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What Happens When A Furnace Runs Without A Filter?


There’s probably a lot that you don’t know about the furnace in your Riverside, California home. For instance, did you know that its filter is actually for protecting its internal components rather than cleaning your indoor air? Furnace air filters are essential for keeping residential heating systems running as they should. Thus, if you think that your indoor air quality (IAQ) is the only thing that will suffer if you fail to put a filter in, you couldn’t be further from the truth. Read on to find out what happens when furnaces are turned on without the protection that air filters provide.


To start, it’s important to think about why furnace manufacturers deliver their products with filters pre-installed. When furnaces are produced, they’re immediately equipped with standard air filters that have modest maximum efficiency reporting value (MERV) ratings. These filters can only pick up relatively large-sized airborne particulates such as:

  • Dust
  • Pet hair
  • Dander
  • Pollen

Volatile organic compounds (VOCs), bacteria, viruses, and other micro-sized particulates pass right through them. If you want air filtration that’s capable of picking these things up, it’s your job to find and install a higher-rated filter.

The sole goal of the filters that manufacturers provide is to prevent coatings of dust from causing your furnace to overheat. This allows furnace producers to deliver products that can live out their expected lifespans and provide the performance promised by their efficiency ratings without sustaining undue wear. Although letting your furnace run with a dirty, clogged filter can cause a host of problems and may even result in outright system failure, failing to put a filter in at all can be just as bad.

Failing to keep this important component in or failing to change it on a regular basis will automatically void your product warranty. Heating equipment manufacturers will not pay to replace or repair damaged furnaces that have been operated sans filters. If you have a home warranty that covers your HVAC system, the protections provided for your furnace from this agreement will become void as well.


Furnaces don’t run efficiently without filters. Not only do buildups of dust and other debris settle on their internal components and cause overheating, but they also find their way into HVAC ductwork. Adding dander and other particulates to this environment creates the perfect conditions for mold to thrive. As dirt accumulates within the ribbed, aluminum tubing that HVAC ductwork is composed of, condensation will build. Once mold spores take root and proliferate in this perpetually damp space, your IAQ will indeed go down. Moreover, all heated or cooled air that gets pushed through your ducting will distribute your fast-growing problem to every corner of your home. Having moldy HVAC ductwork and a moldy living environment overall will further decrease the efficiency and health of your furnace.


Dirty, dust-covered filters cause decreased airflow. They make furnaces work harder to push air into and through your ducting. It stands to reason then that removing the obstruction by simply taking your filter out would improve airflow rather than impede it.

Consider the last time you performed an air filter change. Even when homeowners are diligent about changing their filters out, these units are coated in layers upon layers of dark-gray grime. These lint-like accumulations are often riddled with pet hair, carpet fibers, and more. Without a filter, all of this matter will travel through your heating equipment and your HVAC ductwork. Although it won’t get caught on the filter, it will collect on every other surface throughout the system. Eventually, your furnace will again have a hard time forcing air through this accumulated debris, and problems with low airflow and overheating will still occur.

When airflow decreases, furnaces can run for a long time without having a noticeable impact on indoor temperatures. They may short-cycle as they rapidly overheat and then turn themselves off to cool back down. If you run your furnace without a filter for too long, its motor could burn out.


There are several reasons why people run their furnaces without air filters. In rare instances, disposable furnace filters are too dirty to continue using. These filters aren’t made to be cleaned, and replacements are sometimes difficult to find. However, once a clogged furnace filter is taken out, the heating system should be shut down entirely. It shouldn’t be turned back on until a new filter has been put in.

If you’ve just purchased existing construction, you may be surprised to discover that your furnace doesn’t have a filter at all. While it makes sense to assume that property sellers always put fresh filters into their heating equipment before putting their homes on the market, this assumption could cause serious issues if you never check. Before using your heater for the first time, make sure that there’s a properly sized filter in place, that it’s installed correctly, and that it’s free of accumulated grime.

There are also times when furnace filters are incorrectly installed. This might be the result of slipshod workmanship on the part of a hired handyman or HVAC provider, or it could be that the prior homeowner wasn’t quite sure how to replace this component. Sometimes, homeowners choose furnace filters that are too large or too small and then bend them to fit. Improperly sized filters will fall out as soon as furnaces are turned on and they’re subjected to air movement and changes in air pressure. Having an incorrectly sized filter in place or a filter that’s incorrectly installed is much the same as having no filter at all.

Finally, the most common reason for using a furnace without a filter is simply forgetting to replace this component after you’ve extracted a dirty one. You may have been interrupted by an unexpected visitor or phone call during the middle of this task and simply forgot to pick it back up. It’s easy to get distracted during the course of this ongoing maintenance task. This is especially true if you’ve got a busy, bustling household that’s filled with active children and rambunctious pets.


Replacing a dirty air filter can be an annoying, recurring chore. This is a task that most homeowners should perform once every three months. In households with one or more pets, homes in perpetually dusty environments, and buildings with residents who suffer from asthma or allergies, filter changes are best done once each month. In all instances, changing an air filter is always cheaper and easier than either letting dirt accumulate until a blockage occurs or simply taking a filter out and never putting it back in. When airborne particulates aren’t caught by HVAC air filters, the interior of your furnace and all of your HVAC ductwork must be cleaned. More importantly, if you run your furnace without a filter for too long, your entire heating system may need to be replaced.

At Dragonfly Heating & Cooling, we make it easy to stay on top of essential furnace maintenance. We’ve been serving residents of Riverside, California and the surrounding area for many years. We offer heating, cooling, and indoor air quality services. We also provide ductless mini-splits and preventative maintenance plans. Get in touch with us today!

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