What's the Difference Between an Air Filter and a Furnace Filter? Understanding Furnace Filter Sizes

Are you in the market for a new air or furnace filter? Do you know the difference between the two? It's important to understand the distinction between air filters and furnace filters to ensure you're purchasing the right one for your home. In this article, we'll provide you with a breakdown of the two and some tips on understanding furnace filter sizes.

An air filter is typically used in HVAC systems to clean the air that circulates throughout your home. It's designed to remove particles such as dust, dirt, and allergens from the air, providing you with a cleaner and healthier environment. A furnace filter, on the other hand, is specifically created to protect the furnace's blower fan and heat exchangers from any debris that might damage them.

Understanding the size of the furnace filter you need is crucial to ensure maximum efficiency and proper filtration. Purchasing the wrong-sized filter can result in restricted airflow, which can cause your HVAC system to struggle and increase your energy bill. By taking the time to understand the difference between air and furnace filters and their sizes, you can make an informed decision and keep your home's air and furnace systems running smoothly.

Types of Air and Furnace Filters

When it comes to air and furnace filters, there are several types to choose from. Each filter type has its own benefits and characteristics. Here are the most common types of air and furnace filters:

1. Fiberglass Filters: These are the most basic type of filters that have a thin layer of fiberglass to capture larger particles like dust and dirt. They are the most affordable option, but they need to be replaced regularly as they are not very effective in trapping smaller pollutants.

2. Pleated Filters: These filters are made of folded paper or polyester fabric to increase the surface area for capturing airborne particles. They offer better filtration compared to fiberglass filters and are available in different MERV ratings (Minimum Efficiency Reporting Value), which is a scale to measure the filter's effectiveness.

3. Electrostatic Filters: These filters use an electrostatic charge to attract and trap particles like pollen, pet dander, and smoke. They are more effective in capturing smaller particles and don't require frequent replacement like fiberglass and pleated filters.

4. HEPA Filters: HEPA stands for High-Efficiency Particulate Air. These filters are the most effective in trapping even the tiniest pollutants, including viruses and bacteria. They are widely used in hospitals and laboratories, but they may not be suitable for residential use as they require a high-powered fan to push air through the dense filter.

When selecting an air or furnace filter, consider your budget, the level of indoor air contamination, and the size and type of your HVAC unit. It's recommended to check your filter every month and replace it every 90 days or sooner if it's dirty. Regular replacement of air filters not only improves indoor air quality but also helps in reducing energy bills and extending the life of your HVAC system.

Importance of Properly Sized Filters

The size of your furnace filter is crucial to the efficiency of your HVAC system. Using the wrong filter size can cause clogs, reduced airflow, and excess strain on your furnace or air conditioner. This can lead to energy waste, shorter lifespan of your appliances, and increased utility bills.

Using a filter that is too small can allow debris to pass through, which can damage your equipment and decrease air quality within your home. On the other hand, a filter that is too large can reduce airflow, impeding the distribution of air and making your equipment work harder to heat or cool your space.

Additionally, using a filter with a high MERV rating may seem like a good idea for better air quality, but if it’s too high, it can also reduce airflow and negatively impact your equipment’s performance. That’s why it’s important to choose the right size and rating for your specific system.

Consult with a heating and air conditioning professional to determine the appropriate filter size and rating for your HVAC system. Regularly checking and replacing your filters at the recommended intervals can help maintain your equipment’s efficiency and extend its lifespan.

MERV and How it Affects Filtration

MERV stands for Minimum Efficiency Reporting Value, which is a rating system used to classify the effectiveness of air filters. It measures a filter's ability to capture and remove particles of different sizes from the air passing through it.

A MERV rating ranges from 1 to 20, with higher numbers indicating a higher level of filtration. MERV 1 to 4 filters typically catch larger particles such as pollen, carpet fibers, and dust mites. MERV 5 to 8 filters capture smaller particles such as pet dander, mold spores, and fine dust. MERV 9 to 12 filters remove even smaller particles such as lead dust, humidifier dust, and automobile emissions. And, MERV 13 to 16 filters are the highest-rated filters that are commonly used in residential homes, and they trap even the tiniest of particles such as bacteria, viruses, and smoke particles.

While the highest-rated filters may be appealing, they may not be the best option for every situation. The higher the MERV rating, the more airflow is restricted, which can reduce the efficiency of the heating and cooling system. So, it's essential to find a balance between filtration and airflow when choosing an air filter with a MERV rating.

It's also important to note that higher MERV filters may require more frequent replacements than lower MERV filters. This is because they trap more particles, which can cause them to clog up and reduce airflow more quickly.

In summary, understanding the MERV rating system is crucial when selecting an air filter for your furnace. It is best to consult with an HVAC professional regarding the appropriate filter rating to use that balances filtration and airflow, keeping in mind the specific needs of your home and heating and cooling system.

How Often to Change Furnace Filters?

The frequency at which you should change your furnace filter largely depends on the type of filter you are using. Generally, filters should be checked every month while they are in use and replaced as necessary. The following are some common types of filters and their replacement schedules:

  • Disposable fiberglass filters: These filters typically cost less but require more frequent replacement. They should be checked every month and replaced every 1-3 months.
  • Disposable pleated filters: These filters have a higher filtration capacity and can last up to 3 months, depending on the level of dust and pollutants in your home. Check them monthly and replace when necessary.
  • Permanent filters: These filters are washable and reusable, making them more eco-friendly and cost-effective. They should be washed every 1-3 months and replaced every 5-10 years.
  • Electrostatic filters: These filters use static electricity to capture airborne particles and allergens. They are washable and reusable, and should be washed every 1-3 months and replaced every 5-10 years.

It’s important to note that if you have pets, smokers, or respiratory issues, you may need to change your filter more frequently. Additionally, if you notice decreased airflow, increased energy bills, or a decrease in indoor air quality, it’s time to check and possibly replace your filter.

Maintaining a clean furnace filter not only improves your indoor air quality, but also helps your HVAC system operate more efficiently and prolongs its lifespan. Don’t neglect this important component of your home’s heating and cooling system!

Eco-Friendly Filter Options

As we become more conscious about our impact on the environment, it’s essential to consider eco-friendly filter options for your furnace. Here are a few options to consider:

Reusable Filters: Rather than throwing away disposable filters every few months, consider purchasing a washable, reusable filter. These filters can be washed and reused many times, saving you money and reducing waste.

Electrostatic Filters: Electrostatic filters use static electricity to trap particles in the air. These filters can be washed and reused, and they are incredibly effective at capturing allergens and other pollutants.

MERV-Rated Filters: MERV stands for Minimum Efficiency Reporting Value, and it’s a rating system used to determine a filter’s effectiveness. Choosing a MERV-rated filter with a high rating can help improve your indoor air quality by trapping small particles that other filters might miss. Be sure to consult with an HVAC technician to ensure that your furnace can handle the airflow resistance caused by high-efficiency filters.

By choosing eco-friendly filter options, you can save money, reduce waste, and improve the air quality in your home. Consult with an HVAC technician to determine which type of filter is best suited for your furnace and your needs.


After reading this article, you should now have a clear understanding of the difference between an air filter and a furnace filter, as well as the importance of choosing the right furnace filter size. Remember, a properly sized and clean furnace filter can improve your indoor air quality and extend the lifespan of your HVAC system, saving you money in the long run. Make sure to check your furnace filter regularly and replace it as needed to keep your home healthy and comfortable.

Frequently Asked Question

It's important to know the size and type of furnace filter you need before replacing it. This is because different furnaces require different sizes of filters. Your brand of furnace may need an electrostatic or media filter of a particular size - so make sure to check your old filter for these details.

Replacing your filter regularly can save you money in the long run. It helps your HVAC system become more efficient, and also keeps the air quality high. So, what are the most common furnace filter sizes?

The most popular furnace filters come in the following dimensions: 10x20, 14x20, 16x24, 18x30, 12x12, 14x24, 16x25, 20x20, 12x20, 14x25, 18x18, 20x24, 12x24, 14x30, 18x20, 20x25, 12x30, 15x20, 18x24, 20x30, 12x36, 16x20, 18x25, 24x24 and 25x25. You can use your existing filter as a reference guide to find the perfect size for your furnace. The filter dimensions should be printed on its frame as length x width x height.

In conclusion, it's important to know your furnace filter size before you replace it. Doing so will improve your HVAC system's efficiency and save you money in the long run. Plus, a clean filter is key to keeping your air quality high.

When it comes to furnace filters, exact size matters. If you get a filter that is too small or too big, air will flow around it instead of through it, meaning not all of the air will be filtered. That's why it's essential to use the right size filter for your HVAC unit.

Here's what you need to know: the size of the air filter is typically listed in the big, bold font on the packaging, which is known as the nominal size. This is a rounded-up whole-number version of the actual size, which is written in the fine print on the filter's frame. That's usually 0.25'-0.5' smaller than the nominal size, and the actual measurements of the filter may vary slightly depending on the brand.

In some cases, less common sizes come true to size, meaning the nominal size is the same as the actual size. When that's the case, it's usually noted with “actual size” in parentheses on the filter's frame.

So, the verdict is in: the answer to the question 'Do furnace filters have to be exact size?' is a resounding yes! Be sure to get the right size air filter for your system to ensure it runs smoothly and that you reap all the benefits your filter has to offer.

Are you looking for the best furnace filter? Look no further! Our team of experts at This Old House Reviews have researched the top five furnace filters on Amazon to help you find the perfect one for your HVAC system. Plus, these products are available at local home centers or online retailers like Amazon.

A great furnace filter can make all the difference. It will keep your HVAC system running smoothly, potentially leading to lower electricity bills and better air quality. That's why we've taken the time to compile a list of the best furnace filters for you.

At the top of our list is the Filtrete furnace air filter. It promises to last up to three months and outperform filters made with fiberglass. Plus, it can capture 64% of airborne particles like pet dander and pollen. This filter also comes with instructions for easy installation on HVAC systems that require filters 20 by 25 by 1 inches in size.

Customers love this furnace filter. They find it easy to install, sturdy, and a great value. However, some customers have faced difficulties with Filtrete’s app.

When choosing a furnace filter, make sure to take into account its size, filtration level, and material. That way, you can get the best filter for your specific needs.

Changing or replacing your furnace filter is one of the best ways to maintain a healthy, efficient HVAC system. But you need to make sure that you're using the right filter for your system. Here's a step-by-step guide to help you pick the perfect one!

First, measure the dimensions of your existing filter. This will tell you the length, width, and depth you need. The easiest way to do this is to look at the existing filter and buy the same size.

Next, you need to choose the right MERV rating. The MERV scale ranges from 1-20 and measures the filter's effectiveness at catching particles in the air. The higher the value, the more effective the filter is at trapping particles, but it also means less airflow.

Now it's time to decide on the type of filter. There are many options available, so take some time to research the best type for your system.

Once you have everything you need, it's time to install the filter in your furnace/air handler slot. Remove the old filter to make room for the new one.

Finally, make sure to replace your filter regularly. If it clogs up with dust and particulate, it can reduce airflow and become ineffective.

Furnace air filters are essential for your heating system. You should replace your filter at least once every three months. So, how do you select the best furnace filter for your needs? Let us help you out!

Generally, furnace filters are made up of a cardboard frame, fiberglass filter, and metal mesh. These materials work together to strengthen the filter and protect your HVAC components from dust and debris. Plus, they also help improve the air quality in your home. Without a filter, your HVAC system could become dirty and its performance would suffer.

As air passes through the furnace filter, it traps and catches any contaminants. If you don't replace or clean your filter regularly, it can obstruct airflow and cause damage to your HVAC system. In most cases, you should change your furnace filters every three months. However, you may want to replace them more or less often depending on the situation.