10 Reasons Why You Shouldn’t Run A Furnace Without A Filter

House owners forget about the furnace filters until their unit starts behaving abnormally. So, it is important to change the dirty filters regularly. But does the furnace need a filter? What happens if I run the furnace without a filter? Let’s find out.

Running a furnace without a filter can lead to poor indoor air quality, high energy bills, allergies, mold, contaminated ducts, fire hazards, and the shutdown of the entire system. So, it is recommended to regularly change/clean your furnace’s filter and avoid running the system without it.

So, filters are important, and you must change or clean them every 3 to 4 months. This article will explain the risks behind running a furnace without a filter in detail, how often to change the filter, and how to install it correctly. 

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Can I run a furnace without a filter?

Furnaces are an important part of the house, especially during the winter. 

Without the furnace, you won’t be warm and comfortable in the winter. 

The air filters in the furnace are an essential part of the furnace. 

The filters will filter the air the unit sucks in from your surrounding atmosphere. 

After the air is treated, it will send out warm fresh air around your room. 

So, filters play an important role in your furnace. 

Running a furnace without a filter for extended periods can cause several issues like poor air quality, allergies, reduced airflow, and even a fire hazard. 

How long can you run a furnace without a filter?

You may be able to run a furnace without a filter, but it should be temporary for a short period. 

Whether you can run a furnace without a filter depends on what the experienced HVAC team suggests. 

Some house owners have said running a furnace without a filter for one night is fine. 

At most, you may run a furnace without filters for 2 days. 

However, it is better not to run.

Every time you run the furnace without filters, you risk its efficiency and damage its potential to work properly. 

Filters are present in the furnace for a good reason, and running a unit with one will be dangerous and harmful. 

If you do not prefer running a furnace without a filter, even for a single day, wait for one more day until you clean or buy a new filter the next day. 

Why is a filter important in a furnace?

Your house will always be filled with microscopic stuff like hairs, dead skin cells, dust, dirty air particles, etc. 

You cannot do anything about it unless you have an air purifier. 

Your furnace should blow warm air around you to make you comfortable. 

When you start the furnace, it gathers and sucks the air from the surrounding environment through the intake vents. 

The filter traps all these microscopic stuff and stops them from entering your furnace with the air. 

The air gets filtered with the help of the air filter and releases clean warm air. 

It keeps your unit efficient for longer periods.

When a furnace runs without a filter, dirt and debris enters your furnace, gather around the other components, and reduce their lifespan and efficiency. 

Why should you not run your furnace without a filter?

Running a furnace without a filter can lead to several issues. 

Here are the risks of not running a furnace without a filter and why you should not run them without a filter:

1. Poor air quality

First, pet hair, bacteria, and other harmful particles wander in the air, and ultimately gravity allows them to settle on the floor. 

When the furnace sucks in the air to treat it, it will irritate these particles and mix them up with the air again. 

As a result, these particles will travel inside the furnace. 

The furnace’s heat may destroy some particles, but the rest will pass inside the furnace through the vents if you do not have a filter. 

When this air releases into the surrounding air, it deteriorates indoor air quality. 

So, it is no longer healthy for you or your family.

They are now airborne and will deteriorate the health of the family members. 

Your lungs cannot tolerate dust particles and pet hair, and you will have severe breathing issues and allergies. 

It can be more dangerous for people with lung diseases, asthma, and COPD (Chronic Obstructive Pulmonary Disease). 

Since the air filters will filter the air by trapping these air particles, you must have a filter in your furnace.

2. Increased illness in the house

Along with breathing and lung issues, you may also have several other illnesses due to running a furnace without filters. 

When the dirt and air particles mix with the breathing air, your body constantly fights them, reducing your immune system. 

A reduced immune system means you will easily become susceptible to all the general sicknesses and viruses.

You will suffer the most during winter. 

The combination of a reduced immune system and plenty of bacteria around you will give you a perfect storm of illnesses.

So, make sure that your furnace is running with a filter. 

3. Risk of fire hazards

The small dust particles could be flammable. 

Therefore, your unit will catch fire when these particles slowly accumulate inside a furnace without a filter. 

A fire can damage your furnace beyond repair. 

But if the fire spreads into your house, it will also endanger you and your family. 

The filters can trap these flammable particles, prevent them from entering the furnace, and stop this dangerous fire. 

So, remember to add a filter to your furnace. 

4. Contaminated ductwork and mold growth

When you run a furnace without a filter, dirt particles and air toxins travel through the ductwork to reach your furnace. 

The debris and other toxic particles can accumulate inside the ductwork because they get trapped by the ribbed aluminum tubes. 

This accumulation of dirt particles will gather moisture inside the ductwork, creating a great environment for mold growth. 

Over time, these mold spores will start traveling through the ducts to the furnace and circulate your house through the air. 

It can be extremely dangerous for you and your family, especially if people with lung disease, asthma, and allergies. 

So, do not run your furnace without a filter. 

5. Reduced airflow  

Running a furnace without a filter will reduce the airflow over time. 

Without a filter, the dust particles gather inside the furnace around the other areas, like, the air ducts, vent covers, blower motors, heat exchangers, etc. 

Since these are the major components behind heating and releasing the air, they won’t be able to push the air out of the unit due to the dust cakes, reducing airflow. 

Fixing these parts is quite difficult and expensive. 

But changing the filters is easy. So, install filters in your furnace. 

6. Short cycling and overheating

When the furnace turns on and off repeatedly due to overheating, it is called short cycling. 

When the furnace does not reach its target temperature, it will start back up after cooling down. 

Repeated turning on and off will wear out your furnace faster and reduce its efficiency and lifespan. 

Sometimes, it can also blow off the motor and compressor. 

The unit and the furnace components accumulated with dust will need to struggle to spread the warm air out, and the unit will overheat.

7. Reduced efficiency

As I said earlier, without a filter, the other parts, like the evaporator coils, blower fan motors, ducts, and vents, will get covered with dust particles. 

As a result, they lose their efficiency and struggle to give you the desired temperature. 

Hard work will overheat the unit, release less airflow, and increase the energy bills. 

Over time, these parts will stop working.

The HVAC system will be taxed much more than normal, mostly during winter. 

It is the time when the furnace works overtime to give you the desired temperature. 

For example, the evaporator coil is a winding aluminum tube and part of the air conditioner. 

It helps in removing the heat from the air. 

If you do not have a filter in your furnace, this tube will play the role of the filter due to its metallic fins and tubes. 

As a result, the coil will be caked with dust and reduced efficiency. 

8. High energy bills

As I mentioned earlier, the furnace parts, for example, will have motor and compressor issues due to the lack of air filters.

The dirt will accumulate in the motor and compressor and create malfunctions due to straining. 

So, the parts will need to work harder, and the furnace will keep running to reach the desired temperature, increasing energy bills.

9. Damaged furnace

Only a fire will not damage your furnace. 

Other things can also damage your furnace due to the absence of a filter inside the furnace. 

The air filters trap these particles and keep them from accumulating inside furnace components. 

Without a filter, the dust particles will accumulate in other furnace components, causing malfunction and ultimate breakdown. 

You will sometimes see the furnace not working or hear weird sounds from the belt. 

Sometimes, most of the parts fail together. 

If nothing serious is yet to happen, you are fortunate enough to have time to correct this problem by cleaning the furnace and installing a filter. 

If the problem persists, you will require an expensive repair or replacement of the whole unit. 

The furnace will always choose an excessively cold day to malfunction due to heavy usage. 

You must pay extra during an emergency repair, especially if the timing is outside the normal working days or hours. 

10. Water leakage

The condensate drain will clog when you run a furnace without a filter. 

The drain helps to drain the condensation and extra humidity removed by the air conditioner from your house. 

When you do not put a filter, the air particles will accumulate in this pan. 

As a result, when the moisture does not get a place in the condensate drain, it will come back to your unit and start leaking around it. 

Adding the filter will trap these particles and stop them from accumulating in the condensate drain. 

Therefore, the removed moisture will go to the condensate drain to be drained outside your house. 

How often should I change the furnace filters?

Generally, you can change the furnace filters every 3 to 4 months. 

But, if you are too busy for that, change them at least once per year to encourage the furnace’s smooth functioning. 

The frequency of the filter change depends on the filter size. 

The thinner the size, the more frequent change it requires. 

If the filter is 1 inch thick, you should replace it every 1 to 2 months. 

You should replace filters measuring 2 to 3 inches in thickness every 3 to 6 months. 

If the filters are 5 to 6 inches, replace them every 9 to 12 months. 

You can look at the filter size and estimate a good time for a replacement. 

Some filters do not need to be changed but only cleaned. 

Some filters are washable. You must wash them with water, dry them, and put them back in place. 

If you do not change the filter for a long time, your furnace will lose its proficiency. 

Over time, the safety switch will get activated, and the system will frequently shut off. 

Signs of a dirty air filter

One of the common signs behind a dirty air filter is no heat from the furnace, but the blower keeps working. 

The furnace will work harder but won’t give you the desired temperature. As a result, the energy bills will keep increasing. 

Sometimes, your furnace may give you the desired temperature after a long cycle. 

The energy bills will still increase, indicating that a dirty filter has made the furnace work harder than normal. 

Another sign is decreased performance. 

Your furnace will not work up to your expectations. 

It may struggle to provide warm air, blow cold air, overheat easily, leak water, or produce weird noises. 

A dirty air filter could be the problem.

You will also receive hot and cold spots in your house because the unit struggles to distribute the warm air to every room. 

When a filter is dirty, your furnace circulates allergens and dust from the dirty filter. 

You might experience frequent headaches, allergies, and breathing issues. 

What does it mean if you have a black furnace filter?

It is common for the filter to turn gray. 

It is a sign that your filter has been kept the same for a long time, and dust and debris have settled on it. 

But if the filter is black, something is seriously wrong with it. 

It could be an incomplete combustion cycle, for which the unit has produced toxic carbon monoxide and stained the filter. 

You need an HVAC expert to handle the matter. 

Other causes could be mold or excessive amounts of dirt accumulation. 

Candles and fireplaces could be the reason behind mold as they produce soot, hence staining the filter with black color. 

When you see this, turn off the unit and call your local HVAC to look into the matter.

How to change furnace filters?

  • Shut off your furnace at the breaker box. 
  • The filter will be inside the furnace or return air vents. Remove the service panel or check the furnace manual for guidance. 
  • The old filter’s frame will show you the filter size. The size varies based on the type of furnace. 
  • Use the furnace manual for the filter’s type, size, and thickness. 
  • Remove the old filter and dispose of it. 
  • Slide the filter in its position and ensure the arrows on the frame point are in the same direction as the previous one.
  • If the filter is reusable, wash it off and place it back. 
  • Close the service panel and turn on the furnace to check.

A short fix

Suppose your furnace filter has become very dirty and requires immediate replacement, but the right filter size for your furnace is unavailable.

In that case, either run the furnace without the filter or let the filter stay for a few more days. 

If you are concerned about the consequences of running the furnace without a filter or letting the dirty filter stay, you can try a short fix. 

You can buy 1-2 oversized filters and cut them to fit your furnace. 

Use duct tape to keep them together and install them in the filter slot.

Running a unit with this kind of filter is better than nothing. 

When you get the filter suiting your furnace, install it immediately.

Also check:

Final thoughts

It is very unsafe to run a furnace without a filter.

It can lead to various issues, like reduced airflow, low efficiency of the furnace, high energy bills, leakage, fire hazards, poor air quality, etc. 

Some suggest running a furnace without a filter, but that is temporary. You may run it for only 1-2 days. 

But, I do not suggest taking the risk, even for a day. You reduce the unit’s lifespan and performance whenever you turn on the furnace without a filter. 

Getting a new filter for your furnace is better when you find it very dirty. 

Gray filters, no warm air, high energy bills, and hot and cold spots in the house are common signs of a dirty filter.

Depending on the thickness, change the filters every 3 to 12 months. 


Should I change the filter more often in winter?

During the winter, the furnace runs over time. So, there might be more chances of dirt and soot buildup. 

So, you might have to change the filters frequently in winter. 

Experts suggest changing them once in winter or every 2-3 months. 

Reference: Wikipedia.

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Arthur Smith

Howdy! I am Arthur Smith, an electrical engineer who is extremely passionate about electronics. I have lived in different parts of the US and currently reside in Wisconsin. I am one of those rare children who knew what he wanted to study and do in life right from the start. I was a curious child who wanted to know how switches work and how the AC works, and I would always observe my dad whenever he would handle the wires and fix things around the house. I currently work as an electrical engineer at a reputed company and write for this blog. And I read loads of books or play video games in my free time.

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