Can I Run My Furnace without A Filter Temporarily?

Many homeowners think that it is okay for their furnace to operate without a furnace filter, also called an air filter. And they are right – in some cases. This is a PickHVAC FAQ guide - the purpose of the many similar guides is to provide a quick answer first and then give you complete details. 

Can I Run My Furnace without a Filter?

The air filter is designed to trap dirt, dust, hair and other “gunk” you don’t want gumming up your furnace or flying around your home.  

It definitely isn’t a good idea to run the furnace without a filter for a long time – for a day probably won’t cause major problems. But running the furnace or air handler without a filter for days, months and even for years, as some homeowners do, will cause damage.

Let’s get specific about this important question. 

What Will Happen if I Run My Furnace Without a Filter?

If you decide to run your furnace for a day or more without a filter, there might be minor problems such as more dust than usual in the air or coming out of the air vents. If anyone in your household has breathing issues – asthma, allergies, C.O.P.D or other problems – they might experience worse symptoms than usual.

Now, running your furnace without a filter for a year is not recommended at all!!  Any extended amount of time of operating your heating and cooling without a furnace filter can cause damage to your HVAC system. Below is a list of possible problems that can happen if you run your heating and cooling system without an air filter:

  • Damage to the heat exchanger – Excess dust in the combustion box and heat exchanger will burn, and the burning might weaken the metal even more than normal heat will. If the heat exchanger or combustion box cracks, it is time for a new furnace.  An air filter can cost as little as $2.  Replacing a furnace will cost you over $3000.
  • Poor indoor air quality - The quality of air within your home will be less pure if you decide to run your furnace without an air filter.  No one wants to breathe in dirty air for a day, for a month or for years.
  • Polluted ductwork - Without a furnace filter, the unwanted airborne particles will gather in your ductwork.  This will cause possible mold and/or air pollution inside of your home. Eventually, it will also cause low air flow, higher static pressure and potential mechanical issues. See the next answer and the FAQ “What is the purpose of an air filter?” for more on mechanical issues from running your furnace without a filter. Our guide called Low Air Flow Through Vents has more details. This is also one cause of ductwork needing to be cleaned.  
  • Total System Failure - If you happen to operate your HVAC system without an air filter for a long period of time your entire heating and cooling system may shut down and be damaged beyond repair due to unneeded stress and strain on the furnace system.

Frequently Asked Questions

You might want to learn more about this topic through questions we often receive and have answered.

How often should I change my air filter?

Each air filter for your heating and cooling system will have a recommended length of use.  Some air filters should be changed every month, but others can be used for six months.  Make sure you follow the recommended times for your air filter and replace the filters when they should be changed. We’ve created a full page that answers this question.

How much do air filters cost?

Air filters range greatly in cost, as well as in how well and how long they work effectively.  The cheapest air filters for your furnace cost around $2-$3, but they need to be changed every month and this type of filter also does not filter airborne contaminants as well as more expensive air filters.  The higher-end air filters cost $25 or more, but they last up to six months and keep most of the airborne particulate matter out of your heating and cooling system as well as improve the air quality within your home.

What is worse…no air filter or a dirty furnace filter?

Neither option is not a good idea, but running your heating and cooling system without a filter will cause more damage than if you ran it with a dirty furnace filter. But again, neither is recommended.

What is the longest I can run my furnace without an air filter?

Running your furnace without an air filter for a short period of time, like for a few hours or for a day, will not cause much damage.  If you operate your heating and cooling system without an air filter for long periods of time, then the furnace could be damaged or run very poorly.

What is the purpose of an air filter on a furnace or air handler?

The primary purpose of an air filter in your furnace is to trap unwanted airborne particles and keep your heating and cooling systems free of any airborne contaminants.  As a result, the filter should improve the quality of the air you breathe.  Most air filters are one inch thick, but you can also purchase thicker air filters that trap even more contaminants.

However, your furnace or air handler is made with a filter slot for a specific width of filter. If you want to use a thicker filter with your current system, then we have two key considerations for you.

  1. The slot will have to be replaced with one that will accommodate a thicker filter. We recommend hiring an HVAC technician to do the work rather than DIY. The job isn’t easy, and if the fit of the new sheet metal isn’t really tight, you’ll leak air and diminish the efficiency of the system.
  2. Thicker filters often, but not always, restrict airflow more than 1-inch filters. The blower in your furnace or air handler might not move air effectively if a restrictive air filter is installed. Two results are possible. You will experience low air flow – not as much air will come out the vents, and your home will be properly heated and cooled. And the other result might be that your blower motor fails prematurely from having to work too hard. And if the entire system works harder because the thermostat temperature is never met, then the compressor in your AC might fail too.  

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