The Essential Guide to Furnace Filters

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Last updated: 
March 26, 2024

The Essential Guide to Furnace Filters: Choosing the Right One for Your Home


Furnace filters may not be the most exciting home improvement item, but they play a vital role in saving money and improving indoor air quality. According to the Environmental Protection Agency (EPA), indoor air can be up to five times more polluted than outdoor air, making furnace filters a crucial tool in preventing harmful particles from circulating in your home. These particles can cause asthma attacks and allergies if inhaled. This guide will help you understand how furnace filters work and how to choose the appropriate one for your home.

How Does a Furnace Work?

To understand how a filter works, you must first have a basic knowledge of your home's furnace. Traditional forced-air furnaces draw in air through return ducts, warm it over a heat exchanger, and use a blower fan to push the heated air through ducts that distribute it throughout your house. The furnace continues running until your home reaches the desired temperature, which is typically controlled by a thermostat.

The Role of a Furnace Filter

The furnace filter is connected to the blower fan and is responsible for keeping dust, hair, and other debris away from the fan. As the return duct pulls in air, it also draws in these potentially harmful elements. Since the air recirculates through the furnace, the filter ensures that the circulated air is clean. However, it's important to note that while furnace filters significantly improve air quality, their primary purpose is not to clean your air.

Furnace Filter Ratings: Understanding MERV

Furnace filters are rated using the Minimum Efficiency Reporting Value (MERV) system, with ratings ranging from 1 to 16. Higher MERV ratings indicate better filtration, as the filter can remove a greater amount of potentially harmful particles from the air. However, it's crucial to consider that high-rated filters may restrict airflow, causing your furnace to work harder and possibly overheat or malfunction. Most homes require a filter with a MERV rating between 8 and 11. Before purchasing a filter, consult your furnace's manual to determine the maximum MERV rating recommended for your specific model.

Types of Furnace Filters

There are three main types of furnace filters available on the market:

1. Disposable Pleated Filters

Made from paper and polyester, these filters are the most common and come in various sizes and MERV ratings. Prices range from $2-$3 for basic models to $30-$40 for higher-end options. These filters should be checked monthly and replaced every three months to maintain cleanliness and efficiency.

2. Fiberglass Filters

Fiberglass filters are the most affordable option but are considered flimsy and have lower MERV ratings compared to disposable pleated filters. They require more frequent checks and replacements to maintain effectiveness.

3. Electrostatic Filters

Available in both disposable and washable versions, electrostatic filters charge themselves as air flows through, enhancing their ability to trap small particles, dust, and hair. They are more efficient than the other two categories, making them ideal for homes with small children, pets, or smokers. However, it's essential to consult your furnace's manual to ensure compatibility with electrostatic filters.

Choosing the Right Filter Size

Furnace filters are sized according to their thickness (depth), height, and length. The most common thickness is 1", but 4" filters are also available for larger systems. Height and length combinations range from 10"x10" to 30"x30", with the most popular sizes being 14"x25", 16"x20", 16"x25", 20"x25", and 25"x25".

To determine the appropriate size for your furnace, remove and examine your current filter, as the size is typically printed on the frame. Alternatively, consult your furnace's manual. If your furnace requires a custom-made filter, contact the manufacturer or inquire about replacement options.

Replacing Your Furnace Filter in Three Simple Steps

To ensure your furnace and filter operate safely and efficiently, replace the filter approximately every three months. Follow these three steps for a successful filter replacement:

Step 1: Identify Your Filter Type

  1. Turn off your furnace to prevent accidents.
  2. Remove the existing filter, typically located inside the furnace or air vent.
  3. Look for an arrow indicating the direction of airflow and use a permanent marker to draw a line on the outside of the furnace for future reference.
  4. Write down the filter's size, which can be found on the cardboard frame.
  5. If the filter has a plastic frame, it is reusable. Clean it with a vacuum and water, allowing it to dry completely before reinstalling.

Step 2: Purchase the Replacement Filter

Furnace filters can be found at hardware stores, home centers, and online retailers such as Home Depot, Amazon, or Walmart.

Step 3: Install the New Filter

  1. Look for markings indicating which side of the filter should face the furnace.
  2. Slide the new filter into its designated slot and replace the cover, if applicable.
  3. Keep a record of filter replacement dates to maintain a consistent maintenance schedule.

Top 5 Furnace Filters on the Market

Choosing the right furnace filter can be challenging, as high-efficiency filters may restrict airflow while low-efficiency filters may not adequately clean the air. Based on user experiences and reviews, here are the top 5 furnace filters available:

Filter NameKey FeaturesPriceReplacement Frequency
PUR5000 Whole House Furnace FilterElectrically charged particle collection system; removes 99.97% of particles$700Monthly
Blueair Central Furnace Filter System with MX2 Particle ChargerIonizer for enhanced particle capture; includes two filters for 6-month filtration$199.95Every 6 months
Envirosept Electronic Furnace Filter Air Cleaner SystemPermanent filter with enhanced media and dielectric polarization; removes sub-micron particles$319.95 and upLow maintenance
Guardian Clean Air Furnace Filters by AerusDesigned for maximum airflow; captures 95% of particles down to 0.3 microns; MERV 12 rating$59.95As needed
Aircleen 1500Removes 97.7% of particles down to 0.3 microns; easy-to-replace pads available in 16 sizes$259.95Monthly


While furnace filters may initially seem unexciting, understanding their importance reveals a multi-layered world dedicated to improving indoor air quality. By investing in the right furnace filter and maintaining a regular replacement schedule, you can significantly reduce the amount of dust, mites, dander, and pollen in your home, leading to a healthier living environment.



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