What Will Happen if You Don’t Change Your Furnace Filter

The inside of you home will always have particles such as dust, lint, pollen, animal fur, and mold among others that reduce the quality of your indoor air. Those particles are removed from the house by your HVAC system and trapped by the furnace filter.

It therefore helps to understand the airflow direction in and out of the furnace. The furnace pulls air from the house through the return air duct and after heating/cooling it sends it back to the house through the supply air duct.


The furnace filter is therefore installed in the return air duct. That way, the filter removes the particles before they can reach the furnace therefore ensuring that the air entering the house is not only heated/cooled but also clean.

Furnace filters are however not permanent (some are washable though). They need to be replaced regularly to allow easy passage of air to the furnace.

Ideally, a furnace filter should be changed after every 90 days (3 months). However, the filter may be changed sooner or later than that depending on the number of people and pets in the house, the filter’s MERV rating, its thickness, health condition of family members among other factors.

So, what will happen if you don’t change your furnace filter? And how long can you go without changing a furnace filter?

If you don’t replace your furnace filter, it will become clogged and restrict the flow of air to the furnace. That will force the furnace blower to work harder to force air through the filter, resulting in high energy bills, short-cycling, reduced airflow and low quality indoor air. The furnace’s lifespan will also reduce drastically.

You can go for as long as 12 months without changing your furnace filter. That is however the case for a house that is rarely occupied like a cabin or holiday home. Thick furnace filters (5 to 6 inches) can also last as long as 12 months.

Failure to change the furnace filter can even make you sick. You and members of your family will experience frequent headaches, respiratory infections, dizziness, headaches and itches eyes and throats while allergies and asthma will worsen.

What Happens if you Forget to Change Your Furnace Filter?


The following are some of the things that will happen if you don’t or forget to change your furnace filter:

1. Increased Energy Bills

A furnace filter is a good thing when it is working properly but it can be a nightmare if it is not changed for a long time. One of the first sign of a dirty and clogged filter that needs to be changed is a sudden increase in your electricity bills.

You see, air is designed to flow through the filter effortlessly. The air to the furnace is pulled in by the blower, which is powered by an electric motor.

As particles are deposited on the filter, it restricts the flow of air, forcing the blower to work harder to compensate for the reduced flow rate.  If you don’t replace the filter at that time, the blower motor will work even harder, resulting in consumption of more energy and thereby an increase in your energy bills.

However, there are lots of appliances in the house (electric water heater, washing machine etc.) which can have the same effect and is therefore hard for the homeowner to correctly deduce the source of the high energy bills.

2. Short-Cycling

In an HVAC system, short-cycling refers to a situation where the furnace keeps coming on and off in quick succession. But why does the furnace short-cycle.

As mentioned, the clogged furnace filter prevents air from passing through it forcing the blower to run for a long time.  A furnace that runs for a long time will definitely cause the motor to overheat.

The unit is however fitted with a limit switch (basically a heat sensor) which turns the motor off to prevent it from overheating. As soon as the motor has cooled down, it comes back on. That is how failing to change a furnace filter causes short-cycling.

3. Inadequate Temperature Regulation

As you already know, the function of an HVAC system is to control indoor temperature (either by heating or cooling) and a furnace is central to that function. For that to happen however, air needs to be able to move freely through the filter.

When you don’t change your furnace filter, air will struggle to enter the furnace meaning that your HVAC system will be unable to regulate the indoor temperature.

Anytime you notice a reduction in airflow from the supply air duct and you haven’t replaced the furnace filter in a long time, that could be the source of your problem.

4. Health Issues

When the furnace blower starts to spin, it pulls air from the room, which also carries with it suspended impurities that are then trapped by the filter. If the filter is clogged and can no longer trap more particles, they are likely to move back to the house.

Apart from reducing the quality of the indoor air, health issues like asthma and allergies will worsen since the triggers will be readily available in the air.

There will also be an increases in respiratory infection, sinuses, dizziness, headaches, itchy eyes and/or throat. So in case you have been wondering, failing to change a furnace filter can actually make you sick.

5. Reduced Furnace Lifespan

Although the furnace is fitted with a limit switch to turn off the motor and thus prevent overheating, the constant running of the motor will still cause it to wear out faster than an otherwise moderately running unit.

The end result of that is that you will need to conduct expensive repairs and/or replacements. Failing to change your furnace air filter will therefore reduce the lifespan of your furnace.

How Often Should You Replace Your Furnace Filter?


The rule of thumb is to change your furnace filter after 3 months. However, you many need to change it as frequently as monthly or even once a year depending on some factors.

For instance, the number of people living a house will dictate how often you change your furnace filter. The more the people a house has, the more they bring in dirt from outside and the quicker the filter clogs necessitating a replacement.

Number of pets in a house will also determine if you will change the filter monthly or after 3 months. As you know, pets are always shedding fur and dander which end up trapped on the furnace filter.

Ideally, if you have a single pet, change the filter after 60 days. If you have more than one pet you should change the filter after every 30 days. Homes with no pets should have the filter changed after 90 days.

You will also need to change the furnace filter in your house on a monthly basis if you or another member of the family suffers from allergies or asthma. If not, 90 days is a good time to wait.

Your furnace filter’s MERV rating is another thing to consider. The higher the rating the more often you need to replace it.

Furnace filters also come in different sizes (thickness). Thicker filters don’t need to be replaced as often as the thin filters. Here is how often you should replace furnace filters in accordance with the size:

  • 1 to 2-inch furnace filter – 1 to 3 months
  • 3 to 4-inch furnace filter – 6 to 9 months
  • 5 to 6-inch furnace filter – 9 to 12 months

In homes where the HVAC system is used heavily, the furnace filter will clog at a faster rate than a home where the HVAC system is moderately used. In such a home, the filter will need to be replaced more often.

To make sure you are not caught off-guard, HVAC technician recommend changing HVAC filters at the beginning of each season (since there are 4 seasons in a year).

Also, I would advise that you remove and inspect your furnace filter every month. Have it changed if it is dirty. Remember to always turn off the furnace before removing the furnace.

If you are having problems locating your furnace filter, check out this post.

Signs You Need to Change Your Home’s Air Filter

So, what are the tell-tale signs that your furnace filter is dirty and that it needs to be changed? Here are some of them:

  • Increase in energy bills
  • Short-cycling of the furnace
  • Reduced airflow
  • Worsening of asthma and allergies
  • Burning smell in the house (due to blower motor overheating)
  • Poor quality of indoor air
  • An increase in respiratory infections


And basically those are the things that will happen if you don’t change your furnace filter. Make sure that you replace your furnace filter at least quarterly to avoid these problems.