What is a Furnace Filter Box?

April 27 2017
furnace filter box

What is a Furnace Filter Box?

Quick Answer: A furnace filter box holds a filter in a perfect position to allow air to flow through from the furnace to the filter. Without a furnace filter box, gaps may allow dirt and dust to bypass the filter and spread in your home via furnace filter duct leaks.


The filter of your furnace is responsible for ensuring optimal operation and preventing unwanted contaminants from entering the machine. What you might not know is that there is a furnace filter box for placing the filter.

Your furnace or HVAC system comes equipped with varied sizes of filters for promoting superior filtration, healthier breathing environment, and cleaner overall air. They are designed to protect the furnace system from dirt and debris, along with growth of mold and mildew, which can negatively affect air quality causing numerous respiratory diseases. A common HVAC filter installation technique encountered in homes is the use of filter boxes.

These boxes expose some part of the filter, allowing for easier access. The box holds the filter in a perfect position, which allows for better spreading and circulation of air from the furnace to the filter and then throughout your entire home. Installing a filter without a box will result in restrictive airflow, as the air filter won’t be settled and installed properly. Running your furnace on high temperatures and fan speed could result in the air filter being moved out of the ideal position.

Not only can this result in the furnace filter getting out of its place, but it could force dirt and debris to be spread in the air. Without a furnace filter box, you can easily notice a large return duct leak. Since harmful elements are being released in the air, it could result in some unintended consequences that may even impact the health of your furnace system. Therefore, if you want to secure the air filter correctly in your furnace, the use of a filter box is essential.

However, before choosing a filter box, it is imperative that you measure the size of your air filter. Keep in mind different furnace systems from varied brands and models will have diverse sized air filters. Always make sure to read the owner’s manual of your furnace to get a better idea on which air filter size is the perfect. However, if you’ve lost the manual, you can always consider assessing the size yourself using a length and width calculator.

Once you’ve determined the size and found the perfect box for your filter, you can have it installed in your furnace. This allows for efficient airflow, as it takes on a path of the least resistance. Since the filter box settles the air filter in the perfect position, the air will go through smoothly rather than directing towards the return grille. This clears up the dirt and debris inside the furnace, and prevents it from entering the air outside.

A filter box also allows filtered air to directly pass through the filter. Since there is no gap on the sides of the filter box, the hot/cool air created will only move through the air filter as intended. Without a filter box, unfiltered air bypasses the filter. This results in dirt and dust spreading throughout the air, as the potential to be distributed by the furnace system gets disturbed. Dirt and dust will accumulate on the first surfaces they encounter.

Cleaning this mess out can be a painful and time-consuming task. Dirt and debris can even enter inside the heat pump and water coils, resulting in poor performance from your furnace system. Since your furnace will be forced to work harder, this may even result in an increase of your electricity bills. To prevent this from happening, make sure to always install air filters with a box. Otherwise, you’ll have to deal with a lot of cleaning and maintenance expenses later on, as the moist air mixes with the return air in the ducts. Therefore, be safe rather than sorry and make sure to equip your air filter with a box.

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