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Furnace Filter Guide: Everything You Need to Know


February 21, 2022

Choosing the right furnace filter for your heating system is essential to keeping your home comfortable and clean. Furnace filters are responsible for trapping the airborne dust, allergens, dirt, and bacteria that travel freely in your home and ensure your furnace is in working order.

Replacing furnace filter

It is not enough to buy the right air filters for your system but you also have to maintain them and install them properly. Our furnace filter guide shows you what you need to know to keep your system running smoothly all season long.

What Is a Furnace Filter?

A furnace is a unit in your home that is an integral part of your heating or HVAC system (sometimes referred to as a central air or forced air system). It is used to heat, cool, or circulate air inside your home and distribute it through the ductwork. Whether you know it or not, a furnace filter is hard at work protecting your furnace from debris and getting dirty while filtering particles from your home’s air.

If you want to keep your furnace’s efficiency high, you need to replace or clean your filter regularly. Now, with so many types of furnace filters available, it can be hard to know which one to use. Finding a compatible one with your system can be the difference between a high-efficiency system and a system malfunction.

How Does a Furnace Work?

Dirty air filter
A standard forced-air furnace draws air in through its return ducts. Then, your appliance heats the air over a heat exchanger. A blower fan pushes the warm air through the ductwork that goes into the rest of your home. The furnace runs until the temperature inside your home reaches your desired thermostat setting.

In homes with whole-home air conditioning, the cooling system’s process works similarly to the heating process. The drawn air is cooled by an outdoor compressor unit and cooling coils inside the furnace.

What Does a Furnace Filter Do?

Contrary to popular belief, a furnace filter is not primarily designed to improve the indoor air quality of your home. Its primary purpose is to ensure the continuous and efficient function of your system.

A furnace filter helps keep the blower fan from accumulating debris such as dust and other airborne particles that are pulled in by the return duct. While the primary purpose is to protect the system, filter technology can improve the air quality of your home as a secondary benefit.

What to Look For In a Furnace Filter

Changing intake filter

If you are in need of a new furnace filter, here are a few important factors to consider before making the purchase.

Filter Size

Filter sizes can be as small as 10” x 10” and as large as 30” x 30”. Generally, the most common filter sizes are 14” x 25”, 16” x 20”, 16” x 25”, 20” x 25”, and 25” x 25” but there are other options too.

If you are not sure about your filter size, check your furnace filter cabinet on the side of the door or the frame of the old filter for a filter size. Finding the right size is crucial to your system’s efficiency. You do not want a filter that is too loose and unsecured in the filter cabinet.

Note: In some cases, filter manufacturers may use custom furnace filter sizes along with cut to fit filters. If you are not sure the filter size, you can always contact the manufacturer to confirm the size(s) that are appropriate for your system.

Shop for furnace filters at Wilco.

Filter Thickness

Filter thicknesses range between 1” to 5” thick. Generally, the thicker the filter, the more efficient and longer-lasting it is. However, the thickness you will use will be determined by your particular system. Standard units usually use filters with a thickness of 1” but a 4” thickness is also common among bigger units.

Filter Efficiency (Minimum Efficiency Reporting Value)

The Minimum Efficiency Reporting Value (MERV) is the standard rating system developed by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE). MERV ratings range from 1 to 20.

Essentially, the higher the MERV rating is, the more efficient the filter is at removing airborne particles. For a standard household, we recommend a MERV 10 filter. For homes with occupants with respiratory issues, allergies, or asthma, a MERV rating of 12 or higher is suggested. Keep in mind, you must use a MERV rating compatible with your furnace or risk making the blower’s job more difficult.

Pro tip: If improving indoor air quality is your main concern, pair a high-efficiency filter with an equally efficient vacuum cleaner and air purifier. Practice proper cleaning and maintenance of the vacuum filter.

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Filter Material

Pleated Filter

Furnace filters can be made out of a variety of materials including fiberglass, cotton, and polyester. The material used can affect the efficiency of your filter’s filtration.

Fiberglass Filters

  • Fiberglass filters, made of spun fiberglass, have MERV ratings between 2 to 4 and are usually about 1” thick. While they are affordable, they may only be good for trapping large dust particles and not very well at improving indoor air quality.

Disposable Pleated Filters

  • Disposable pleated furnace filters are made of cotton paper sheets or polyester. Disposable filters have a MERV rating between 6 and 13. Cotton filters, in particular, have MERV ratings starting at 10 and up. While they may be a bit more expensive than traditional pleated filters, the cotton fibers can remove small airborne particles.

Washable Filters

  • Washable filters are not as common but have a MERV rating of 8. They are usually made with an aluminum or plastic frame. These reusable filters can last anywhere from 6 to 8 years. While they can be convenient, they are known to grow bacteria and fungus when not properly cleaned and dried and do not have the highest MERV ratings.

HEPA Filters

  • High-Efficiency Particulate Air (HEPA) filters have the highest MERV ratings ranging from 17 to the highest, 20. However, these thicker filters are not compatible with most HVAC systems. These high-efficiency air filters would require the blower to work harder to push air through them. Ensure that your units are rated for HEPA filters. These air filters are recommended for use in homes with people who have severe allergies, asthma, and other respiratory conditions.

Disposable and washable furnace filters are available in electrostatic filter models that self-charge as air passes through them. The self-charging capability enables the fibers to collect small particles. Before using electrostatic filters, ensure that your system can safely use them.

Pleated vs. Non-Pleated

Should you go with pleated or not pleated air filters? Traditional fiberglass, non-pleated filters can get the job done but pleated furnace air filters have a greater surface area that collects and traps a higher concentration of airborne particles. In addition, their smaller pores can trap those tiny particles without obstructing airflow.

Understanding MERV Ratings

pleated filter with metal wire

The best air filters for your furnace depend on your household needs and type of unit. For instance, some filters are better at catching pet dander and mold spores than others. Refer to this rating guide to determine the best furnace filter for you.

  • MERV rating 1-4: Captures particles over 10 microns; Good for trapping pollen, dust mites, carpet and textile fibers, and sanding and spray paint dust
  • MERV rating 5-8: Captures particles between 3 and 10 microns; Good for trapping mold spores, testing acids, cement dust, hairspray, and fabric protector
  • MERV rating 9-12: Captures particles between 1 and 3 microns; Good for trapping milled flour, humidifier and lead dust, car emissions, welding fumes, and legionella
  • MERV rating 13-16: Captures particles between .3 and 1 microns; Good for trapping bacteria, tobacco smoke, and droplet nuclei (sneeze)
  • MERV rating 17-20: Captures particles under 0.3 microns; Good for trapping attached viruses, carbon dust, and combustion smoke

Replacing a Furnace Filter

Furnance with Filter Showing
Replacing your system’s filter when it needs it can improve the purity of your indoor air, extend the longevity of your HVAC system, and reduce your energy use. Proper replacement not only removes airborne pollutants and contaminants from inside your home but also keeps the ductwork of your HVAC system free from debris.

How to Replace a Furnace Filter

If you are due for a furnace filter replacement and you have determined the compatible filter with the appropriate MERV rating, you can easily replace an air filter in a matter of minutes. Here is a step-by-step process on how to safely replace your air filter:

  1. Before changing furnace filters, turn off your furnace before you replace or clean your filter. This ensures the appliance does not turn on while you are in the process of changing your air filter.
  2. Open the air filter compartment door. Generally, you can find this between the furnace and the air intake. Pull out the old and dirty air filter and immediately and gently place it into a plastic bag and seal the plastic bag shut. Do not shake the filter since it can release dust particles.
  3. If you have a washable air filter, remove it and vacuum as much of the dust and debris from it before thoroughly rinsing it with water to clean it. Let it dry completely before placing it back in the filter compartment.
  4. Check the cleanliness around the filter opening. If you notice a lot of hair, dust, and other debris, make sure to vacuum the area around the furnace and inside the filter opening before inserting a new air filter.
  5. When placing the replacement filter into the compartment, filters will have an arrow on the top of their frame indicating the direction of the airflow and proper placement to ensure the airflow is going through it correctly. Ensure you are properly inserting the air filter into your furnace so that the arrow is facing the furnace side of the compartment.
  6. Close the filter compartment and turn your furnace back on.
  7. Check the quality of your air filter every month and replace or clean your filter every 3 months for best results. If necessary, contact an HVAC professional to perform these maintenance and cleaning services.

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When Should You Replace Furnace Filters?

Dirty air filter

The frequency of your furnace filter replacement depends on a variety of factors. Generally, an average home may need to replace or clean their furnace filter every 3 months. However, the frequency of replacement can be longer or shorter depending on the following factors:

  • Type of filter used
  • Frequency of use of HVAC system
  • Number of pets in the home
  • Household members with allergies or medical conditions
  • Amount of dust in the home
  • Length of heating and cooling cycles
  • Environmental occurrences like wildfire smoke and ash; swap filters during firee season when there is a lot of smoke

Pro tip: We recommend doing a monthly check of your pleated furnace filter quality, especially if you have new units. Check more frequently with disposable fiberglass filters. Look for any build up and grey color.

Where Is the Furnace Filter Located?

The location of your furnace filter depends on the type of HVAC unit you have in your home. Generally, furnace filters can be found in the blower compartment. Check for a grill opening that draws air into it. If you are unsure where to find the filter location, refer to your furnace manufacturer’s manual or website for more information.

Do All Furnaces Have Air Filters?

Yes. All furnaces, including gas furnaces, are fitted with a filter. Without one, they would not be able to trap the airborne contaminants and pollutants from entering your house, improve air quality, and maintain high system efficiency. For this reason, every furnace needs a filter and regular replacements.

What Happens If You Do Not Replace the Furnace Filter?

Dirty Filter

Dirty furnace filters do not just lose out on their filtering capabilities but also can lead to a number of system failures if the filter is not adequately replaced or cleaned. In some cases, a very dirty filter can reduce the efficiency of your system or worse, cause it to freeze up. When the frozen moisture thaws, the condensate drain can flood the system and cause water damage.

In addition, dirty furnace filters can lead to issues with your system’s heat pump and air compressor. Ultimately, a clogged filter can lead to costly repairs. It is much more cost-effective to invest in an air filter and its proper and regular replacement and maintenance compared to the total cost of repairing your damaged unit.

Shop for High-Efficiency Furnace Filters at Wilco Farm Stores

Do not wait before it is too late to replace or clean your furnace air filters. Be prepared for the colder months with the right furnace filter size and MERV rating for your HVAC system. Explore a wide range of air filter materials and sizes at Wilco Farm Stores.

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