated air filters to achieve efficiencies between MERV 8 and 14. While they cost more to buy than flat filters, they give you measurably better air quality and last longer, too. There’s less risk of inadvertently reducing system efficiency because you forgot to change the filter.
  • High-efficiency particulate air (HEPA) – Chances are you’ve heard about these filters from ads for vacuum cleaners or air purifiers. HEPA filters are the most efficient available, able to trap at least 99.7 percent of particles 3 microns or larger. They come with MERVs of 17 and above.This means they trap many of the finest air contaminants, such as smoke particles and some bacteria, which pose a health threat because they’re small enough for you to breathe in. Be aware that “HEPA-type filters” aren’t true HEPA filters and are usually only 80 to 90 percent efficient.

If you could use a little guidance on choosing the right air filters and other air quality equipment for your home, contact us at AirConditioningSouthEast.com for help finding a knowledgeable, trustworthy contractor in your area.