Furnace Filter Bending – How To Fix The Root Cause


A furnace filter can bend if it is too large for the furnace. Excessive pressure from the blower fan due to overheating, electrical damage, or an obstruction in the airflow can also bend the filter.

Air filters are essential to keep airborne contaminants from accumulating inside the furnace. If your furnace filter keeps bending, its filtration ability is reduced, which eventually damages the appliance.

This article explains all the potential reasons why your furnace filter keeps bending and how it impacts the heating process. We then discuss quick troubleshooting tips to resolve the problem.

  • Furnace filters often bend when the blower fan applies excessive pressure because of a malfunction.
  • Dirty filters or incorrectly sized filters can also bend in the furnace.
  • Air filters should not be deliberately bent, as it can allow airborne contaminants to enter the furnace, and damage it.

Furnaces and their filters

Before we jump to the causes and effects of a collapsed furnace filter, let’s first discuss how a furnace works and why it needs air filters.

In general, a furnace works using the following mechanisms:

  • Once the unit is switched on, it starts pulling in cold air.
  • A rotating blower fan circulates and propels the air through the furnace.
  • Air passes through the filter at the side or floor of the appliance.
  • The metal chamber carries out combustion to generate heat energy which is used to warm up the air.
  • The heated air is dispensed into the room.
  • Condensing furnaces process the exhaust gases from combustion in a secondary heat exchanger to extract the remaining heat. The consequent cold gases then liquefy and drain through the pipe.
  • In conventional furnaces, the exhaust gases are vented through a flue without passing through a secondary heat exchanger.

What Is The Role Of Furnace filters?

Filters are important components of furnaces. The air filters of a furnace protect the overall unit, and improve the quality of the air.

Protect The Furnace

Air is dense with several different contaminants like dust, dander, pet hair, germs, poisonous gases, etc. These pollutants are part of the air stream that is drawn into the furnace and can easily build up anywhere in the system.

This may lead to clogging in the drain pipe, formation of mold and bacterial colonies (and eventual damage to the furnace material), obstruction in airflow, and overheating.

Air filters trap the majority of the pollutants before the air circulates in the unit and help improve the optimal life span of the furnace.

Improve Air Quality

Furnace filters help improve air quality by helping emit clean air from the system.

This plays a significant role in preventing various allergic reactions, diseases, chest pain, respiratory distress, and other health conditions which otherwise occur upon exposure to airborne pollutants.

Plentiful Air has more information on how furnace filters help reduce allergic reactions available here.

3 Main Reasons Why Your Furnace filter Keeps Bending

Here are the main reasons why your furnace filter keeps bending:

1. Strain From Blower Fan

The blower fan applies pressure to pull air into the furnace system. If the fan becomes damaged or requires extra force to draw air, it exerts strain on the filter, causing it to bend.

A blower fan can become damaged from:

  • Overheating: Dust build-up in the motor or problems in combustion can cause the furnace to overheat, which primarily affects the fan motor. If the root issue is not fixed quickly, the motor can be damaged permanently.

While in most cases, this results in a non-functioning motor, rarely, this may also lead to extra pressure. This is because dust build-up can force the fan to exert more pressure to ensure sufficient air flow.

  • Electrical Problems: Electrical issues like voltage surges damage the wiring and can cause the fan to rotate at high speed. While the damage and effect might not be permanent, the force generated is often enough to bend the filter.

Here’s why the blower fan might need extra force to generate and maintain the airflow:

  • Blockage: A clogged filter constricts the airflow, which forces the fan to produce more pressure to draw in air
  • Improper Installation Of Furnace: The furnace should be installed in an open area, ensuring that the side (with the inlet and filter) is exposed to enough open space. If walls or any surrounding furniture or objects block the airflow, the motor has to exert higher pressure to maintain a continuous and even airflow.
  • High MERV Rating Filter: MERV is a rating scale used to determine a filter’s potential to capture particles measuring between 0.3 to 10 microns. The scale ranges from 1 to 20, and the highest value suggests maximum efficiency.

Theoretically, you should go for the highest MERV rating. However, the MERV rating is also proportional to the filter’s thickness, which can obstruct the airflow (eventually causing the filter to bend).

To prevent the problem, it is always better to buy a filter with an 8 to 11 MERV rating as it guarantees smooth airflow and also effectively removes allergens.

MERV rated filters and their removal efficiency of different particle sizes. Furnace filters should have a MERV rating between 8 and 11.
Particle size efficiency of filters (in microns) as MERV (Minimum Efficiency Reporting Values) ratings. Data sourced from the U.S. Environmental Protection Agency. Created for Plentifulair.com.

2. Oversized Filter

If your furnace filter is not exactly the right size, it may bend to fit in the furnace.

Installing the right-sized filter is crucial for the efficient removal of airborne pollutants. Larger filters bend or don’t fit into the system properly, while smaller ones allow air to move around and past them (with contaminants).

Air filters are measured by their thickness, height, and length.

Check your furnace’s user manual for the filter size suitable for your model. To avoid possible problems, it is better to consult your furnace’s manufacturer for a replacement filter.

3. Old Or Clogged Filter

Air filters have pores that trap contaminants. Over time, dust and debris build up in the pores, and they eventually become saturated. This makes the filter unable to trap further pollutants until it’s cleaned.

If the filter becomes old, excessively clogged, or is not cleaned and maintained adequately for a long time, it becomes swollen and eventually collapses, appearing bent.

Here’s how to properly maintain furnace filters:

  • Turn off the furnace, and unplug it from the power source.
  • Refer to your user manual to locate and remove the furnace filter.
  • Use a clean cloth to dust off the filter. If the collected debris is hard and difficult to remove, you may also use a handheld vacuum (at minimal pressure to avoid damaging the filter).
  • Now wash the filter with lightly running tap water or war water. Before this, make sure to check if washing the filter would damage its media.
  • Allow the filter to dry in the open air for a few hours before placing it back in the system.

Furnace filters come in various types, including disposable pleated filters, fiberglass filters, and some generic reusable filters. Rarely, advanced models may also have True HEPA, electrostatic filters or carbon filters.

If you are using a non-washable filter like HEPA and activated charcoal, or your reusable filter is swollen and does not work well (indicated by inadequate heating and frequently overheating motor), here’s how to replace it:

  • Turn off the furnace, and disconnect its power supply.
  • Locate and remove the old filter.
  • Take the replacement filter, and locate its arrow. The arrow points in the direction you should install the filter. Slide the filter in slots, making sure that the arrow faces towards the furnace and away from the air duct.

It is important to mention that the replacement filter should be the right size and, ideally, should be bought from the original brand. Metal framed filters are also available, and are more robust against bending.

Frequently Asked Questions

What Causes A Furnace Filter To Collapse?

A furnace filter can collapse because of increased strain from the blower fan. This may also occur due to excessive clogging of filter pores, which causes them to swell and eventually collapse.

Replacing the furnace filter is usually necessary to fix this problem. However, be sure to confirm the original cause of the collapse in case the blower fan also needs some maintenance.

Can You Bend Air Filters?

Air filters should not be bent. Bending an air filter compromises its efficiency and makes it easy for airborne contaminants to escape into the system, and should be avoided.

People tend to bend air filters when they accidentally purchase one in a larger size. While bending the filter might help fit it in its allotted HVAC space, this practice can seriously damage the appliance.

What Happens If You Put A Furnace Filter in Backwards?

Installing the furnace filter improperly, such as in the opposite direction (backwards), negatively impacts filtration and allows the filter to saturate quickly. This also causes furnaces to generate extra pressure to maintain the airflow, which increases energy costs.

Here’s how to correctly install the furnace filter:

  • Turn the furnace off, and disconnect its power supply.
  • Locate the arrows on the filter, and line the filter up accordingly.
  • Carefully slide the filter in the slots, and make sure it is secured in place.
  • Switch the furnace on.

Russell Singleton

Russell holds a Bachelor of Science (Environmental and Marine Geoscience) with Class I Honors. He is currently completing his doctorate in science and is passionate about all earth processes, especially isotope geochemistry and paleohydrology.

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