What is a Furnace Filter?

How much do you know about the filter in your furnace? Do you know enough to even ask what is a furnace filter? In addition to heating and cooling your home, HVAC systems also clean the air in your home — which they can only do with the help of an air filter. This article provides a general overview of the different types of furnace air filters and how to maintain them.

What Does Your Furnace Air Filter Do?

The filter is a simple but important component of the furnace that few people think about. However, it serves two important purposes:

  • It protects your furnace: The air filter protects the blower fan of your furnace from debris, dust, hair and various other pollutants that come in through the return duct.
  • It protects your lungs: The furnace filter also helps improve the indoor air quality (IAQ) in your home.

Your furnace cycles air through your home, ducts and furnace. Cold air travels through the return ducts in the system, where the heat exchanger heats it. The newly heated air then goes through the ducts and is delivered into the various rooms in your home.

This warmer air, which is now in the rooms of your home, pushes the existing cooler air back in through the return ducts, where the process starts over. As this air travels around your house, it picks up dirt, debris, dust and various other pollutants along the way. However, your furnace filter blocks these pollutants, preventing them from circulating around your home.

What Is a MERV Rating?

The Minimum Efficiency Reporting Values (MERV) rating describes a filter's ability to block and capture bigger particles ranging from 0.3 microns (µm) to 10.0 µm. Filters with higher MERV ratings are better at capturing certain particle types. If you want to compare the performance of different furnace air filters, this rating is a very helpful metric.

Here are what specific MERV ratings mean.

  • 1-4: A filter with a 1-4 MERV rating traps less than 20% of particles between 3.0 µm and 10.0 µm in size.
  • 6: An air filter with a 6 MERV rating traps 49.9% of particles ranging between 3.0 µm and 10.0 µm.
  • 8: Air filters with 8 MERV ratings trap 84.9% of particles between 3.0 µm and 10.0 µm.10: A filter with a 10 MERV rating will filter out between 50 and 64.9% of particles measuring 1.0 µm to 3.0 µm and at least 85% of particles measuring 3.0 µm to 10.0 µm.
  • 12: Filters with a 12 rating will filter between 80% - 89.9% of particles between 1.0 µm and 3.0 µm and at least 90% of particles measuring between 3.0 µm and 10.0 µm.
  • 14: An air filter with a 14 MERV rating filters out 75% - 84% of particles ranging from 0.3 µm to 1.0 µm and at least 90% of particles between 1.0 µm and 3.0 µm.
  • 16: A rating of 16 means the filter captures 75% of particles between 0.3 µm and 1.0 µm.

High-efficiency particulate absorbing (HEPA) air filters are pleated mechanical air filters that can theoretically take out 99.7% or more of pollen, dust, bacteria, mold and airborne particles measuring 0.3 µm. Particles of 0.3 µm are the most penetrating and hardest to trap — smaller or larger particles are trapped even more effectively.

Keep in mind that air cleaners such as furnaces need cleaning and new filters periodically to work effectively. Refer to the manufacturer's recommendations when it comes to filter replacement and furnace maintenance.

Types of Furnace Filters

You have several options to choose from when it comes to furnace filters. Some of the most common options include the following:

  • Disposable fiberglass: The most popular and affordable filter type, disposable fiberglass filters cannot do much more than block large particles of debris, lint and dust. They are not suitable for cleaning the air itself. If you or someone in your household suffers from allergies or asthma, disposable fiberglass filters are not recommended.
  • Disposable pleated: These filters are another popular option and consist of polyester and cotton paper. Reasonably priced and made from eco-friendly materials, these filters can remove certain tiny particles like spores and mites. However, you should change them frequently to prevent your HVAC system from clogging.
  • Disposable electrostatic: Featuring self-charging electrostatic paper or cotton fibers, these filters can capture tiny, potentially harmful particles. Standard versions are reasonably priced, but if you need a custom size, expect to pay more. These filters are an option to consider if you're a smoker or have pets.
  • Permanent electrostatic: Permanent electrostatic filters also feature self-charging electrostatic fibers but can be taken out, washed and reused for many years. Permanent filters are more expensive than disposable filters, but they can be more cost-effective in the long term because you don't need to buy new filters as often.
  • High-efficiency pleated: These filters feature pleated synthetic cotton and are connected to a solid metal grid that helps prevent leaks. These exceptional filters can filter out the tiniest particles and are highly recommended for people with breathing issues or autoimmune conditions. High-efficiency filters are often used in hospitals and other healthcare settings.

Is a Furnace Filter the Same as an Air Conditioner Filter?

If your house has both air conditioning and central heating, the furnace filter and the air conditioner filter are the same. Even if your home has separate heating and cooling units, they probably still share the same air handler, which means they also share the same filter. Air travels through the air handler and the filter whether you're cooling or heating your home.

Many more recent homes and those in temperate regions that only need moderate heating and cooling use heat pumps, which function as both AC and heating. WIth higher efficiency than traditional separated furnace and AC systems, heat pumps use the same air handler and same compressor. If your home has one of these heat pumps, you only have one filter for both your AC and heating.

When the seasons change, it's important to clean or replace your air filter to ensure your system will be ready for the hot or cold weather. Air must be able to flow through the system freely. Any impedance or constriction can strain your furnace, AC unit or heat pump, resulting in less efficient operation and possible damage to the system.

Can I Run a Furnace Without a Filter?

Technically, you can run a furnace without a filter, but we strongly discourage it. If you don't have a filter and decide to run your furnace anyway, make sure you are aware of the potential consequences, which include:

1. Damage to Your System

One of the main functions of an air filter is to remove airborne particles such as dust from accumulating on the interior components of your furnace. Due to the continuous stream of air through your furnace, airborne particles can quickly accumulate if you don't have a filter, and this accumulation can lead to furnace malfunction. Restoring the furnace to its former state could require a thorough cleaning or perhaps even expensive repairs. This buildup could also cause fires if it accumulates close to electrical parts or hot components, so you'll want to prevent this unwanted scenario by ensuring you always have a clean filter in place.

2. Damage to Your Health

If no filter is in place, whatever is in the air will continue to travel around your house for all to breathe in. This may cause respiratory issues like allergies and asthma, especially for very old or very young household members.

3. Dirtier Home

More airborne particles floating around your house will result in more dust on your floors and furniture, which means you'll have to vacuum and clean more often.

How Often Should I Change My Furnace Filter?

Changing your air filter regularly is key to keeping your HVAC system in top condition. But how often do you need to change it?

Most experts recommend changing your home's air filter every 30 days if you're using an inexpensive fiberglass filter. Some high-end pleated filters can go uncleaned for up to 6 months. However, these guidelines assume normal use and don't consider filter type and size.

A good general rule of thumb is to replace furnace filters and pleated air filters every 90 days. The longer the filter goes uncleaned or unreplaced, the more dust, dirt and allergens will get trapped. This can clog your filter and decreasing its efficiency. You will likely want to replace your filter more often if any of the following situations apply to you:

  • You or members of your household have asthma or allergies: The Allergy and Asthma Foundation of America (AFFA) states that indoor and outdoor air quality are equally important. If someone in your home suffers from allergies or asthma, they have a much greater sensitivity to airborne particulate than those without these conditions. In this case, you should change the air filters in your home every six weeks.
  • You have pets: If you have cats or dogs in your home, you know odors can build up over time. Shedding can be particularly bad when the seasons change. When you have pets, you should replace your air filter every two months. Consider changing it more often during the transitions to spring and fall.
  • You have younger children: If you have younger kids in your house, you will want to ensure your air quality is the best it can be. Change your filter at least every two to three months.

 Factors That Can Shorten the Lifespan of Your Filter

Several factors can shorten the usable life of your HVAC system filter. That includes when you:

  • Use it constantly: If you're in a moderate climate and use your furnace or AC for only a few hours a day, your filter could last an entire season, perhaps even a whole year. Conversely, if you're running your HVAC constantly, you will need to change the filter every few weeks, especially if you use fiberglass filters.
  • Have a larger home: If your home is larger, your HVAC unit will have to work harder for the same drop in temperature change. This could require more frequent filter replacements.
  • Have poor air quality: If the air quality outside your home is poor or you have pets inside, you will need to change your filter more frequently for cleaner air.

Changing a Furnace Filter

Many homeowners are unaware that their HVAC system has a filter. When they learn they should replace the filter regularly, their first question is, "What is a furnace filter?" Their next question is often, "How do I change it?" Follow these steps to complete this easy but important task:

  1. Turn off the furnace: As noted above, you don't want to run your furnace without a filter. Remember to turn it off before you do anything else!
  2. Locate your filter: Most air filters are housed either inside the furnace or inside the air return vent.
  3. Remove your filter: Before you remove the filter, note the arrow on the filter, which tells you which direction the air flows in. It's a good idea to mark the direction of the airflow on the edge of the filter housing with a permanent pen or marker. This will make it easier to remember the correct direction to install filters in the future. Once you've done this, you can take out your existing filter.
  4. Make note of the size: When you remove your existing filter, check the filter frame for the size. If there is no existing filter, measure the height, width and depth of the slot. Then you'll be able to purchase the right furnace filter size.
  5. Purchase a quality filter: Buy a high-quality filter that is sure to remove pollen, dust and smoke. That ensures you and your family can breathe the cleanest air possible when using your furnace.
  6. Install your new filter: If your new filter is wrapped in plastic, make sure to remove the wrapper before the installation. Look for the marks that indicate the direction of the airflow, then just slide the new filter into place. Replace or close any cover that was over it previously. To ensure you'll know when you have to replace the filter again, we recommend that you note the date you changed it. Write it right on your filter and in your records.
  7. Turn the furnace back on: Once you've replaced the filter, you can turn the furnace back on to heat your home.

Shop Furnaces and HVAC Air Filters From Ingram's Water & Air

Ingram's Water & Air has over 30 years of experience selling home comfort, purification and filtration equipment. Browse our wide selection of HVAC air filters here.

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